THE UNIVERSITY OF NEW SOUTH WALES
THE OBSERVATION AND MODELLING OF WINDS OVER SOUTH EASTERN AUSTRALIA
KENNETH L. BATT
A thesis submitted in fulfillment of the requirements for the degree of Master of Science (Mathematics), 2005
School of Mathematics, Faculty Of Science, University of New South Wales Sydney 2052, Australia
PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Project Report Sheet Surname or Family name: BATT First name: KENNETH LESLIE Abbreviation for degree as given in the University Calender: MSc School: MATHEMATICS Faculty: SCIENCE Title: THE OBSERVATION AND MODELLING OF WINDS OVER SOUTH EASTERN AUSTRALIA
Abstract 350 words maximum: (PLEASE TYPE)
This study uses a very high resolution numerical weather prediction (NWP) model to investigate the complex structure and behaviour of cold fronts along the New South Wales coast during the warmer months of the year, the complex interaction between the wind flow and coastlines and elevated areas as well as the lee-trough effect, particularly the way it affects waters off the east coast of Tasmania, The study also investigates the utility of the higher resolution NWP model to better predict wind fields compared to a lower resolution model.
The University of New South Wales very high resolution model (HIRES), nested in the Australian Bureau of Meteorology's coarse NWP model (GASP), was run at various horizontal resolutions (from 15 to 25km) in order to investigate the above-mentioned features. It was found to have very good skill in resolving the features and was also found to be very accurate in the prediction of surface wind fields for various yacht race events out to at least four days ahead.
It can be concluded that there is considerable skill in the ability of high-resolution NWP models such as HIRES, to predict the major features of the wind fields over the ocean out to several days ahead. Moreover, it was also able to more accurately simulate the complex structure of the summer-time cool change as it progressed along the NSW coast than the lower resolution model runs. The influence of coastlines, particularly ones with complex topographical features, on the wind flow was demonstrated to a limited extent throughout the study. Finally the following concepts were also verified as a result of the study:
air flow takes the path of least resistance the shape of topography can help generate local turbulence the orientation of the wind flow to a mountain range is important in determining turbulent effects. under certain airflow and stability situations, standing wave activity and a lee trough can be observed in the lee of mountains, hills or even high coastal cliffs.
Declaration relating to disposition of project report/thesis I am fully aware of the policy of the University relating to the retention and use of higher degree project reports and theses, namely that the University retains the copies submitted for examination and is free to allow them to be consulted or borrowed. Subject to the provisions of the Copyright Act 1968, the University may issue a project report or thesis in whole or in part, in photostat or microfilm or other copying medium. I also authorise the publication by University Microfilms of a 350 word abstract in Dissertation Abstracts International (applicable to doctorates only).
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FOR OFFICE USE ONLY Date of completion of requirements for Award:
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THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS
DEDICATION
To my adorable spouse Helen and our beautiful children Christie and Georgia who helped make this possible, my love and thanks forever.
To my mother and late father who allowed a passion to blossom.
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ACKNOWLEDGEMENTS
Professor L. M. Leslie, for providing the opportunity, guidance and tools for this thesis.
Professor J. Middleton for guidance and supervision.
Mr R.P. Morison for technical assistance.
Dr B. W. Buckley for guidance and motivation.
Dr M. Speer for technical assistance.
Dr L. Qi for technical assistance.
Mr D. Williams for technical assistance.
Ms Helen Smith for technical assistance.
My family Helen, Christie and Georgia for allowing me the time to sail, research and write.
The Bureau of Meteorology for allowing me to take up the opportunity and offer the time off to research and write this thesis.
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SUPPORTING PUBLICATIONS
Journals
Batt, K.L. (1994). The Meteorology of the 1993 Sydney to Hobart Yacht Race. Offshore
Yachting Feb/Mar 1994, Journal of the Cruising Yacht Club of Australia, Jamieson Publishing.
Batt, K., and James, J. (1994). Weather dampens Sydney to Hobart yacht race. Mariners
Weather Log, 38: 8-11.
Batt, K.L. (1994). The 1993 Sydney to Hobart Yacht Race “Weather News” (in-house journal of the Australian Bureau of Meteorology).
Buckley, B., and Batt, K. (1997). The Weather Factor-Sydney to Hobart Yacht Race 1996.
Offshore Yachting Feb/Mar 1997, Journal of the Cruising Yacht Club of Australia, Jamieson
Publishing.
Batt, K.L., and Hainsworth, A. (2001). Tricky Tassie Coast. Offshore Yachting: December
2001, Journal of the Cruising Yacht Club of Australia, Jamieson Publishing.
Batt, K.L., and Leslie L.M. (1998). Verification of output from a very high resolution
numerical weather prediction model: the 1996 Sydney to Hobart yacht race. Meteorol. Appl.
5: 321-327
Batt, K.L., Morison R.P., and Speer, M.S. (2000). Direct verification of forecasts from a very
high resolution numerical weather prediction (NWP) model. Meteorol Atmos Phys 74: 117-
127
Batt, K.L, Qi, L., and Morison, R.P. (2002).The Modeling and Observation of a Lee Trough
event over Eastern Tasmania. Meteorol Atmos Phys 80: 177-187.
4 Conference Proceedings
The Verification of Output from a High Resolution Numerical Weather Prediction Model: The
1996 Sydney to Hobart Yacht Race. Paper presented at AMOS-APOC Conference 1997,
Macquarie University.
A Modelling and Observational Study of Sea Breeze Regimes over the Greater Sydney Coastal
Area. Paper presented at AMOS-APOC Conference 1997, Macquarie University.
Verification of Output from a Very High Resolution Numerical Weather Prediction Model:
The 1996 and 1997 Sydney to Hobart Yacht Races. Paper presented at Australian
Atmospheres and Oceans ’98, 9-12 February, 1998 Victoria University, Wellington, New Zealand.
Course Attendance
Numerical Weather Modelling, MATH5295 UNSW 1996.
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CONTENTS
Chapter 1 Introduction 11
1.1 General and Aims 11 1.2 Data sources used in studies 15
Chapter 2 Theory 19
2.1 The greater accuracy of a very high resolution model over a lower resolution global model for the same domain. 19 2.2 The Summertime Cool Change over Southeast Australia 20 2.3 Coastal Wind Flows 27 2.4 Airflow in the vicinity of Complex Orography 31
Chapter 3 Description of the Model 35
Chapter 4 Verification of output from a very high resolution numerical weather prediction model: The 1996 Sydney to Hobart yacht race
Overview 39 4.1 Introduction 40 4.2 The meteorological setting 42 4.3 Model performance 44 4.4 Discussion and verification of results 48 Figures 53
Chapter 5 Direct verification of forecasts from a very high resolution Numerical Weather Prediction (NWP) model.
Overview 69 5.1 Introduction 70 5.2 The meteorological setting 71 5.3 Model performance 73 5.4 Discussion and verification of results 79 Figures 83
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Chapter 6 The modelling and observation of a lee trough event over eastern Tasmania.
Overview 97 6.1 Introduction 98 6.2 Theory 101 6.3 Synoptic and Mesoscale overview 104 6.4 Results 107 6.5 Discussion 109 Figures 111
Chapter 7 Concluding Remarks 126
References 130
Appendix 1 135
A1.1 Introduction 135 A1.2 A brief chronology of events 136 A1.3 Conclusion 144
7 List of Figures
Chapter 2
Figure 2.1 Classic “textbook” picture of a cold front. Figure 2.2 Cold front with an associated frontal transition zone. Figure 2.3 A roll cloud over Botany Bay, NSW. Figure 2.4 Distorted shape of a summertime cold front as it moves along the NSW coast. Figure 2.5 A vertical cross-section of a front along the NSW coast. Figure 2.6 Topography and location map of Sydney Harbour, NSW. Figure 2.7 An idealized lee trough and mesoscale heat low event in Tasmania.
Chapter 4
Figure 4.1 Map of southeastern Australia showing locations of places mentioned in this chapter. Figure 4.2 The HIRES model domain including topography. Contour interval is 100 m. Figure 4.3 Australian region mean sea level pressure analyses (4hPa spacing) at 1000 EDST for (a) 26 December 1996 and (b) 27 December 1996 and (c) 28 December 1996 and (d) 30 December 1996. Figure 4.4(a) HIRES model forecast winds for 1600 EDST on 26 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison. Figure 4.4(b) Wind speed plot for Bellambi Point automatic weather station on 26 December 1996 (wind speed in knots, time in EDST) with the 25km horizontal resolution model forecast winds (smoothed black line) overlain. Figure 4.5(a) As in Figure 4.4(a) except at 15 km resolution. Figure 4.5(b) As in Figure 4.4(b) except at 15 km resolution. Figure 4.6 As in Figure 4.4(a) except for 1000 EDST on 27 December 1996. Figure 4.7 As in Figure 4.4(a) except for 0400 EDST on 28 December 1996. Figure 4.8 As in Figure 4.4(a) except for 2200 EDST on 28 December 1996. Figure 4.9 As in Figure 4.4(a) except for 2200 EDST on 29 December 1996. Figure 4.10 As in Figure 4.4(a) except for 1000 EDST on 30 December 1996. Figure 4.11 Graph of V wind component error. Figure 4.12 Graph of U wind component error.
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Chapter 5
Figure 5.1 Map of southeastern Australia showing locations of places mentioned in this chapter. Figure 5.2 The HIRES model domain including topography. Contour interval is 100 m. Figure 5.3 Australian region mean sea level pressure analyses (4hPa spacing) at 1100 EDST 26 December 1997 inclusive. Figure 5.4 The wind triangle (After Chisnell, 1992). Figure 5.5 The ground wind and tide wind combine to form the true or sailing wind, whose direction and speed can be calculated from the boat speed, compass heading, apparent wind speed and angle, as above (After Chisnell, 1992). Figure 5.6 HIRES model forecast winds at 12 metres for 1700 EDST 26 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are shown for the position of the yacht Nicorette and for selected Bureau of Meteorology (BoM) coastal observation stations. Figure 5.7 As in Figure 5.6 except for 2300 EDST 26 December 1997. Figure 5.8 Wind velocity data for Nowra AWS for 26 December 1997. Times are for Eastern Standard Time (EST). Add 1 hour to obtain EDST. Figure 5.9 As in Figure 5.6 except for 1100 EDST 27 December 1997. Figure 5.10 As in Figure 5.6 except for 1100 EDST 28 December 1997. Figure 5.11 As in Figure 5.6 except for 2300 EDST 28 December 1997. Figure 5.12 As in Figure 5.6 except for 1100 EDST 29 December 1997. Figure 5.13 As in Figure 5.6 except for 1700 EDST 29 December 1997. Figure 5.14 Graph of V wind component error. Figure 5.15 Graph of U wind component error.
Chapter 6
Figure 6.1(a) Topography of Tasmania including place names used in this chapter. Figure 6.1(b) West-east cross-section through Tasmania Figure 6.2 An idealized lee trough and mesoscale heat low event in Tasmania (after Bureau of Meteorology, 1991) Figure 6.3 Australian region mean sea level pressure analysis (4hPa spacing) at 1100EDST from 27 to 30 December 1995 inclusive. Figure 6.4 Upper wind profiles for both Hobart and Launceston Airports from 0900 EDST 28 December to 1500EDST on 29 December 1995 Figure 6.5 Aerological sounding for Hobart Airport for 1000EDST 27 December 1995 Figure 6.6 Aerological sounding for Hobart Airport for 1000EDST 28 December 1995 Figure 6.7 Streamline mesoscale analysis valid 1400EDST on 28 December 1995. Wind observations from yacht Ragamuffin are circled and in standard format. Figure 6.8 Streamline mesoscale analysis valid 1400EDST on 29 December 1995. Wind observations from yacht Ragamuffin are circled and in standard format.
Figure 6.9 HIRES model forecast winds for 1100EDST on 28 December 1995 at 10km
9 resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled. Figure 6.10 As in Figure 6.9 except for 1400EDST on 28 December 1995. Figure 6.11 As in Figure 6.9 except for 2300EDST on 28 December 1995. Figure 6.12 As in Figure 6.9 except for 0500EDST on 29 December 1995. Figure 6.13 As in Figure 6.9 except for 1100EDST on 29 December 1995. Figure 6.14 As in Figure 6.9 except for 1400EDST on 29 December 1995.
Appendix 1 Weather dampens Sydney to Hobart yacht race
Figure A1 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 26 December 1993. Figure A2 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 27 December 1993. Figure A3 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 28 December 1993. Figure A4 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 29 December 1993. Figure A5 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 30 December 1993. Figure A6 Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 31 December 1993.
List of Tables
Chapter 3
Table 3.1 Key HIRES model features.
Chapter 4
Table 4.1 Comparison between 135km GASP, 25km HIRES and actual conditions. Table 4.2 t-test: paired two sample for means- U wind component. Table 4.3 t-test: paired two sample for means- V wind component.
Chapter 5
Table 5.1 Forecast 135km, 25km and 10km resolution model winds compared with actual wind velocities. Table 5.2 t-test: paired two sample for means- U wind component. Table 5.3 t-test: paired two sample for means- V wind component.
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CHAPTER 1
INTRODUCTION
1.1 Background And Aims
Since the 1970s Numerical Weather Prediction (NWP) has progressed in quantum leaps with very high resolution prediction models running routinely at horizontal resolutions in the range 500m to 5000m and with vertical resolutions anywhere between 20 and 60 levels.
Some of these models are able to run on laptop computers as well as desktop PCs.
Initial interest in the numerical simulation of the marine environment commenced during the
1950s with the forecasting of the wave spectrum (Pierson et al, 1955). It wasn’t until the
1980s that the importance of wave modelling (The Swamp Group, 1985) became really evident and this was quickly followed by the development of the ocean-atmosphere coupled model (The WAMDI Group, 1988).
With the increasing quantity of both surface and remotely sensed data that became available during the late 1980s and 1990s, came one of the biggest breakthroughs in NWP, namely that which led to the improvement in data assimilation techniques. These improved techniques allowed for the direct assimilation of infrared and microwave radiances which impact upon analysed temperature and humidity fields.
11 These huge improvements in data assimilation techniques and computing technology over time, coupled with a greater understanding of the more complex physical processes of the atmosphere (and oceans), has allowed NWP models to run at much faster speeds and at much higher resolutions than, say, 10 to 15 years ago. For a greater understanding of how NWP grew in Australia, the reader is referred to Leslie and Dietachmayer, 1992.
The motivation for this study stemmed primarily from my participation in my third Sydney to
Hobart Yacht Race during late December 1993. It was in the lead-up to and during the race that I had decided to explore the world of the very high resolution NWP model.
Up until this time, high to very high resolution NWP models were mostly used to study local meteorological phenomena such as the sea breeze circulation (McPherson, 1970, Pielke,
1974, Physick, 1976 and 1980, Abbs, 1986 and Abbs and Physick, 1992) and other mesoscale phenomena as outlined in Noye (1987). From an ocean yacht racing perspective, the interpretation of global and regional NWP model output was primarily used to aid in the construction of yacht race forecasts, such as the Sydney to Hobart and others around the globe. This essentially was the case until the HIRES NWP model was developed and became an operational model.
The work presented (to the author’s best knowledge) in this thesis was the first accurate very high- resolution weather simulation of any major ocean yacht race event in the world.
The very encouraging results from this study has since prompted the use of this model in most major Australian offshore yacht races in the construction of very detailed forecasts for the duration of the race. The duration of these, on average, range from 2 to 5 days. Following on from this, the HIRES model was also used by the author to aid in the construction of a
12 race forecast for the Auckland (New Zealand) to Rio de Janeiro leg of the 1998 Whitbread
Round the World race (now known as the Volvo Ocean Race). The model was run out to 14
days and the moving domain ranged from Auckland to Cape Horn. This was a world first and
the results were overall very pleasing.
The weather outlook for the 1993 Hobart Race was prepared a few days before the race
commenced. It was based upon the output from the Bureau of Meteorology’s low -resolution
global model GASP (Global ASsimilation and Prognosis). Although GASP performed very
well indeed, I thought at the time that more detail, particularly with respect to the forecast
wind fields, would have been very helpful.
The quality of forecasts that were received on the yacht during the race were generally
mediocre and this greatly inhibited the prediction of the exact location and the intensity of a
low pressure system that subsequently developed. This weather system produced gale to
storm-force winds for much of the race and enhanced my desire to try to improve the
forecasting not only for this race in the future, but for marine forecasting generally.
A peer reviewed paper that was submitted to the ‘Mariner’s Weather Log’ after the race hopefully gives an idea of the Sydney to Hobart Yacht Race, particularly when the weather becomes severe. This paper appears as Appendix 1.
Subsequent Sydney to Hobart Yacht races and other races along the New South Wales coast
increased my desire to better understand a number of complex interactions and hence to
undertake this thesis, which investigates the following:
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1. The utility of a high-resolution model over a global model in resolving finer scale
meteorological features. It is hoped to demonstrate that there is considerable skill in
the ability of high-resolution NWP models such as the University of New South
Wales HIRES model, to predict the major features of the wind fields over the ocean
out to several days ahead over lower resolution global models, such as GASP.
2. The behaviour of cold fronts along the New South Wales coast during the warmer
months of the year. The complex structure and behaviour of cold fronts along the
New South Wales coast during the warmer months of the year is to be investigated by
using HIRES run at very high resolutions. A very high resolution model should be
able to more accurately simulate the complex structure of the cool change as it
progresses along the NSW coast than the coarser resolution model runs.
3. The influence of coastlines, particularly with complex features, on the wind flow. It is
generally accepted in the meteorological community that unless the numerical model
is run at horizontal resolutions of 5 km or less, a model will normally fail to detect the
very localised changes to the wind velocity fields (e.g. speed enhancement/reduction
along coastlines and around headlands and the evolution of the sea/land breeze)
along/over complex coastlines and the coastal zone generally. It is hoped from this
study to prove this hypothesis; and
4. The marked effect of a mountain barrier on the general wind flow. In particular to
investigate the following theoretical concepts that:
14 • Airflow takes the path of least resistance. During thermally stable conditions air will
flow around obstructions and through valleys or narrow channels between high
topography. During thermally unstable conditions air will flow up and over
obstructions.
• The shape of topography can help generate local turbulence.
• The importance of the orientation of the wind flow to a mountain range in
determining turbulent effects.
• Under certain airflow and thermal stability situations, standing wave activity and a
lee trough can be observed in the lee of mountains, hills or even high coastal cliffs.
1.2 Data Used In Studies
1.2.1 Data sources used in the computer model
The three studies presented in this thesis used Bureau of Meteorology archived real-time data
as the basis for initialising the model. In addition, a weekly averaged, high-resolution sea-
surface temperature data set for the Tasman Sea, which was derived from satellite radiances,
was incorporated into the model. This was used in preference to the much lower resolution
climatological data that had been used in the Bureau’s operational GASP model at that time.
1.2.2 Observational Data
Orlanski (1975) first proposed atmospheric scale definitions according to the different
processes with characteristic time and horizontal scales. A mix of synoptic scale (2000 to
10000km), meso-scale (2 to 1000km) and microscale (0 to 2km) data were used in the three
studies presented below. These data was were used since windflow involves very complex
15 processes that bridge the above scales. The above definitions were used in the studies presented in this thesis.
It must be stressed that surface conditions being monitored by land stations may differ quite markedly from those being experienced by a vessel at sea, even as close as 2km from the coast.
Problems surrounding land stations primarily arise from the nature of the orography surrounding the station (surface roughness), elevation of the station, proximity to the coast, and in the case of wind, the exposure of the wind sensors. The standard height for wind measurement at Bureau of Meteorology observing stations is 10 metres above ground level with a clearance of ten times the height away from any obstruction(s). At times, features such as river valleys, mountain ranges and hills can interfere with the general wind flow at the station, even though the wind sensors are mounted some 10 times the height away from the particular obstruction. The stability of the planetary boundary layer can also be a factor at times. Another very important factor is that the reported wind velocity is taken as the average over a ten-minute period.
At sea the measurement of wind velocity on a vessel may introduce errors. The most obvious one of these is that the effect of the vessel’s motion is included in the measured wind speed.
Some anemometers will remove this via a simple vector calculation. This is fine, assuming the vessel is moving steadily. Caution is required when the vessel is ploughing through heavy seas – slowly rising up a crest then surging into the following trough. If the anemometer computer is averaging the vessel’s speed over any more than a couple of seconds the vessel’s
16 movement will not be removed reliably from the reported wind speed. The technique by which the vessel speed is being determined can also be suspect.
What anemometers actually measure depends upon a number of factors. Most important of these is how the anemometer is exposed. A typical vessel mounting would be on a short spar close to the top of the mast. The wind speed measured is, therefore, dependent upon the height of the mast. An anemometer on a 6-metre mast will measure a much lighter wind than that measured on top of a 20-metre mast. The up-wash effect of the wind blowing upwards around the mast itself (if the anemometer is mounted too close to it) is also important. This effect will vary depending upon whether the vessel is sailing into the wind or down wind, being greatest into the wind. Also important is the response time, averaging period and calibration of the anemometer. Most commercial anemometers are only calibrated up to around 30-knot mean wind speed, which is sufficient for most purposes. What they measure beyond this is anyone’s guess.
The response time of the anemometer relates to how quickly the cups spin up and spin down as the wind speed varies. An anemometer that is very responsive to slight changes in wind speed in light wind conditions can be prone to overspin in strong wind conditions, over- reading the wind speed. Any anemometer that does not spin down sufficiently rapidly will under-report the wind lulls, effectively over-reporting wind speed. The sampling interval of the anemometer is also to be considered. Small, responsive anemometers may report a one- second-wind gust. The anemometers the Bureau of Meteorology use, are designed to handle wind speeds up to 120 knots and report a 3-second wind gust that will be lower than the 1- second gust.
17
The average wind speed is more meaningful, but requires an accurate averaging technique.
Humans have a natural tendency to pay more attention to the stronger gusts and ignore the lulls, leading to an upward bias in estimated average wind speed. The best anemometers allow the selection of the averaging period, with 10 minutes being the period that reflects the wind field produced by identifiable weather systems rather than individual cloud lines or topographic features.
There are a few other factors that affect wind speed. The heel of the vessel can initially increase wind speed, then decrease it once it becomes too great. In rough conditions where the vessel is pitching or rolling significantly, the acceleration and deceleration of the masthead through the air can add considerably to the reported wind speed. Some anemometers can compensate for the mast movement by averaging over a few seconds.
However, very few can handle extreme excursions of the mast that last several seconds. The stability of the planetary boundary layer over the sea, as with the land case, is also an important factor.
The surface observational data used were obtained from the Bureau of Meteorology’s archives. Vertical soundings of temperature and wind that are presented in Chapter 4, were crucial in the study. These data were again sourced from the Bureau archives. The author performed micro-scale mobile single station observations of wind onboard a yacht utilising a
10 minute averaged true wind velocity (see Figures 5.4 and 5.5).
18 CHAPTER 2
THEORY
The studies below were designed to examine a number of theoretical concepts thought to be of great use for marine meteorologists involved in the forecasting of windfields over southeastern Australia. These were as follows:
2.1 The greater accuracy of a very high resolution model over a lower
resolution global model for the same domain.
As a way of achieving greater forecasting accuracy for a limited domain, a very high resolution model, such as HIRES, may be nested in a lower resolution model. This is achieved by obtaining archived operational data at the required lower horizontal resolution. In the case of the studies below, the Bureau of Meteorology’s global model GASP was used with a horizontal resolution of 135km. These analyses were interpolated at the same resolution as the HIRES model, and passed through its dynamic initialisation scheme. A HIRES forecast was produced at this resolution covering the Australian continent and adjacent oceans. A nested forecast was then run over the required domain at the required higher resolutions. For the studies below the model was run at resolutions between 10 and 25km, but it can be run at higher or lower resolutions.
The greater accuracy of HIRES over the lower resolution model GASP is ably demonstrated in the studies presented in this thesis.
19 2.2 The Summertime Cool Change over Southeast Australia
A feature of the weather map at anytime of the year is the cold front. This front mostly affects southern Australia, but from time to time, especially in the winter and spring months, can penetrate north into tropical areas.
A front is simply the boundary between two differing air masses. It may be moving, in which case the front is named for the advancing air mass - cold or warm - or stationary. Other than this there is nothing simple about fronts. The classic “textbook” picture of a cold front
(Figure 2.1) is an advancing air mass, cold in the case of a cold front, pushing into the existing air mass, warm in this case, causing the air at the boundary to rise and consequently form cloud and rain.
Figure 2.1: Classic “textbook” picture of a cold front.
20 This simplistic picture seldom applies in NSW, especially in the summer. Features of the
situation that can cause variations from the classic case include the properties of the existing
air mass - it may not be moist or unstable enough to form cloud or rain even when forced to
rise - and the depth of the advancing air mass. In NSW during summer this is often very
shallow, with interesting effects. The idea that a front is a single boundary is also frequently
found to be false. The concept of a transition zone or frontal zone is often more appropriate.
It is important also to realise that the boundaries which are called fronts are seldom
unchanging in their characteristics, but more often strengthen and weaken as they move along, and frequently dissipate or regenerate. For example, winds on each side of a front which act to increase the existing temperature difference, can markedly strengthen a front.
All these factors make forecasting the arrival of a “change” (although vague and ambiguous, arguably a better term than “front”) an infinitely more complex matter than simply timing it
and extrapolating its movement into the future.
2.2.1 Summertime Cold Fronts over South Australia and Victoria
An intensive field study of summertime cold fronts, called the Cold Fronts Research Program
(CFRP), was conducted in 1980, 1981 and 1984. Two types of cold front were identified.
The first consisted of a cold front with an associated frontal transition zone, and was the most
common type observed during the CFRP (Figure 2.2). It typically extends for approximately
300km ahead of the front and contains several discontinuities in wind, pressure, temperature
and humidity. These discontinuities are usually associated with bands of convective cloud.
21 CFRP studies indicated that the speed of movement of the final front depends on the winds in
the post-frontal air and the component of the winds opposing the front.
100 km 200 km 300 km
10 km Clouds
5 km
Clouds Rain
t jet e rly Frontal transition zone j he e ly ut z m r so e r te ld t e a o on r s C fr b W e ld - w Co a - e th S r o n
COREL\R3361-2 Discontinuities
Figure 2.2: Cold front with an associated frontal transition zone (after Garratt et al 1985).
The second type of front (see Figure 2.5) has a structure akin to a shallow layer of cold air -
sometimes only about 900 metres thick - sliding under warmer air ahead of it, and can be
thought of as a “gravity current” flow whose speed depends on the density difference
between the cold and warm air and the depth of the cold air. Pre-frontal winds also influence
the frontal speed. A very strong pressure gradient occurs in the immediate post-frontal air.
Usually there is little precipitation associated with events of this type. This kind of front, sometimes accompanied by a roll cloud (Figure 2.3) and nearly always with strong winds, moves through Bass Strait over the warmer months. At times it will precede the first kind of front by a number of hours, say, from 3 to 6 hours.
22
Figure 2.3: A roll cloud over Botany Bay, NSW (Photographer: Richard Jardine, BoM)
2.2.2 Fronts over New South Wales
The second type of front is often not the result of a front originating in the Southern Ocean; many of them form ahead of a major front somewhere over Australia, even as far east as
Victoria or close to the NSW coast, and are sometimes associated with small scale low pressure systems. In any case, these shallow fronts are the most common and difficult “cool change” forecasting problem in the summer in NSW. Their development can cause errors in forecasts of front arrival time and in forecasts of pre-frontal conditions.
The most difficult problem with shallow fronts is that the mountains of southeastern
Australia retard their movement, which then develops a distorted shape as seen on the surface synoptic chart (Figure 2.4), with locations close to the coast receiving the wind change long before the air behind the front becomes deep enough to move into the mountainous areas.
Over inland New South Wales, frontal evolution may also be most complex. West of the mountains the front can move quite slowly but may still “overtake” the front on the
23 mountains, and may often degenerate as an identifiable change somewhere over the interior of the state. Frequently wind change lines develop ahead of a major cold front. These are very difficult to predict and, because of the sparse observational network in the west of the state, are also difficult to detect.
148o E 149o E 150o E 151o E
2400 2100 1800 1700
1600 34o S
1500
1400 35oS
Canberra 1300
1200
36oS 1100
1000
0900
0800
0600 0700 0600 0700
COREL\R3361-1 Figure 2.4: Distorted shape of a summer-time cold front as it moves along the NSW coast (After Colquhoun, 1981).
24 The most dramatic of these shallow fronts are those which develop into the well-known phenomenon known as the Southerly Buster, which is a change involving the sudden onset of strong southerly wind squalls near the coast. A number of past Sydney to Hobart yacht race fleets have experienced this phenomenon a short time into the race period. The southerly winds are cool, originating from over the sea, and often replace northwesterly to northeasterly winds ahead of the front. The winds are most gusty just after the change and frequently reach at least 30 knots. Wind speeds of up to 72 knots have been recorded. The strong gusty winds may last for several hours. Temperature changes can be dramatic. A fall in temperature of 10-15°C in a few minutes is common. Southerly Busters commonly occur during the afternoon since this is when the temperature difference between the prefrontal and postfrontal air will be greatest and this is one of the contributing factors to the speed and strength of the front.
2.2.3 Important Features of Shallow Fronts
Besides the retardation of the front by the mountains , the following are the main features of these shallow fronts;
• the afternoon frontal speed of movement can be double those of the morning.
• post-frontal winds can vary quite markedly as one moves inland. Near the coast,
winds can be strong southerlies. However on the Blue Mountains west of Sydney, the
winds can be more from the east-northeast and lighter. West of the mountains, post-
frontal winds are generally southwesterly. Meteorological experience suggests that
these wind direction variations are common to most shallow fronts. The difference in
post-frontal winds east and west of the mountains can sometimes mean that locations
25 in the mountains and in between may receive either an easterly or a westerly wind
change depending on which change moves through first.
• the air behind southerly busters can exhibit horizontal roll vortices - rotating “cylinders”
of air which have their axes parallel to the ground. Figure 2.5 shows a vertical cross-
section of a front along the coast. (Streamlines show flow relative to the circulation
centres. The front is moving towards the left hand side of the page). There is ascending
motion immediately to the south of the front at the head of the first roll vortex, and
coincident with the greatest instability. The horizontal portions of the vortices can
produce occasional brief lulls or short-lived increases in the surface wind speed. It must
be emphasised, however, that roll vortices are not associated with every front or
Southerly Buster.
Figure 2.5: A vertical cross-section of a front along the coast. (Streamlines show flow relative to the circulation centres. The front is moving towards the left hand side of the page which is north).
The meteorological conditions ahead of fronts can vary considerably; strong northwesterly winds and hot conditions are quite common, and if these are strong enough they will extend to the coastal areas; if they are weak however, sea breezes may still develop close to the coast, which may moderate temperatures by as much as 10 to 12 degrees Celsius.
26 After a shallow front moves through a location, there is frequently little or no precipitation. A reasonably common occurrence, however, is for a shallow layer of low cloud to appear with the change. Once the change deepens sufficiently, this low cloud may thicken and drizzle or showers may form, depending on the characteristics of the post-frontal air.
In summary, although the main features of summertime cool changes in NSW are relatively constant, the actual weather (including wind) produced by them can vary enormously. This is a chronic peculiarity of weather forecasting – although in general terms two situations may look very similar, the details or fine scale structure of the situation can encompass a huge range of conditions, and it’s all too often the fine scale features which decide the weather.
The complex behaviour of summer-time fronts along the NSW coast is demonstrated in
Chapters 5 and 6.
2.3 Coastal Wind Flows
The orography of a coastline and the proximity to the coast of a mountain barrier can have serious implications on local wind velocities.
There are three separate categories of factors that act together to form the wind fields experienced across the coastal zone (considered to be the area within 20 nautical miles of the coast) that must always be assessed. The first of these is the broad scale flow of air produced by the large pressure systems - the highs and lows - that are always present. Forecasters refer to these as synoptic influences, which determine the general flow pattern of the winds across large areas. The second category of factors to consider are the local effects caused by topographic influences close to coastal parts and the third category is the temperature difference between the land and the sea. The third category also can affect the gust structure
27 and short-term variability of the winds. These include such things as the different heating
rates over various surfaces, the location, size and shape of man-made structures, the more
detailed variations in vegetation cover and general uneven-ness of the land surface (known to
forecasters as roughness), and the location of convective types of cloud, such as cumulus and
cumulonimbus clouds. The contributions from these three different categories of effects must
be combined, with different effects sometimes assisting each other and sometimes opposing
each other.
Particular attention needs to be given by coastal (as well as ocean) sailors and forecasters to
the expected passage of a significant synoptic feature, such as a cold front, southerly change
or a trough line, as this has the potential to dramatically alter the wind field in a very short
period of time. Also, on some occasions a strong offshore wind can develop above a surface temperature inversion overnight, following a day when there has been a fresh onshore sea breeze. In this case a shallow sea breeze can persist overnight and into the early morning, only to be pushed offshore by the strengthening offshore breeze, normally by mid morning.
Regular sailors can identify the local effects of the winds on their area of the coast quite readily. They are most obvious when the synoptic flows mentioned earlier are relatively
weak, typically below about 12 knots. The hills and valleys around coastal parts cause these
normally light to moderate breezes to vary in speed and direction, these variations altering as
the day progresses and as the low level stability of the troposphere (the rate at which the
troposphere cools with increasing height above the ground) changes. The breezes will tend to
flow around headlands and ridges until the depth and strength of the breezes increases to a
point where they will commence to flow over, as well as around, these topographic features.
28
This type of flow, at times, can lead to super-geostrophic winds being generated with Gabo
Island (on the south east part of the Australian mainland), Wilson’s Promontory (on south
coast of Victoria) and Maatsuyker Island (on the southwest tip of Tasmania) produce wind
effects that are good examples of this flow, particularly in broad westerly synoptic situations
over southern Australia.
The two most common local flows affecting the coastal zone deserve a mention. Overnight,
when the air over the land cools faster than the air over the sea, cold air will drain from
inland areas down the main river valleys towards the sea. As an example, take the area of
Sydney Harbour (Figure 2.6). This drainage flow tends to be almost a true westerly, or marginally north of west, at the Sydney Harbour Bridge, caused in part by the constriction in the Harbour between the high rise buildings of central Sydney and those of North Sydney.
The breeze turns into a west to south westerly around Clark and Shark Islands and becomes a south westerly, or even a south to south westerly, near Middle Head. The drainage flow
entering the main Harbour from Middle Harbour tends to be a westerly, which turns
southwesterly as it leaves the Harbour between Sydney Heads or across the Manly Gap.
29 Topography - Sydney Harbour
M
i d
d
l e
Manly Gap
H a r b o u r Sydney North Head Harbour Middle Bridge Head Sydney Heads North Sydney South 1000 m Head 800 Central Fort Denison 600 Sydney Clarke I Shark I 400 300 200 150 100 75 50 25
COREL\R3361-4
Figure 2.6: Topography and location map of Sydney Harbour, NSW.
During the day, outside of the winter months, a sea breeze is likely to develop, provided conditions are suitable. This circulation is essentially the opposite of the cold air drainage flow. As the land surface heats, its temperature rises above that of the water, the air over the land rises and sea air flows in to replace it. The strength and direction of the sea breeze will be determined by such factors as the temperature difference between inland areas and the sea, the synoptic pressure pattern at that time and the amount of cloud cover. A pure sea breeze along, for example the Sydney coastline, is a north easterly, although weaker breezes will
30 have a more easterly component when they first arrive. The shape of Sydney Harbour causes this sea breeze to appear as an east to south easterly in the Fort Denison area for the first few hours. The breeze will, on most occasions, back more northerly as the day progresses.
The third category of factors that affect the gust structure and local variations over the coastal zone are by far the most complex and difficult to predict. The factors that produce these effects mentioned earlier require a better understanding of micro-meteorology than can be covered by this thesis. Nevertheless, these effects all have a physical reason, an understanding of which can assist a racing sailor make the right tactical decision during the course of a yacht race. Given two similar synoptic situations and with a similar set of local meteorological parameters, the nature of the gust structures and wind variations experienced across the coastal zone will be similar. Identifying these similarities is, unfortunately, not a trivial task and considerable investigations must be undertaken to identify them and hence the author’s great interest in them. Some of these complex coastal influences on wind flows are demonstrated throughout the studies presented in the following sections.
2.4 Airflow in the vicinity of Complex Orography
Air forced to rise over hills and mountains can result in the formation of waves above and downstream of the obstruction.
Formation of the waves requires the following conditions:
• thermal stability
• orientation of the wind to the mountain range (within 30 degrees of normal to ridge)
• wind speeds ( at least 15 knots for small mountains less than 1 km in altitude and 25
knots for mountains about 4 km in altitude)
31
Under suitable humidity conditions the waves may be marked by rotor and lens shaped
clouds. These waves/clouds are quasi-stationary and may extend as high as 30 km in the
atmosphere. Wavelengths are typically 5 to 50 km and depend entirely on the characteristics
of the air-stream. The amplitude of the waves is largely dependent on the shape of the
obstruction. Mountain and lee waves can produce light and variable winds at ground level in
the coastal zone which can pose problems for boaters.
2.4.1 Lee Trough
Air flowing onto extensive mountain ranges increases the air pressure on the windward side.
Pressure is correspondingly reduced on the lee side resulting in a lee trough. Wind velocities can vary dramatically across the axis of the lee trough. Fig. 2.7 illustrates a lee trough caused by the Tasmanian Highlands.
32 REPORTED WIND DIRECTION 10 REPORTED WIND SPEED IN KNOTS 1012 ISOBARS (MB) POSSIBLE WIND DIRECTION 1013
1012 ROUGH SEAS 30 MODERATE SEAS MODERATE SEAS 20 20 1011 20 15 MODERATE SEAS 10 1010 SMOOTH 15 SEAS
5 SMOOTH kts SEAS LOW 1009 1008 mbs
10
10
1008
45 SLIGHT SEAS
ROUGH SEAS 1006
CMAN/R3311-4
Figure 2.7: An idealized lee trough and mesoscale heat low event in Tasmania (after Bureau of Meteorology, 1991)
The effects of mountain orography on wind flow can be summarised as follows:
• causes maximum surface roughness and hence wind flow is generally turbulent.
• acts as a solid barrier diverting and channelling the wind and generating eddies.
Channelling of wind can have serious consequences for boaters along affected coastlines. For example, the Hunter Valley in NSW causes significant channelling in a west to northwesterly wind causing considerably higher wind speed in eastern parts of the Hunter than would be expected from the synoptic scale pressure gradients.
33
Similar effects occur in other places along the NSW coast as well as other parts of the
Australian coastline.
• air flow takes the path of least resistance. During stable conditions air will flow
around obstructions and through valleys etc. During unstable conditions air will flow
up and over obstructions.
• the shape of topography can help generate local turbulence.
• the orientation of the wind flow to a mountain range is important in determining
turbulent effects.
• under certain airflow and thermal stability situations, standing wave activity and a lee
trough can be observed in the lee of mountains, hills or even high coastal cliffs.
Chapter 6 provides an excellent example of the complexities of airflow around complex terrain and also offers some more in-depth theory.
34
CHAPTER 3
DESCRIPTION OF THE MODEL
This chapter provides a brief description of the NWP model used for the studies presented in this thesis. For more detailed model specifications, see Leslie et al (1985). The model used in all the three studies presented in this thesis is the high resolution limited area model (HIRES) developed at the University of New South Wales by Professor Lance Leslie (Leslie and
Skinner, 1994). The application of HIRES involved using it in its current state of development and extending the model in a variety of ways. The most notable application was the extension of the model to very high resolution. This is very noticeable in both Chapters 4 and 5 where there is a dramatic difference between the forecast timing and the forecast wind field across a Southerly Buster as the resolution was increased from 25km to 15km, in the case described in Chapter 4, and from 25km to 10km in the second study outlined in Chapter
5. In Chapter 4, the advantage in using a very high resolution model run at a horizontal resolution of 10km to resolve the “lee trough effect” is demonstrated.
HIRES was initially developed in the 1980s to complement the Bureau of Meteorology’s operational NWP models. It has, over time, undergone a number of upgrades and modifications, including its physics and topographical packages.
The HIRES model possesses state-of-the-art numerics and representation of physical processes in the atmosphere. The integrations are carried out on a staggered Arakawa C-grid using a split
35 semi-implicit time differencing scheme. This scheme possesses high order differencing
algorithms which are very efficient in that they split the slow atmospheric modes from the dynamically unimportant high frequency gravity wave modes. The physical processes represented have been greatly extended compared to simple K-theory. Higher order closure
(level 2.25) representations of the turbulent vertical transfers of heat, momentum and moisture in the boundary layer. A new, efficient radiation scheme is applied in the body of the atmosphere and the surface as well as the sea-surface temperatures are real-time 5 day means rather than climatological means. Key model features are listed in Table 3.1, and some previous applications are given in Leslie & Skinner (1994) and Speer at al. (1996).
The model uses as its base the governing Navier-Stokes equations for fluid motion. These equations are given below in height co-ordinates. Refer to Bureau of Meteorology (1995) for a further in-depth treatment.
The two horizontal equations of motion are: Du ∂ p vf −= α , (1) Dt ∂ x
Dv ∂ p uf −−= α . (2) Dt ∂ y
The vertical equation of motion is Dw ∂ p −= α − g , (3) Dt ∂ z
36 but this equation is almost always replaced by the hydrostatic equation ∂ p −= ρ g . (4) ∂ z The continuity equation is ∂ ρ 0 . ()ρ u =∇+ 0 . (5) ∂t
The thermodynamic equation is D( lnθ ) Q& = , (6) Dt CTp where Q& represents a rate of diabatic heating. The equation of state is α = TRp , (7) while the potential temperature is given by
R ⎛ p ⎞Cp θ = T⎜ 0 ⎟ . (8) ⎝ p ⎠ Where the symbols have their usual meaning.
Sigma co-ordinates are used in the vertical. A cubic spline is implemented to convert pressure to
sigma levels.
37
MODEL FEATURE HIRES Horizontal resolution Variable down to 1km
Numerical scheme Split semi-implicit (high order)
No. of vertical levels Variable, usually 16 to 31
Assimilation scheme 6-hourly cycling
Initialization Dynamic
Orography 2 minute resolution
Boundary layer scheme Mellor-Yamada, Level 2.25
Radiation scheme Fels-Schwarzkopf
Convective scheme Fritsch-Chappell
Sea-surface temperatures 5-day average
Lateral boundary conditions From BoM global model
Table 3.1: Key HIRES model features.
38
CHAPTER 4
VERIFICATION OF OUTPUT FROM A VERY HIGH RESOLUTION NUMERICAL WEATHER PREDICTION MODEL: THE 1996 SYDNEY TO HOBART YACHT RACE
Overview
For the 1996 Sydney to Hobart Yacht Race the HIRES model was run at 25 km horizontal resolution to produce a 5 day forecast. The model output was subjected to detailed verification by the author who carried out an observational program aboard the yacht AMP
Wild Oats.
The model-predicted winds were verified on a six-hourly basis utilising instrumentation on the yacht. The yacht carried wind sensors which were situated on top of the mast at a height of 17.5 metres above the water. The author was interested in determining the accuracy of both the wind directional trends and the wind speeds forecast by the model.
It was found that the model was particularly accurate early in the race when a major wind change known locally as a Southerly Buster occurred just after the start of the race. Later in the race from about the fourth day, the quality of the forecasts decreased in accuracy.
This study was carried out with Professor Lance Leslie from the School of Mathematics,
University of New South Wales. Professor Leslie performed all the model runs whilst I as the
39 lead author, identified the event, provided all the background information, including
observations, the synoptic setting and the interpretation and discussion of the results.
The following work in this chapter has been published in an international peer-reviewed
journal: ‘Meteorological Applications’.
4.1 Introduction
Commencing on 26 December each year, the Sydney to Hobart yacht race is one of the
major highlights of the Australian sporting calendar. The race covers a distance of 1,167
kilometres (630 nautical miles) through the Tasman Sea and eastern Bass Strait, which can
subject the yachts to some of the roughest waters in the world (see location map, Figure 4.1).
In most years more than 100 ocean racing yachts from around the world enter the race. Their
length ranges from a compact 9.5 metres to the 25 metre maxi-yachts (Batt & James, 1995).
The race record which had stood since 1975 was 2 days, 14 hours, 36 minutes, 56 seconds. It
was broken in the 1996 race by the German maxi-yacht Morning Glory, and the record now
stands at 2 days, 14 hours, 7 min, 10 sec.
.
Each year for the last 20 years or so, the NSW Regional Office of the Australian Bureau of
Meteorology, in Sydney, has provided a pre-race briefing on 24 December, briefing facilities
on race day (26 December), and special race forecasts for the duration of the race.
In the pre-race briefing for the 1996 race, the author warned of the likelihood of a strong
Southerly Buster not long into the race. This was expected to be rapidly replaced by a period
of light winds. Beyond that, the predictions needed to be treated with care as they were
forecasts out to 5 days in advance, which was at the usual limit of skill of NWP models in the
40 Southern Hemisphere at that time.
Early on the morning of the race day (Thursday, 26 December), Bureau of Meteorology
forecasters at the starting point, which was the Cruising Yacht Club of Australia in Sydney
Harbour, handed out the latest predictions for the race. A strong wind warning had been
issued for the Southerly Buster, which was forecast to arrive at 1600 hours Eastern Daylight
Saving Time (EDST), with light winds ahead of the change. The southerly was forecast to
give way to light winds on the Friday morning with freshening north easterly winds predicted
to develop in the afternoon and to persist until the next change, a fresh westerly, late on
Saturday, 28 December (Buckley & Batt, 1997). This sequence was predicted well by the
HIRES model being qualitatively very close to what actually occurred, as will be discussed
below.
The purposes of this study were, first, to describe the meteorological conditions covering the
race period applicable to AMP Wild Oats, namely, 26 December to 30 December 1996 and, second, to compare observed wind data with those forecast from HIRES. To achieve these aims the high resolution model was run at 25 km horizontal resolution (an additional run at
15 km resolution was also undertaken), over a region between 32deg. S 144deg. E and 32 S
158 E to 44.5 deg. S 144 deg. E and 44.5 S 158 E, thereby comfortably covering the race course (see Figure 4.2).
4.2 The meteorological setting
A pre-frontal trough had moved through Sydney at around 0530 EDST on Thursday 26
December, the morning of the race. Between the trough and a cold front that had reached the
41 far south coast of NSW, the pressure gradient had become very small. Light to moderate south to southeast winds behind the trough became light and variable, especially as a weak rainband associated with the cold front moved over Sydney as shown in Figure 4.3(a), which is the sea level pressure (SLP) chart valid for 1000 EDST on 26 December.
With the clearance of the cloud band just before the start of the race at 1300 EDST, there was sufficient heating of the land to allow a weak northeast sea breeze to develop. This sea breeze persisted, although it was very patchy in Sydney Harbour, until the cold front and associated southerly wind change arrived.
The southerly change arrived at Bellambi Point, just north of Wollongong (see Figure 4.1), at around 1500 EDST with gusts to 47 knots (kn) being recorded, and was an excellent example of a Southerly Buster. The Buster spread northwards throughout the fleet with southerly winds averaging 25 to 30 kn for 4 to 5 hours. The Buster arrived at Sydney at around 1600
EDST with gusts to 47 kn, and a total of 8 yachts were dismasted when it hit the fleet
(Buckley & Batt, 1997).
Winds had moderated overnight and had backed to the southeast by 1000 EDST on Friday
27 December (Figure 4.3(b)) as a high pressure system, with a central pressure of 1024 hPa had become centred to the east of Gabo Island. A cold front was approaching the Great
Australian Bight. The high centre had moved towards the southeast over the ensuing 24 hours and allowed a ridge of high pressure to develop over the entire race area.
By the afternoon of the 27 December, the wind direction had backed further into the
42 north/northeast and the speed had increased to average around 15 to 20 kn. By evening, the speed had built up to average 20 to 25 kn with gusts to 30 kn being experienced, persisting overnight and into much of the next day.
By 1000 EDST on Saturday 28 December (Figure 4.3©), the cold front was situated just to the west of Tasmania, whilst the ridge of high pressure remained centred over the southeast of Australia. With the passage of the front over Tasmania, the winds had slowly abated and the direction backed into the westnorthwest. The abatement in the winds was due mostly to de-coupling near the surface associated with near-overcast cloud conditions, together with a weakening frontal system.
The front had moved through north-eastern Tasmania during the late evening of 28
December and had initially produced 15 to 20 kn southwest winds which had quickly abated to 10 to 15 kn and backed into the south. By the afternoon, the winds had become light to moderate southeast to northeast, owing to the lee effect of the Tasmanian mainland.
Meanwhile, boats rounding Tasman Island on the southeast coast of Tasmania and those in
Bass Strait were experiencing 15 to 20 kn southwest winds.
By 1000 EDST on Sunday 30 December (Figure 4.3(d)), a ridge of high pressure had developed off the east coast of Tasmania. This ridge, which was coupled with a weak trough of low pressure lying over Tasmania, produced a northeast airstream over the east coast, which was very favourable for the development of leeward reduction of wind speeds and afternoon coastal sea breezes.
43 4.3 Model performance
The performance of the HIRES model over the period 26 to 30 December, 1996 was assessed by comparing the model output with the observations taken for the same time by the author who was sailing in the race on the yacht AMP Wild Oats. It should be noted that the nearest model level to the mast height of 17.5 m is at about 12 m. This height difference would contribute to a discrepancy in the wind speed and direction between the observation and the model output. Moreover, the verifying boat observations are at single locations and are taken over the ocean at widely separated time intervals. This presents a very difficult forecasting challenge. Table 4.1 below is a comparison of forecast wind velocities from the low resolution model GASP and the very high resolution model HIRES with the actual wind velocities as measured on AMP Wild Oats.
44
Date/Time Wind Wind Wind Wind Wind Wind
EDST Direction Speed (kn) Direction Speed (kn) Direction Speed
GASP GASP HIRES HIRES Observed Observed
135km 135km 25km 25km Degrees Knots
26/1700 200 30 180 30 170 32
26/2300 200 30 175 30 150 25
27/1100 180 20 160 18 025 09
27/2300 160 10 030 13 015 21
28/0500 120 10 010 19 010 24
28/1100 320 15 340 15 030 24
28/2300 260 15 250 16 300 23
29/1100 260 20 260 12 090 05
29/2300 260 10 260 02 100 12
30/1100 220 15 220 11 111 02
Table 4.1: Comparison between 135km GASP, 25km HIRES and actual conditions.
4.3.1 Day 1: Thursday 26 December, 1996
The model run, which was initialised at 1000 EDST on 25 December, captured the passage of the Southerly Buster at the beginning of the race, on the afternoon of the 26 December, exceptionally well from the point of view of both timing, and wind velocity. (Figs.4.4a and
4.4b). From then on the model had some difficulty keeping pace with the timing of the wind directional changes but the trend was good. The wind speeds forecast by the model were
45 same as those observed onboard Wild Oats. See Table 4.1 above.
Figure 4.4a shows the model forecast winds over the entire forecast domain for 1600 EDST on 26 December at 25 km resolution. It can seen that the Southerly Buster had overtaken the leading yachts, such as Morning Glory, with a wind direction of 180 deg. and a ten minute average speed of 30 kn or more. These yachts were positioned just 10km south of Botany
Bay.
According to the log of AMP Wild Oats, the change arrived at 1550 EDST with a mean speed of 32 kn with gusts to 40 kn whilst abeam Botany Bay. Figure 4.4(b) shows a time series plot of wind speed from the automatic weather station at Bellambi Point (just north of
Wollongong), with the model output at 15 minute intervals overlaid as a direct comparison.
Figure 4.5(a) is the HIRES model forecast winds for the same time as Figure 4.4 but at a horizontal resolution of 15 km. When compared to the 25 km run, this higher resolution forecast had the front situated a little further north, around Botany Bay, shows more structure in the frontal zone, and provides a time sequence that is closer to the observed anemograph trace (Figure 4.5(b)). The forecast wind direction corresponding to the actual position of
Wild Oats is 180 deg at 25 kn. The 30 kn isotach on the prognosis chart at that time is situated a little further to the south.
4.3.2 Day 2: Friday 27 December, 1996
Figure 4.6 shows the HIRES model forecast winds for 1000 EDST on 27 December at 25 km resolution. The wind velocity measured onboard Wild Oats at position 35.46 deg. S 150.48 deg. E was 041 deg. at 10 kn. The model forecast velocity is 160 deg. at 18 kn. The model
46 had predicted the velocity with an error in direction of 119 deg. and the speed was 8kn too high, but the weakening trend is well forecast.
4.3.3 Day 3: Saturday 28 December, 1996
In Figure 4.7, which shows the model forecast winds for 0400 EDST on 28 December, the actual wind velocity at a position of 38.23 deg. S, 150.11 deg. E was 010 deg. at 24 kn. The corresponding model forecast wind velocity was 010 deg. at 19 kn. The directions match perfectly but the model speed is 5 kn too low.
Figure 4.8 shows the model forecast output for 2200 EDST on 28 December. The actual wind velocity on Wild Oats at position 40.21 S 149.13 E was 274 deg. at 16 kn. The model was forecasting 260 deg. at 16kn, so its predictive skill remained very high. The model was predicting the cold front to move through eastern Bass Strait at around 1200 EDST on 28
December. According to the actual observations it arrived at approximately 1900 EDST on the 28 December, which was an error of only 7 hours after almost 96 hours into the forecast period.
4.3.4 Day 4: Sunday 29 December, 1996
From Day 4 on, the quality of the model output began to diminish as might be expected given the typical limit of NWP skill for the Southern Hemisphere ocean areas at the time of this race was being approached. At 2200 EDST on 29 December, the position of Wild Oats was
42.40 deg. S 148.19 E. The observed wind velocity was 109 deg. at 13 kn. The model output was 230deg. at 15 kn (Figure 4.9). The agreement in speed was good with the model forecasting 2kn more than was observed, but the direction forecast was some 21 degrees in
47 error. However, given that the model was run at 25 km horizontal resolution, it cannot be
expected to resolve the boundary between the broader flow and that induced by the lee
effects of the Tasmanian Highlands. This is exacerbated by the fact that the model was now
more than 96 hours into the forecast. The model needed to be run at a much higher resolution
to rectify this deficiency.
4.3.5 Day 5: Monday 30 December, 1996
Figure 4.10 shows the model forecast output for 1000 EDST on 30 December. At this time
Wild Oats was in the Derwent River experiencing very light easterly winds. In contrast, the
model was forecasting a wind velocity of 230 deg. at 11 kn. The model was nearly at the end
of its run (5.5 days), the major factor contributing to this decrease in skill.
4.4 Discussion and Verification of Results
It has been shown, in the context of the 1996 Sydney to Hobart Yacht Race, that there is
potentially considerable skill in the ability of high resolution NWP models such as the
University of New South Wales HIRES model, to predict the major features of wind fields
over the ocean several days ahead.
The comparison of the model predictions (see Table 4.1 above) with point observations over
the open ocean is an extremely difficult test and the accuracy shown in these simulations was
very encouraging, especially given the model resolution of 25km over 135km. If the model
predictions for the race period were run at a higher resolution, say 10 km or below, far more
structure would be evident, especially the structure of cold fronts and the leeward wind
effects of Tasmania on the general wind flow. An additional limited duration model run (to
48 30 hours) at a horizontal resolution of 15 km was performed after the event. This run showed
better skill in the timing and the structure of the Southerly Buster that moved through the
fleet on the first day.
During the 1996 Sydney-Hobart race 10 observations were able to be compared with the
corresponding forecasts, based on a model run prior to the commencement of the race. The
winds were converted into their ‘U’ and ‘V’ components representing E/W and N/S
directions respectively. For this study with winds from the E and N are given a positive
orientation.
Initially the model went for a strong southerly flow. The verifying observations agreed well,
though with a tendency for the winds to start to shift around a little to east of south. By
2200hr EDST on 27th the model was starting to indicate this shift towards a wind a
little east of south, shifting around to a NE wind by 1000hr EDST. The corresponding
observations indicated that this process occurred quite a deal quicker, with a NE wind already
established by 2200hr EDST and freshening further by 1000hr EDST when it had largely
continued around to the N. The northerly flow persisted through 1600 hr EDST on 28th before
it was forecast to back around to the NW and W and eventually SW over the next 2 days. The
corresponding observations again continued to suggest that the model was too weak on the
northerly and easterly components of the wind. At 2200hr EDST on 28th the wind persisted east of north before shifting around to the WNW by 1000hr EDST. Thereafter the winds were observed to come from the east, and not the west as the model suggested. This situation was a result of the lee trough over eastern Tasmania.
There appeared to be a rather regular pattern between the forecast winds and the
49 corresponding observations that suggested the model over-estimated the southerly/under-
estimated the northerly components, and also over-estimated the westerly/under-estimated
the easterly components. It was possible to test for this via a paired t-test (see Tables 4.2 and
4.3 below). To achieve this, the winds had to be converted from direction and speed into their
‘U’ and ‘V’ components.
A comparison of each of the 10 pairs of forecasts/observations for the ‘U’ and ‘V’
components of the wind was then conducted. The paired t-test takes the difference between
each data pair, which are denoted as model (Variable 1) and observed (Variable 2) here, and
aims to conclude if this difference is zero or not. If it is close to zero, statistically speaking,
then the forecasts exhibit no bias. To test whether this was the case the following hypothesis
was proposed:
H0: mean difference = 0
H1: mean difference not equal to 0
The mean difference was approximated by the mean of values in the series and the variance
by the population variance divided by the number of observations in the series (10
observations). In the case of the ‘V’ component of the wind, the average forecast = -4.96 kn and the average observation = +2.84 kn. This indicated that the average forecast was for a wind with a southerly component, but the observations tended to average a northerly component. The overall difference was -7.80 kn. The variance of these differences amounted to 66.04. In the case of the ‘U’ component, the average forecast was -2.54 kn and the average observation was +4.22 kn. This suggested that the average forecast was for a wind with a westerly component whereas the average observation had an easterly component. The population variance was 70.98. From the ratio of the mean value and its variance a t-statistic
50 was derived which measures the number of standard deviations this difference is away from
zero. Using standard statistical theory it can be concluded that this difference was significant.
For ‘V’ component:
T = -7.80/(sqrt(66.04/10)) = -3.035 (1)
For ‘U’ component:
T = -6.76/(sqrt(70.98/10)) = -2.539 (2)
Both of these values were compared against theoretical values to determine the probability
that the differences of -7.80 and -6.76 are indeed different from zero. They were found to be
1.4% and 3.2% respectively (2-tailed test). This is known as the ‘level of significance’ of the
test and both of these values are strongly suggestive that there was indeed a bias in the
original forecast towards favouring winds from the south and west more than was observed.
This the case since the model did have problems trying to resolve the mostly east to west
flow set up by the more localised effects induced by the Tasmanian mainland and the sea
breezes down the NSW coast. Because of this H0 was rejected and it was concluded that H1 was correct, and the mean difference was not equal to zero. Figures 4.11a and b are plots of the Error against the Forecast and support the conclusion above. In the case of the ‘V’ component (Fig 4.11a), 8 of the 10 forecasts had a negative error, indicating that the forecast had too much southerly/too little northerly compared to the observations, and the only two positive values were marginally so. In the case of the ‘U’ component (see Figure 4.11b) there were 3 occasions when the value was positive and one very marginal negative value. Of the negative values, 3 events were significantly negative i.e. < -15. Both results support the claim of a significant bias.
51 Table 4.2: t-Test: Paired Two Sample for Means – V wind component
Variable 1 Variable 2 Mean -4.95861 2.839621 Variance 290.553 331.8852 Observations 10 10 Pearson Correlation 0.895883 Hypothesized Mean Difference 0 df 9 t Stat -3.0346 P(T<=t) one-tail 0.007071 t Critical one-tail 1.833114 P(T<=t) two-tail 0.014141 t Critical two-tail 2.262159
Table 4.3: t-Test: Paired Two Sample for Means- U wind component
Variable 1 Variable 2 Mean -2.54074 4.222933 Variance 55.51012 86.56299 Observations 10 10 Pearson Correlation 0.512774 Hypothesized Mean Difference 0 df 9 t Stat -2.53866 P(T<=t) one-tail 0.01589 t Critical one-tail 1.833114 P(T<=t) two-tail 0.03178 t Critical two-tail 2.262159
52
Figure 4.1 Map of southeastern Australia showing locations of places mentioned in the text.
53
Figure 4.2 The HIRES model domain including topography. Contour interval is 100 m.
54 Figure 4.3(a) Australian region mean sea level pressure analysis (4hPa spacing) at 1000 EDST 26 December 1996.
55
Figure 4.3(b) Australian region mean sea level pressure analysis (4hPa spacing) at 1000 EDST 27 December 1996.
56
Figure 4.3© Australian region mean sea level pressure analysis (4hPa spacing) at 1000 EDST 28 December 1996.
57
Figure 4.3(d) Australian region mean sea level pressure analysis (4hPa spacing) at 1000 EDST 30 December 1996.
58
Figure 4.4(a) HIRES model forecast winds for 1600 EDST on 26 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
59
Figure 4.4 (b) Wind speed plot for Bellambi Point automatic weather station on 26 December 1996 (wind speed in knots, time in EDST) with the 25km model forecast winds (smoothed black line) overlain.
60
Figure 4.5(a) HIRES model forecast winds for 1600 EDST on 26 December 1996 at 15 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
61
Figure 4.5 (b) Wind speed plot for Bellambi Point on 26 December 1996 (wind speed in knots, time in EDST) with the 15km horizontal resolution model forecast winds (smoothed black line) overlain.
62
Figure 4.6 HIRES model forecast winds for 1000 EDST on 27 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
63
Figure 4.7 HIRES model forecast winds for 0400 EDST on 28 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
64
Figure 4.8 HIRES model forecast winds for 2200 EDST on 28 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
65
Figure 4.9 HIRES model forecast winds for 2200 EDST on 29 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
66
Figure 4.10 HIRES model forecast winds for 1000 EDST on 30 December 1996 at 25 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocity barb in standard meteorological format is also shown at the position of the yacht AMP Wild Oats, for comparison.
67 V Wind Component
5 0 -5 1 2 3 4 5 6 7 8 9 10 -10 -15
Error (Kn) -20 -25 -30 Forecast
Figure 4.11(a): Graph of V wind component error
U Wind Component
10 5 0 -5 1 2 3 4 5 6 7 8 9 10
Error (Kn) -10 -15 -20 Forecast
Figure 4.11(b): Graph of U wind component error
68 CHAPTER 5
DIRECT VERIFICATION OF FORECASTS FROM A VERY HIGH RESOLUTION NUMERICAL WEATHER PREDICTION (NWP) MODEL
Overview
This chapter is an extension of the study presented in Chapter 4. The opportunity was taken
to run the HIRES model for a different synoptic situation to that of the previous chapter and
at both 25km (real-time mode) and 10km (hindsight mode) horizontal resolutions for the
duration of the race. In December 1997, the HIRES model was run for the period spanning
the annual Sydney to Hobart race yacht area. The model output was verified against
observations from onboard a competing yacht and land based observations.
For this race the model was run at 25km horizontal resolution and the output was made
available to all competitors on the morning of the race, 26 December. It was also decided to expand the verification procedure to include all observations available both from land and sea within the model domain as well as those available from a yacht.
After the event, the model was run once in hindcast mode out to five days at an increased horizontal resolution of 10 km. Both model runs were subjected to a detailed verification
procedure aboard the maxi-yacht Nicorette according to observations taken by the author
utilising a pre-arranged observational program throughout the race.
Surface synoptic weather maps prepared in the NSW Office of the Bureau of Meteorology
69 were also consulted in order to extend the verification scheme. The model predicted winds
were verified on a six-hourly basis utilising instrumentation on the yacht as well as surface observations plotted in standard World Meteorological Organisation (WMO) format on surface synoptic weather maps.
The yacht’s wind sensors were mounted on top of the mast at a height of 30 metres above the water. The accuracy of the wind velocity forecast by the model was of great interest. The verifications reveal that the model gave overall forecast guidance of very good to excellent
quality. The predictions were particularly accurate early in the race when a Southerly Buster
event occurred during the evening of the first day.
This study was carried out in conjunction with two other researchers: Mr Russel Morison
from the School of Mathematics, University of New South Wales and Dr Milton Speer from
the Sydney office of the Bureau of Meteorology. Mr Morison performed all the model runs
whilst Dr Speer provided some valuable meteorological advice. My contribution was to
identify the event, collect and collate all the background information, including observations,
identify the synoptic setting and provide the interpretation and discussion of the results. The
following work in this chapter was published in an international peer-reviewed journal:
‘Meteorology and Atmospheric Physics’.
5.1 Introduction
This study extends the work described in Chapter 4, utilising the HIRES model at a much
higher resolution of 10km after the race yacht event, as well as 25km before the
70 commencement of the yacht race. The study includes a more comprehensive verification network. The higher resolution model output was also compared to that from the Bureau of
Meteorology’s global model GASP, the only other comparable dataset available in real-time.
The purposes of this study were, first, to describe the meteorological conditions covering the race period applicable to the maxi-yacht Nicorette, from 26 to 29 December 1997 and, second, to compare observed wind data with those from the 10 km (hindcast) and 25 km
(forecast) resolution output of HIRES, as well as the wind forecasts from the GASP global model. The model was run over the domain shown in Fig. 5.2 encompassing the race course shown in Fig. 5.1.
5.2 The meteorological setting
The extreme western flank of a ridge of high pressure shown in Fig. 5.3 was evident over north eastern NSW on the morning of Friday 26 December 1997, which resulted in light to moderate north to northeast winds along much of the NSW coast. These winds freshened during the afternoon of 26 December as the ridge slowly eroded and moved eastwards while a trough of low pressure was moving slowly northwards up the NSW south coast. The pressure gradient was moderate behind the trough due to the rapid strengthening of a ridge of high pressure off the south coast of NSW following the passage of the trough.
A cold front was also situated in western Bass Strait at 1100 EDST 26 December 1997.
Winds behind the trough initially were light to moderate southwest to southeast but had increased to fresh to strong south to southwest by the evening. Of interest was the fact that the winds over the land, as measured by the automatic weather station (AWS) at Nowra
Royal Australian Naval (RAN) Base, which is located inland and approximately 35 km NW
71 of Jervis Bay, and at other places along the NSW coast close to Nowra were lighter than
those measured onboard Nicorette whilst the yacht was positioned south-eastward of and 70 km seaward of the AWS. This was especially the case between 2100 and 2200 EDST 26
December 1997.
By 1100 EDST 27 December 1997 (Fig. 5.3), the ridge of high pressure had become firmly established over much of the race area. This produced a fresh to strong south to southwest
flow over the NSW south coast as well as eastern Bass Strait. A cold front was situated to
the south of Tasmania and was moving towards the northeast. The passage of the front
through eastern Bass Strait maintained the fresh to strong southwest flow into the evening of
the 27 December. During the early morning hours of Sunday 28 December 1997, the winds
had abated as the centre of the high pressure system had moved eastwards into Bass Strait as
shown in Fig. 5.3. As the wind speed moderated, the wind direction backed from the
southwest to the northeast by the early evening of 28 December. As this high pressure system
moved eastward, a cold front was moving in from the west and was positioned on the west
coast of Tasmania by 1100 EDST 29 December 1997 (Fig. 5.3). The wind speed ahead of the
front had freshened and the direction had backed into the north and was from the northwest
by mid-morning.
The winds off the east coast of Tasmania became light and variable over the afternoon of 29
December under the influence of a lee trough which was induced by the Tasmanian
Highlands. As Nicorette reached the southeastern part of the Tasmanian coast the wind speed increased and the direction changed to the southwest as the cold front moved through eastern parts of Tasmania. When the yacht had reached the Derwent River, the wind speed had
72 decreased and the direction had backed to the southeast as local influences took control.
5.3 Model performance
The performance of the HIRES model over the period 26 to 29 December 1997 was assessed
by comparing the model output with observations taken from the official surface synoptic
weather stations for the corresponding time archived by the NSW Regional Office of the
BoM. The model output was also compared to observations taken during the same period
onboard Nicorette.
It should be noted that the nearest model level to the mast height of 30 m is at about 12 m and would contribute to any discrepancy in the wind speed and direction. An attempt was made to
relate the 30 m wind speed measured on the yacht to that of the model height wind speed of
12m. This was achieved by using the following simple interpolation (Laughlin 1997), based
on a standard logarithmic wind profile over the ocean and assuming neutral stability
conditions:
U(z) = U(zref) x (z/zref)* (1) where U(z) = HIRES wind speed height at 12m, U(zref) is the height of yacht’s anemometer
at 30m, and the symbol * denotes the aerodynamic surface roughness length of 0.10m
(assumed to be a reasonable average for the race area).
It should also be noted that the onboard instrumentation used to output the sailing wind velocity for the masthead height of 30m was produced by a vector calculation using the wind
triangle (Chisnell, 1992), see Figure 5.4. The instrumentation accesses data from different sensors: boat speed; compass heading; apparent wind angle and apparent wind speed. The
apparent wind velocity is the wind that a person senses whilst on a moving boat. It is one that
73 can be measured onboard directly and is a product of the following three components:
• the wind blowing over the earth’s surface, known as the true wind velocity to
meteorologists but is the ground wind velocity to sailors.
• the wind produced by water motion relative to the land, known as the tide wind. It is
equal in speed and opposite in direction to the near surface water movement.
• the wind produced by the boat’s motion, which is known as the motion wind. Its
speed is equal to the boat speed and its direction will blow directly onto the bow of
the boat.
The vector sum of the above three components is called the apparent wind. This is the only one that can be measured directly. It is the wind triangle that allows the other wind component to be calculated. The sailing wind as it is referred to by Chisnell in his wind triangle, is a vector sum of the ground wind and the tide wind and is the wind that a yacht actually sails in (see Figure 5.5).
On Nicorette, the observed absolute wind velocity was an attempt to take into account the
tide wind by the resolution of the leeway of the yacht. It was the absolute wind velocity that
was logged by the author as it was the nearest approximation to the wind velocity being
predicted by the HIRES model.
The performance of the model is assessed on a day-to-day basis in the following sections.
Table 5.1 summarises the comparisons between the forecast and the observed wind velocities. It must be stressed that only charts representing significant meteorological
74 changes that occurred during the race are discussed below. It is also important to realise that locations such as Wilson’s Promontory and Gabo Island are over-exposed and, as such, observed wind speeds will generally be super-geostrophic.
5.3.1 Day 1: Friday 26 December 1997
The model run, which was initialised at 2300 EDST 25 December 1997, had captured the passage of the Southerly Buster along the south coast of NSW during the early evening of the race, on the 26 December, exceptionally well from the point of view of both timing and wind velocity. From that time on, the model had very good to excellent skill with both the timing of significant wind direction and speed changes. These are summarised in Table 5.1.
Figure 5.6 shows HIRES 18 hour forecast winds over the entire forecast domain for 1700
EDST 26 December at 10km resolution. It can seen that the Southerly Buster had not yet hit the leading yachts, such as Nicorette which was leading the fleet at that time. Observations further south along the NSW south coast and elsewhere are in very good agreement with the model output. According to the log of the yacht at 1700 EDST, the modified wind velocity was 040 deg. true at 18 kn. From Figure 5.9 and Table 5.1 it can be seen that the model was predicting 030 deg. at 16 kn.
From the yacht observations, it was evident that a weak pre-frontal trough had hit the leading boats at 1845 EDST with a wind velocity around 235 deg. at 10 kn. The leading yachts at this stage were positioned to the east-northeast of Jervis Bay on the south coast of NSW, but about 40 km to seaward. The main change with gusts to 52 kn hit the leaders at around 2100
EDST.
75
Colquhoun (1981) presented a case study of a Southerly Buster on 1 February 1977 similar to the event that occurred on 26 December 1997. From Figure 5.7 and Table 5.1 it can be seen that the model forecast winds were from the south at 25 kn at the yacht’s position for
2300 EDST 26 December 1997. The wind data logged on Nicorette, for this time was 190 deg at 31 kn. Observations from other locations are shown on Fig. 5.7 and are again in very good agreement with the forecast output.
Figure 5.8 is a plot of wind velocity data for the automatic weather station (AWS) at Nowra.
This AWS is approximately 30 km directly inland from the coast. The data shows that as the change passed through Nowra it lacked the intensity that the yachts experienced some 30 to
40 km off the coast. This phenomenon has also been observed by Reid (2000) in her studies on Southerly Busters along the NSW coast and also by Mass and Albright (1987) during their studies of North American coastal southerlies and alongshore surges.
The initial change arrived in Sydney at around 2100 EDST, which was followed by the
Southerly Buster at approximately 2200 EDST 26 December 1997. From Figure 5.7 it can be seen that the model captured this event, including the structure of the front, with remarkable skill.
5.3.2 Day 2: Saturday 27 December 1997
Figure 5.9 is the 36 hr model forecast winds for 1100 EDST 27 December at 10km resolution. The wind velocity which was measured onboard Nicorette at position 36.32 deg.
S 150.25 deg. E was 242 deg. at 18 kn. The forecast model velocity was 220 deg. at 15 kn.
76 The model therefore forecast the velocity very well. Again observations from other locations were in very good agreement with the model forecast winds.
5.3.3 Day 3: Sunday 28 December 1997
In Figure 5.10, which shows the 60 hr HIRES forecast winds for 1100 EDST 28 December, the observed average wind velocity at a position of 38.35 deg. S, 150.20 deg. E was 180 deg. at 14 kn. The forecast wind velocity was 180 deg. at 8 kn. The directions matched perfectly but the model speed was 6 kn too low. The wind speed trend over this time was decreasing and was indicated by the model.
From 1100 EDST 28 December, the wind velocity was changing quite rapidly, especially the direction, and the model was capturing this very accurately. Under a light wind regime, one would expect local winds to dominate. Over Tasmania on this day, this situation was forecast very well by HIRES. Forecast light and variable winds match the observed wind field fairly closely.
Figure 5.11 shows the 72 hr. model forecast output for 2300 EDST 28 December. The actual wind velocity on Nicorette at position 40.26 S 149.30 E was 010 deg. at 14 kn. The model was forecasting 020 deg. at 11kn, some 3kn less in speed and 10 degrees more in direction than observed. Over the greater domain, the model forecast output matched closely the observed wind field.
5.3.4 Day 4: Monday 29 December 1997
From Day 4 on, the quality of the model output was expected to diminish as the typical limit
77 of NWP predictability for the Southern Hemisphere ocean areas was being approached. In
this case the overall skill remained very high with the forecast wind field very closely
matching that of the observed.
At 1100 EDST 29 December (Figure 5.12), the yacht’s log was indicating a wind velocity of
340 deg. at 9 kn at a position near Maria Island. The model was forecasting 320 deg. at 10
kn, which indicates very good skill, especially considering that the lee effects of the
Tasmanian highlands were marked at the meso-scale.
From a study by Whitehead (1981), and the experience of the author (8 Sydney to Hobart
Yacht Races), these lee effects manifest themselves as light, fluky winds persisting along the
Tasmanian east coast and out approximately 60 km to seawards before the flow returns to
near geostrophic. The wind direction on the coast north of Maria Island was from the
northeast. South of Maria Island, the wind direction commenced as a north-easterly, but as
the yacht moved south, the direction veered quickly into the south and eventually ended up as
a south-westerly near and around Tasman Island. The model didn’t resolve the lee effects
particularly well at 10 km resolution, but future higher resolution simulations may achieve
this.
The HIRES model was predicting the cold front to move through southern Tasmania at
around 1600 EDST 29 December. According to the actual observations it arrived over southeast Tasmania at approximately 1500 EDST 29 December, an error of only 1 hour after almost 4 days into the forecast period. Credit must be also given here to GASP in which
HIRES was nested for this model run given the proximity to the southern boundary of
78 HIRES.
At 1700 EDST 29 December, the position of Nicorette was at 43.14 deg. S 148.00 E (close
to Tasman Island). The actual wind velocity was 240 deg. at 23 kn. The forecast model
output was forecasting 240deg. at 10 kn (Figure 5.13). The agreement in direction was
excellent but the speed forecast was not good, 13kn too low. However, given that the model
was run at 10 km resolution, it cannot be expected to differentiate precisely the broader flow
from the localised convergent flow induced by the orography of the Tasman Peninsula and in
particular Tasman Island. The prediction was also well into the forecast period. It has been
the author’s experience that the orographic induced convergent flow around Tasman Island is
very localised. Nicorette completed the race at approximately 2100 EDST 29 December
1997.
5.4 Discussion and Verification of Results
It has been shown, in the context of the 1997 Sydney to Hobart Yacht Race, that very accurate wind velocity prediction skill was exhibited by the 10 km horizontal resolution model run over the 25 km NWP model run from HIRES (see Table 5.1). The results further extend the usefulness of high-resolution model predictions over global model predictions for resolving finer scale meteorological features.
Nevertheless, it must be said that the BoM’s global model GASP (run at 135km resolution), which set the lateral boundary conditions for HIRES, performed well, see Table 5.1. It would
be a very unfair test to directly compare the results from a high-resolution numerical model
with those from a coarser grid global model. It also highlights the fact that higher resolution
models should be used whenever possible in forecasting offices. This capability to predict
79 detail in the major features of the wind fields over the ocean several days ahead, was verified
using a direct comparison between the model predictions and observed point wind velocities over the open ocean, as well as coastal observations.
If the model was run at even higher resolution, say 5 km or below, even more structure may have been evident in the wind fields, especially of cold fronts and the leeward effects of
Tasmania.
Date/Time GASP HIRES HIRES Observed
135km 25km 10km velocity (boat)
EDST degrees/knots degrees/knots degrees/knots degrees/knots
26/1700 040/12 030/08 030/16 040/18
26/2300 030/10 030/08 180/25 190/31
27/1100 120/10 180/09 220/15 242/18
28/1100 080/10 215/08 180/08 180/14
28/2300 020/15 020/12 020/11 010/14
29/1100 315/25 310/20 320/10 340/09
29/1700 250/25 250/20 240/10 240/23
Table 5.1: Forecast 135 km, 25 km and 10 km resolution model winds compared with actual
wind velocities.
80
In the case of this study, 13 forecasts were verified in the same manner as the 1996 data in
Chapter 4 . From Table 5.2 below, the average forecast ‘U’ component was -4.80 kts
compared to an observed -7.03 kns, with a population variance of 56.50. For the ‘V’
component the values were -2.70, -4.28 and 23.25 respectively (Table 5.3). This resulted in
the following t-statistics:
For ‘U’ component:
T = -1.070 (2)
For ‘V’ component:
T = -1.180 (3)
In both these cases the t-values are quite a deal smaller in magnitude and are less suggestive
that the differences are significantly different from zero. In statistical terms 30.6% and 26.1%
are well above any level of significance that would support such a claim. So in this instance
the null Hypothesis H0 is accepted, and the mean difference between the forecasts and observations is zero. It is worth noting that the t values are again negative, so in both cases the model had a tendency to favour more of a southerly and westerly component in the wind than was actually observed.
81
Table 5.2: t-Test: Paired Two Sample for Means: U wind
t-Test: Paired Two Sample for Means: U wind
Variable 1Variable 2 Mean -4.80225 -7.03365 Variance 173.3415 339.298 Observations 13 13 Pearson Correlation 0.940429 Hypothesized Mean Difference 0 df 12 t Stat -1.070351 P(T<=t) one-tail 0.152759 t Critical one-tail 1.782287 P(T<=t) two-tail 0.305518 t Critical two-tail 2.178813
Table 5.3: t-Test: Paired Two Sample for Means: V wind
t-Test: Paired Two Sample for Means: V wind
Variable 1 Variable 2 Mean -2.69806 -4.27605 Variance 35.20567 86.96546 Observations 13 13 Pearson Correlation 0.893838 Hypothesized Mean Difference 0 df 12 t Stat -1.179832 P(T<=t) one-tail 0.130464 t Critical one-tail 1.782287 P(T<=t) two-tail 0.260928 t Critical two-tail 2.178813
82 :
Figure 5.1 Map of southeastern Australia showing locations of places mentioned in the text.
83
Figure 5.2 The HIRES model domain including topography. Contour interval is 100 m.
84
1008 L 1004 1008 1008 1004 T. C . " S I D " 1008 10 1008 10 10 10 1000 1004 1004 L 1000
1008 1008 1008 1008 20 20 20 20 1008 L 1012 1012 1012 1008 1012 1012 130 140 130 140 120 150 120 150 1020 1016 1012 1012 1 1 1016 110 1016 60 110 1008 1012 1016 1012 60 30 30 30 1016 30 1024 1020 L 1020 1016 H 1016 1012
1024 1024 1020 1012 H 1012 1012 1020 1016 40 1020 40 40 40 1016 1012 1008 1004 1016 1008 1012 1012 1016 1000 26 1008 1008 1004 1000 1004 1008 27 1004 1008 1012 1012 1008 1000 1004 CMAN/R3239-1 CMAN/R3239-2
1008 1008 T. C . "S I D " 10 10 10 10 1008 1008 1004 1000 1004 1000 1008 T. C . "S E L W YN " L L 1008 1008 1004 1008 1004 1008 1008 20 20 20 20 L 1012 1012 1008 1012 L 1012 1012 1012 1008 130 140 130 140 120 150 120 150 1016 1016 1012 1016 1016 1 1016 1 110 60 110 60 1016 1016 1020 30 30 30 30 1020 1020 1020 1024 1016 1020 H
1012 H 1012 1024 H H 1020 40 L 1020 40 40 1020 40 1016 1016 1008 1016 1016 1008 1016 1016 1020 1004 1000 28 1012 1008 1004 1012 1020 1020 1012 29 1012 1012 1020 CMAN/R3239-3 CMAN/R3239-4
COREL/R3361-5
Figure 5.3: Australian region mean sea level pressure analyses (4hPa spacing) at 1100 EDST 26 to 29 December 1997 inclusive.
85
Figure 5.4 The wind triangle (After Chisnell, 1992).
86
Figure 5.5 The ground wind and tide wind combine to form the true or sailing wind, whose direction and speed can be calculated from the boat speed, compass heading, apparent wind speed and angle, as above (After Chisnell, 1992).
87
Figure 5.6 HIRES model forecast winds at 12 metres for 1600 EDST 26 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and for selected BoM coastal observation stations.
88
Figure 5.7: HIRES model forecast winds at 12 metres for 2300 EDST 26 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and selected BoM coastal observation stations.
89
200 20
180 X 18
160 X X X X 16 X
140 X 14 X 120 X X X X X X 12 X N (degrees)
O X I 100 X X X X X 10 T
C X X X E R 80 X 8 WIND SPEED (knots)
WIND DI 60 6
40 X Wind Speed 4
Wind Direction 20 2
0 0 11:55 12:25 12:55 13:25 13:55 14:25 14:55 15:25 15:55 16:25 16:55 17:25 17:55 18:25 18:55 19:25 19:55 20:25 20:55 21:25 21:55 22:25 22:55 23:25 23:55
TIME (EST)
CMAN/R3160-3
Figure 5.8 Wind velocity data for Nowra AWS for 26 December 1997. Times are for Eastern Standard Time (EST). Add 1 hour to obtain EDST.
90
Figure 5.9: HIRES model forecast winds at 12 metres for 1100 EDST 27 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and for selected BoM coastal observation stations.
91
Figure 5.10: HIRES model forecast winds at 12 metres for 1100 EDST 28 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette for selected BoM coastal observation stations.
92
Figure 5.11: HIRES model forecast winds at 12 metres for 2300 EDST 28 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and for selected BoM coastal observation stations.
93
Figure 5.12: HIRES model forecast winds at 12 metres for 1100 EDST 29 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and for selected BoM coastal observation stations.
94
Figure 5.13: HIRES model forecast winds at 12 metres for 1700 EDST 29 December 1997 at 10 km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). The observed wind velocity barbs in standard meteorological format are also shown for the position of the yacht Nicorette and for selected BoM coastal observation stations.
95
V Wind Component
5 0 -5 12345678910 -10 -15
Error (Kn) -20 -25 -30 Forecast
Figure 5.14 Graph of V wind component error
U Wind Component
10 5 0 -5 1 2 3 4 5 6 7 8 9 10
Error (Kn) -10 -15 -20 Forecast
Figure 5.15 Graph of U wind component error
96 CHAPTER 6
THE MODELING AND OBSERVATION OF A LEE TROUGH EVENT OVER EASTERN TASMANIA
Overview
Due to Tasmania’s complex orography, a lee trough can often occur along the eastern coast of Tasmania. This effect is often observed under a broad westerly airflow which is modified by the topographic barrier of the Tasmanian Western and Central Highlands.
The Tasmanian east coast lee trough seems to be most pronounced when the troposphere is stable with an inversion just above the highest parts of the western and central highlands, known as the Western Mountains and Central Plateau respectively. Blocking of the low-level airflow, coupled with the formation of a windward ridge of high pressure, may also contribute to the formation of the lee trough.
From time to time, mesoscale thermal lows and pronounced standing wave activity have also been observed to form over parts of the state co-incident with a lee trough. These can further complicate the overall wind pattern over parts of the Tasmanian east coast.
This study aims to investigate some aspects of this lee trough by the utilization of HIRES. It was carried out with the support of two other researchers: Dr Lixin Qi and Mr Russel
Morison, both from the School of Mathematics, University of New South Wales. Dr Lixin Qi provided some valuable meteorological advice whilst Mr Morison performed all the model runs. My contribution was to identify the event, collect and collate all the background
97 information, including observations, identify the synoptic setting and provide the
interpretation and discussion of the results. The following work in this chapter has been
published in an international peer-reviewed journal: ‘Meteorology and Atmospheric Physics’.
6.1 Introduction
The orography of Tasmania is quite complex (see Figs 6.1a and b). One of the meteorological
ramifications of this complex orography is the formation of a trough in the lee of the
mountains when conditions are favourable (Fig 6.2). In broad westerly wind flow, the trough that forms along the east coast is of particular importance to recreational and professional boaters, including fishers.
The effect of the lee trough can be complicated at times by the formation of a thermal low.
Whitehead (1981) suggests that these lows develop on days when the surface air temperature
in the region becomes such that the dry adiabatic lapse rate is established to a height of at
least 1200m with an isothermal layer or inversion above. This situation also coincides with
(but is not a pre-requisite for) the formation of a lee trough. This thermal low phenomenon,
will not however, be discussed any further in this paper.
98 The height of the Tasmanian highlands is approximately 1200 metres. Wind flow below this height is considered to be blocked during a lee trough event. Forecasters in the Hobart office of the Australian Bureau of Meteorology assess the wind and temperature structure above and below 850hPa in order to ascertain whether a lee trough event will eventuate
(Hainsworth, personal communication).
The east coast lee trough which forms from time to time, is known to affect the general wind flow in a manner outlined below (Batt &Hainsworth 2001). Under broad westerly (northwest to southwest) wind flow, a lee trough will usually form on the east coast of Tasmania, and can affect waters up 60km offshore. This situation has been verified by the author during several Sydney to Hobart yacht races. Hainsworth (personal communication), a senior forecaster with the Bureau of Meteorology in Hobart, has also noted this during conversations with professional fishers.
However, the effects at the surface of a northwesterly versus a southwesterly wind flow can be quite different. The following sections examine them in more detail.
North West airflow: Surface winds in the morning (up to around 1100 EDST) will generally start as north-westerlies over most of the coastal waters. North of around St Helens Point, winds may even be slightly accelerated as winds funnel through Banks Strait. However, as the day progresses and the Tasmanian land mass heats, pressure falls occur along inland east coastal parts. Consequently, winds will start to veer more northerly and by 1300 to 1400
EDST will veer even more and become northeasterly inshore south of Eddystone Point. At sunset, this breeze will decrease quite quickly and can become quite light and variable for a
99 period until the northwesterly re-establishes itself during the early hours of the morning.
Southwest airflow: Surface winds along the east coast become very variable inshore. To the north of the Freycinet Peninsula, winds will be markedly affected. They often become light and variable during the morning after a light westerly land breeze overnight. During the day there is a good chance of a light east to northeast sea breeze. South of the Freycinet Peninsula though, winds mostly commence as west to southwest then gradually turn more southerly north of around Maria Island and then often southeast during the afternoon due to sea breeze influences. The stronger the southwesterly flow, the further offshore these effects will extend.
During Westerly airflow, the entire east coast of Tasmania can be affected by the lee trough.
The HIRES model was used to investigate some aspects of lee troughs. The model was run at
10km horizontal resolution and was nested in the GASP, which is the Australian Bureau of
Meteorology’s global model.
A lee trough event that occurred between 27 and 29 December 1995 was investigated, when the author was participating in the annual Sydney to Hobart yacht race. Regular observations were taken from the yacht Ragamuffin whilst traversing the east coast of Tasmania during a lee trough event. The onboard observations were supplemented by coastal observations obtained from the Australian Bureau of Meteorology’s official coastal observing stations.
The period 28 to 29 December 1995 was the subject of this study since the yacht was not close to the Tasmanian mainland on 27 December. The yacht successfully completed the race during the evening of 29 December. Some aspects of this trough were investigated and the results presented in this chapter.
100
6.2 Theory
This section is intended to extend the broad theory presented above in Chapter 2 and apply it specifically to a mountain barrier. When a stable airstream flows across a substantial mountain range, a trough, with at times an embedded surface low pressure system, can develop on the lee side. This is known as a lee trough and in the case when a closed system forms, it is termed lee cyclogenesis. This phenomenom can be explained by considering the idea of vorticity, or the “spin”, of a fluid.
As air spreads out horizontally its spin rate decreases. On the other hand, if air contracts in a horizontal direction, its spin rate increases. Thus it can be seen that the rate of change of absolute vorticity is related to divergence.
The relationship is shown as Equation 1 (After Atkinson, 1981). d ζζ ∇−= V (1) dt a ha
Where ζa is the absolute vorticity and ∇Vh is the divergence.
In the case of a lee trough, air close to the earth’s surface is forced to follow the surface contours. Upon encountering a mountain range, this air is forced upwards and eventually will descend its leeward side. Due to the stable nature of the lower troposphere, airflow higher up will flow horizontally. From a vorticity point of view, air in the lower troposphere will contract or shrink as it rises up the windward side of the mountain range. Conversely, as the air descends the leeward side of the mountain it will be stretched. As a result, the absolute vorticity will decrease as the air ascends the mountain range and will return to its initial value
101 as the air descends the lee side. This means that the relative vorticity of the air will be
anticyclonic whilst it is over the mountain area (the absolute vorticity is less cyclonic). This
also implies anticyclonic curvature (left deflection) of streamlines, assuming no horizontal
wind shear. Since lower pressure lies to the right of the streamlines in the Southern
Hemisphere, it implies that lower pressure exists on the lee side of the mountain.
In the case of broad westerly flow onto the Tasmanian highlands, air will have a net
displacement to the north after crossing the mountain barrier. This means that its planetary
vorticity will be less cyclonic that it was on the windward side of the barrier. This implies
that the air possesses cyclonic relative vorticity once it has traversed the mountain barrier.
This in turn leads to cyclonic curvature of the streamlines in the lee of the mountains.
In order to obtain an idea of the horizontal influence of the lee trough, the value of the radii
of curvature of the streamlines in the lee of a mountain were calculated. To achieve this, a
quantity known as the potential vorticity was used. The potential vorticity relates the absolute
vorticity to the pressure difference of the air column in question.
Thus (after Atkinson, 1981)
ζ a = C (2) − pp 12
where ζa : absolute vorticity, p2 - p1 : pressure difference between bottom and top of air column, C: constant.
102 By inserting some known values for the Tasmanian case, we obtain the following:
Effective height of Tasmanian mountain barrier 1200m
Assume east coastal plain is at sea level=1000hPa
Assume height above mountain when flow not affected=300hPa
(f+ ζm)/(880-300)=f+0/(1000-300)
so that
ζm =-0.17f
Now neglecting shear
ζm = V/R
where R is the radius of curvature of the flow at the mountain crest.
Combining last two equations gives
R=10/-0.17f
At 43 degrees south, f =-10-4 s-1, so if V=10m/s then the radius of curvature at the mountain top is approximately 60km. We can presume that the shrinking of the air column occurs more at the base than the top. This being the case, then we would expect smaller radii of curvature at the surface and larger radii at the top.
As stated above, the radius of curvature at sea level is up to approximately 60km. This value compares excellently with the calculated value given above adjusted for sea level.
103
Following the earlier work of Queney, Steenburgh and Mass (Steenburgh & Mass, 1994)
suggested that if the Rossby Number (Ro), defined as
Ro = U/fLm (3)
where U= basic-state wind speed, f= Coriolis parameter and Lm= the half-width of the ridge, is
much less than unity, then the mountain ridge will produce stationary Rossby waves, with
synoptic-scale windward ridging and lee-troughing.
After substituting values for the Tasmanian case, namely, U=10m/s, f= -10-4 m/s and Lm=
1000m, the value of Ro that we obtain is –10. This result is well less than unity and as such would confirm the existence of a lee-trough.
6.3 Synoptic and Mesoscale Overview
The synoptic weather pattern over the period of interest was as follows:
On 27 December 1995 (Fig. 6.3a), a series of cold fronts were passing over Tasmania. This
situation produced a fresh to strong southwest airstream over the state.
Figure 6.3b shows the SLP chart at 1100 EDST 28 December 1995. A cold front had passed
over Tasmania during the early hours of 28 December. At 1100 EDST the cold front was
situated from near Gabo Island to 40 deg S 160 deg E to 50 deg S 165 deg E. Tasmania was
under the effects of a moderate south to southwest airstream. Late and overnight on 28
December, a high pressure system had moved eastwards through Bass Strait. This system
affected northern parts of Tasmania whilst southern parts remained under the influence of a
104 weakened southwesterly airstream.
The upper wind profiles for both Hobart and Launceston Airports at 0900EDST 28 December
(Fig. 6.4) show south to southwesterly flow from close to the surface up to at least 300hPa.
(Both Hobart and Launceston Airports are situated to the east of the effective mountain
barrier). Over the day, the flow below 880hPa became blocked as the lower troposphere
became more stable. Above 880hPa, the flow was gale to storm force southwesterlies. This is
shown by the examination of the wind profiles at 1500 EDST 28 December 1995. The upper
wind profiles would have relaxed somewhat overnight over the state, especially in the north
with the passage of the high through Bass Strait.
On 29 December 1995 (Fig.6.3c) at 1100EDST, the centre of high pressure was located near
35 deg. S 155 deg E. A cold front was passing through southern parts of Tasmania. This front
re-introduced a west to southwest airstream over central and northern parts of Tasmania and
strengthened the existing west to southwest flow over southern parts.
The upper wind profiles for both Hobart and Launceston Airports (Fig. 6.4) at 0900 EDST 29
December and 1500 EDST 29 December 1995 respectively shows blocked flow below
880hPa and a vigorous west to southwesterly flow above 850hPa. On 30 December 1995
(Fig. 6.3d), Tasmania was under the effects of a weak ridge of high pressure.
It would appear that this lee trough event commenced during 26 December. At that stage the
yacht Ragamuffin was situated well north of Tasmania. On this day as well as on 27
December, the author had not been in a position to perform observations close to the
105 Tasmanian coast. It was not until 1100 EDST on 28 December that the yacht was close to the northeastern coast of Tasmania.
Figure 6.5 shows the aerological sounding for Hobart Airport at 1000 EDST 27 December. It indicates a strong inversion extending from approximately 850 to 710hPa (1500metres to
2950metres). This meant that the inversion was situated above the effective height of the
Tasmanian highlands (1200m). The inversion slowly weakened over the next 24 hour period as shown by Figure 6.6 (aerological sounding for Hobart Airport for 1000 EDST 29
December. The upper wind structure, perhaps aided by the presence of the temperature inversion, was conducive to the formation of a lee trough.
It also appears that the “unblocked” flow (flow above 880hPa) was from the south to southwest on 28 December which veered toward the west on 29 December. This indicated that the surface windflow would behave as stated earlier.
Streamline mesoscale analyses are given for both 1400 EDST on 28 and 29 December respectively (Figs 6.7& 6.8). It would appear from the limited surface observational data that lee troughing over eastern and northeastern parts of Tasmania can be quite complex under southwesterly to westerly flow.
On 28 December a lee trough extended from near Wynyard, on the northwest coast to about the Freycinet Peninsula on the east coast. A secondary trough was located close to the southern part of Flinders Island. It would appear that as the general wind flow veered to the west, the main lee trough shifted a little to the east and a mesoscale low formed between
106 Swan Island and Eddystone Point. A weak trough extended eastward from the mesoscale low.
6.4 Results
At 1100 EDST 28 December, the model forecast wind field (Fig.6.9) appears to fit the overall observed surface wind field very well. At this time, the model was 72 hours into its run after having been initialised at 1100 EDST 25 December. The yacht Ragamuffin at this time was 185 km northeast of Tasmania and in “undisturbed” or geostrophic flow, well outside of the effects of the lee trough. The yacht was reporting a southwest wind averaging
15kn which ties in exactly with the forecast wind velocity for the area.
Note that winds at Maatsukyer Island, on the southwest tip of Tasmania, normally over-read with general flow from the northwest through the west, and also from the south to the east.
Consequently surface wind data from Maatsukyer has not been included in the wind analysis discussion in this chapter.
Figure 6.10 is the forecast wind field for 1400 EDST 28 December. At this time, the forecast winds over the northeast part of Tasmania did not agree with the observed winds. The model does, however, indicate lighter and slightly more variable winds in this area generally.
Forecast and actual wind velocities over southeastern Tasmania agree fairly well with each other as they do for the position of Ragamuffin (positioned at around 160km northeast of
Tasmania in geostrophic flow). Over the central east coast, it would be expected from experience that at around this time wind velocities would be light from the south tending southeast.
Unfortunately actual wind data could not be obtained for this time but the model winds are
107 in good agreement with this expectation, apart from a slightly stronger speed.
During the late evening and overnight on 28/29 December a high pressure centre moved through Bass Strait. The effect of this system was modelled well as Figure 6.11 indicates.
The modelled and observed winds over the northeast of Tasmania at this time (2300 EDST) are indicative of the wind flow that one would expect on the western flank of an anticyclone.
At this time the yacht Ragamuffin was situated some 56km off St Helen’s Point on the northeastern Tasmanian coast. This position was close to the observed/theoretical radius of curvature of 60km for the lee trough to operate within. The observed wind direction fits the modelled direction very well, but the modelled speed was about 8kn less than the actual. The southern part of Tasmania was still under the influence of a southwesterly airstream. The modelled winds did not capture the sea breeze component that was affecting the southeast of the state. At 10km resolution it would not be expected that the model would resolve a sea breeze circulation with any degree of accuracy.
By 0500 EDST 29 December, (Fig. 6.12), the centre of the high pressure system was just east of Flinders Island. Winds, both modelled and observed, over the northeast corner of the state were still in good agreement, indicative of the western flank of the high pressure system. At this time, Ragamuffin was situated just off the Freycinet Peninsula and well within the zone of the lee trough. The observed wind velocity of west at 5kn, was representative of a land breeze which was expected around this time under a southwesterly synoptic wind regime.
The modelled wind velocity in the area of Ragamuffin was directionally in excellent agreement but the speed was about 10kn too strong.
108 Further south near Tasman Island, the forecast wind speed was 15kn too high but the direction was again in excellent agreement. Because of local effects, namely speed enhancement due to steep orography, in the area of Tasman Island, any model, unless run at a much higher resolution, would not capture the actual wind speed accurately.
With the re-establishment/strengthening of the southwesterly wind flow over Tasmania on 29
December, the eastern part of the state as a whole came under the influence of the lee trough again. Figure 6.13 is the forecast wind chart for 1100 EDST 29 December. Over the southeast of the state, the forecast winds were about 10kn too strong and the direction field was approximately 90 degrees different to the actual winds. In the northeast corner of Tasmania, the observed winds were close to the forecast wind fields, i.e. light and variable.
By 1400 EDST 29 December (Fig. 6.14), the situation had not changed significantly from the earlier one. This was considered to be a good result since the model was 99 hr into its run.
6.5 Discussion
Based on the sparse observational data (presented on Figs 6.9 to 6.14) and also the streamline meso-scale analyses (Figs 6.7 & 6.8), the eastern Tasmanian lee-trough can be very complex under general southwesterly to westerly wind flow. The trough has two components - a
“major” trough close in the lee of the main mountain chain (in the Midlands, see Fig 6.1b), and a secondary trough that could form over northeastern parts of the state. This is possibly due to the presence of the Northeast/East secondary mountain chain (see Fig 6.1b).
There is good skill in the ability of HIRES (nested in the Australian Bureau of Meteorology’s
109 global model GASP), to predict or hint at this phenomenon so far ahead in time when run
with a horizontal resolution of 10km.
It can also be said that the theoretical value for the radius of streamline curvature at the top of
the Tasmanian highlands, is in excellent agreement with the observed value. The Rossby
Number (Ro) result of –10 would suggest that lee-troughing should have been evident in this particular situation.
110
Figure 6.1a: Topography of Tasmania including place names used in text
111
WEST EAST 1500 1500
750 750
METRES METRES WESTERN CENTRAL MIDLANDS NE & E MOUNTAINS PLATEAU MOUNTAINS
CMAN/R3311-5
Figure 6.1b: A west – east cross-section through Tasmania
112
REPORTED WIND DIRECTION 10 REPORTED WIND SPEED IN KNOTS 1012 ISOBARS (MB) POSSIBLE WIND DIRECTION 1013
1012 ROUGH SEAS 30 MODERATE SEAS MODERATE SEAS 20 20 1011 20 15 MODERATE SEAS 10 1010 SMOOTH 15 SEAS
5 SMOOTH kts SEAS LOW 1009 1008 mbs
10
10
1008
45 SLIGHT SEAS
ROUGH SEAS 1006
CMAN/R3311-4
Figure 6.2: An idealized lee trough and mesoscale heat low event in Tasmania (after Bureau of Meteorology, 1991)
113
1012 1012 10 1012 10 10 10 L 1012
1012 L L 1012
20 20 20 20 L 1016 L 1012 1016 1016 1012
130 140 130 140 1016 120 150 120 1016 150 10 160 10 L 160 1 1012 1 1020 1020 30 30 30 L 30 1016 1020 1024 1024 1016 1016 1012 1028 40 1012 1028 40 1008 40 40 H 1020 H 1004 1000 1008 1008 1012 1012 996 27 1024 1028 1024 1016 1004 1000 1004 28 1028 1024 1020 1016 1008
1012 1012 1012 10 10 10 10
1008 1008 1012 1012 1012 L 20 20 20 L 20 L L 1008 L 1016 1008 1016 1016 130 1012 140 130 140 120 150 1016 120 150 10 160 10 160 1 1016 1 1012 1020 1020 30 1020 30 30 30 1020 L 1016 1024 1024 H 1020 1028 40 40 0 0 4 4 H 1016 1024 H H 1012 1020 1020 29 1024 1020 1016 1012 1008 30 CMAN/R3311-7
Figure 6.3: Australian region mean sea level pressure analyses (4hPa spacing) at 1100 EDST from 27 to 30 December 1995 inclusive.
114
LAUNCESTON AIRPORT 300 300 300 300
500 500 500 500
700 700 700 700
850 850 850 850
877 877 877 877
910 910 910 910
943 hPa 943 hPa 943 hPa 943 hPa
HOBART AIRPORT 300 300 300 300
500 500 500 500
700 700 700 700
850 850 850 850
910 910 910 910
925 925 925 925
943 hPa 943 hPa 943 hPa 943 hPa
27 th 2200Z 28 th 0400Z 28 th 2200Z 29 th 0400Z
CMAN/R3311-6
Figure 6.4: Upper wind profiles for both Hobart and Launceston Airports from 0900 EDST 28 December 1995 (2200 UTC 27 December) to 1500 EDST 29 December 1995 (0400UTC 29 December 1995). The observed wind velocity barbs are given in standard meteorological format with the wind speed in knots. Heights are given in hectopascals (hPa).
115 -100 -90 -80 -70 -60 -50 -40
-30
-20 20 24 28 32 200
-10 o 300
0
400
10 ( C) TEMPERATURE PRESSURE (hPa)PRESSURE 500
600 20 -4 1 2 3 5 8 12 20 700
800 30 900 1000
CMAN\R3314-6
Figure 6.5: Aerological sounding for Hobart Airport for 1000 EDST 28 December 1995
116
-100 -90 -80 -70 -60 -50 -40
-30
-20 20 24 28 32 200
-10 o 300
0
400
10 (C) TEMPERATURE PRESSURE (hPa) 500
600 20 -4 1 2 3 5 8 12 20 700
800 30 900 1000
CMAN\R3314-7
Figure 6.6: Aerological sounding for Hobart Airport for 1000 EDST 29 December 1995
117
o o 145 E 150 E
VICTORIA
Wilsons Promontory
o 40 S o 40 S
Smithton Marrawah Wynyard Devonport
TASMANIA Launceston Airport
Strahan
OVER 900
600 - 900
300 - 600
0 - 300 0300 UTC 28/12/95 Height in metres
145 o E 150 o E
CMAN/R3311-2
Figure 6.7: Streamline mesoscale analysis valid 1400 EDST 28 December 1995. Wind observations from yacht Ragamuffin are circled and in standard format.
118
o o 145 E 150 E
VICTORIA
Wilsons Promontory
o 40 S o 40 S
Smithton Marrawah Wynyard Devonport L
TASMANIA Launceston Airport
Strahan
OVER 900
600 - 900
300 - 600
0 - 300 0300 UTC 29/12/95 Height in metres
145 o E 150 o E
CMAN/R3311-3
Figure 6.8: Streamline mesoscale analysis valid 1400 EDST 29 December 1995. Wind observations from yacht Ragamuffin are circled and in standard format.
119
30S 25 31S
32S
33S 10 5 34S 15
10 35S
36S 10 5 20
37S 10
38S 15 25 15 39S 20 15 40S 10 20 25 41S 15 42S
43S 10 20 44S 15
140E 142E 144E 146E 148E 150E 152E 154E 156E 158E 30 COREL/R3311-8
Figure 6.9: HIRES model forecast winds for 1100 EDST 28 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
120
30S
31S
32S
33S
34S 5 35S 10 10
36S
37S 15 15 38S 10 20
39S 15
40S 15 25 20 41S
42S
43S
44S
15 140E 142E 144E 146E 148E 150E 152E 154E 156E 158E 30 COREL/R3311-9
Figure 6.10: HIRES model forecast winds for 1400 EDST 28 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
121
Figure 6.11: HIRES model forecast winds for 2300 EDST 28 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
122
Figure 6.12: HIRES model forecast winds for 0500 EDST 28 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
123 30S
31S 15 32S 5 15 33S
10 34S
35S 10
36S 10 5 37S
38S 15 39S
20 40S 5
41S 15
42S
43S 10 44S
140E 142E 144E 146E 148E 150E 152E 154E 156E 158E 30 COREL/R3311-13
Figure 6.13: HIRES model forecast winds for 1100 EDST 29 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
124
30S
31S 15 32S 5 15 33S
10 34S
35S 10
36S 10 5 37S
38S 15 39S
20 40S 5
41S 15
42S
43S 10 44S
140E 142E 144E 146E 148E 150E 152E 154E 156E 158E 30 COREL/R3311-13
Figure 6.14: HIRES model forecast winds for 1400 EDST 29 December 1995 at 10km resolution showing wind direction (arrows) and speed (indicated both by the length of the arrows and isotachs in knots). Observed wind velocities in standard meteorological format are shown for selected stations and observations from the yacht Ragamuffin are circled.
125
CHAPTER 7
CONCLUDING REMARKS
In the case studies presented in the previous chapters, an NWP model has been run at very high resolutions in order to investigate a number of theoretical concepts. These are listed and summarized below:
1. The greater accuracy of a very high-resolution model over a lower resolution global model for the same domain.
It has been shown, in the context of Chapters 4, 5 and 6, that there is considerable skill in the ability of high-resolution NWP models such as the University of New South Wales HIRES model, to predict the major features of the wind fields over the ocean out to several days ahead. The comparison of the model prediction with point values over the open ocean is an extremely severe test and the accuracy shown was very encouraging, especially at higher resolutions.
The greater accuracy of the very high-resolution model over the lower resolution global model, was ably demonstrated in the context of the 1997 Sydney to Hobart Yacht Race
(Chapter 5). Very accurate wind velocity prediction skill was exhibited by the 10 km horizontal resolution model run over the 25 km NWP model run of HIRES and the 135km low resolution GASP run . The results from this study along with those from Chapter 4 further extend the usefulness of high-resolution model predictions over global model predictions for resolving finer scale meteorological features.
126
2. The Summertime Cool Change over Southeast Australia
The complex structure and behaviour of cold fronts along the New South Wales coast during the warmer months of the year was demonstrated in Chapters 4 and 5.
This was particularly the case when HIRES was run at very high resolutions. It was able to more accurately simulate the complex structure of the cool change as it progressed along the
NSW coast than the lower resolution model runs.
This was particularly the case in Chapter 4. In this study, the HIRES model was run in hindcast mode at a horizontal resolution of 15 km. It had the front situated a little further north, around Botany Bay, than the 25km run and also showed more structure in the frontal zone, and provided a time sequence that was closer to the observed wind velocities.
3. Coastal Wind Flows
The influence of coastlines, particularly complex ones, on the wind flow was demonstrated to a limited extent throughout the above studies, particularly in Chapters 5 and 6. It is generally accepted in the meteorological community that unless the numerical model is run at horizontal resolutions of 5 km or less, the model will normally fail to detect the very localised changes to the wind velocity fields (e.g. speed enhancement/reduction along coastlines and around headlands and the evolution of the sea/land breeze) along/over complex coastlines and the coastal zone generally. This was noted by the author (working as a yachting meteorologist) during the Olympic Games conducted in Sydney during September
2000. The operational 5km NWP model (MESOLAPS: MESOscale Limited Area Prediction
System) used for the routine forecasting of the wind fields over the sailing areas in and
127 adjacent to Sydney Harbour performed much better than the lower resolution NWP models
that were available, particularly from the point of view of forecasting the evolution of the sea
breeze. Furthermore, the 5km model better resolved some of the more localized effects around Sydney Harbour than did the suite of lower resolution models that were available to forecasters.
4. Airflow in the vicinity of Complex Orography
The following concepts were verified utilizing the study presented in Chapter 6.
• air flow takes the path of least resistance. During stable conditions air will flow
around obstructions and through valleys etc. During unstable conditions air will flow
up and over obstructions.
• the shape of topography can help generate local turbulence.
• the orientation of the wind flow to a mountain range is important in determining
turbulent effects.
• under certain airflow and stability situations, standing wave activity and a lee trough
can be observed in the lee of mountains, hills or even high coastal cliffs.
Furthermore, there was good skill in the ability of HIRES (nested in the Australian Bureau of
Meteorology’s global model GASP), to predict or hint at the lee-trough phenomenon so far
ahead in time at a horizontal resolution of 10km.
128
It can also be said that the theoretical value for the radius of streamline curvature at the top of
the Tasmanian highlands, was in excellent agreement with the observed value. The Rossby
Number (Ro) result of –10 would suggest that lee-troughing should have been evident in this particular situation.
129
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134
APPENDIX 1
WEATHER DAMPENS THE SYDNEY TO HOBART YACHT RACE
A1.1 Introduction
The Sydney to Hobart Yacht Race, starting every year on 26 December, is one of the major
highlights of the Australian sporting calendar. The race is from Sydney, the State capital of
New South Wales (NSW) to Hobart, the capital of the island State of Tasmania - a distance
of 630 nautical miles (1,167 km) through the Tasman Sea and eastern Bass Strait, some of the
roughest water in the world.
In most years more than 100 ocean racing yachts from around the world enter the race. Their
length ranges from a compact 9.5 metres to the 25 metre maxi-yachts. The race record is 2 days, 14 hours, 36 minutes, 56 seconds, set by the American maxi ketch Kialoa in 1975.
Usually, most of the fleet is in Hobart for New Year’s Eve.
The 1993 race proved to be one of the most demanding of all, both on the crews and
equipment. Headwinds prevailed for most of the race, with spinnakers only being used for the
first and final few hours. Of the 104 yachts that started only 37 crossed the finishing line.
135 A low pressure system developed during the first day over Eastern Bass Strait and maintained a central pressure of about 992 hPa for the following three days before moving to the south- south-east and weakening.
The New South Wales Regional Office of the Australian Bureau of Meteorology, provided the pre-race briefing to the yacht crews and media on Friday 24 December, two days prior to the start. This attracted greater interest than normal as the Bureau had informed the organisers of the race that numerical computer models were indicating that a significant low pressure system would develop during the course of the race.
The following two days saw the crews undertaking preparations for what looked like being a very rough trip. The possible development was of even further interest to the author as it was to be his third Sydney to Hobart race. He was a crew member and the weather forecaster on the eventual International Offshore Rule (IOR) handicap winner, Solbourne Wild Oats.
A1.2 A Brief Chronology of Events
The following is a brief chronology of events:
136 1.2.1 The First Day (Sunday 26 December)
The race began at 1300 EDST in a 12 to 15 knot northeast seabreeze which persisted into the late afternoon. A thunderstorm then moved over the race area followed by a period of light and variable winds for about a hour, ahead of a cold front (Figure A1) which brought fresh west to southwesterly winds. These winds slowly increased in speed to become strong to gale force by early Monday morning.
Figure A1: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 26 December 1993.
A1.2.2 The Second Day (Monday 27 December)
By 1100 EDST, the front was located well out in the Tasman Sea with an associated low pressure centre of 992 hPa in the Southern Tasman Sea (Figure A2). A second low pressure centre of similar intensity was located a little further to the west in Bass Strait.
137
Figure A2: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 27 December 1993.
The strongest winds during the day were being reported over Western Bass Strait and to the south of the low pressure complex. Because of the strong winds, two other annual yacht races from Melbourne, the State capital of Victoria, to Hobart and Devonport in Tasmania were postponed
.
The leaders in the Sydney to Hobart Yacht Race at this stage were well off the southern NSW coast and about to enter waters east of Bass Strait. The bulk of the fleet was spread out further northwards well off the NSW coast and east of the rhumb-line. The fleet was now feeling the full effects of strong to gale-force winds which were opposing the southward flowing East Australian Current and creating extremely uncomfortable sea conditions with
138 significant wave heights approaching 10 metres. Yachts bringing up the rear of the fleet were generally closer inshore, receiving some protection from the strong to gale force offshore southwesterlies. However, by evening, more than 30 yachts had retired, either through equipment failure or concern for crew safety.
A1.2.3 The Third Day (Tuesday 28 December)
During the early hours of Tuesday, the Southern Tasman Sea low (Figure A3) had reached its lowest pressure of 986 hPa. The second centre over Bass Strait was quickly losing its identity. As predicted by the computer models some four to five days previously, the deepening of the low had become a reality. This was a result of a dynamic interaction with the upper level flow and the marked surface temperature gradient across the front. The warmer than average sea water temperatures in the area may have also contributed to the deepening.
The low was quickly becoming cut-off as the high pressure system, further to the west of
Tasmania, ridged to the south of the low towards New Zealand. This resulted in the low remaining nearly stationary for the next 24 to 48 hours.
139
Figure A3: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 28 December 1993.
The yachts along the NSW coast were still under the influence of the southwesterly
airstream, but the race leaders entering Bass Strait were experiencing south to southeasterly
winds averaging 35 to 40 knots with wind gusts in excess of 70 knots. Sea conditions were
horrendous. Wave heights at times were over 10 metres with some larger rogue waves being
reported. These rogue waves were apparently responsible for some of the worst hull and
rigging damage as well as crew injuries sustained by the retiring yachts.
140 One man spent 5 hours in the water after being swept off the yacht he owned. Two boats were abandoned by their crews due to fears of sinking. By evening a further 32 yachts had retired, bringing the total retirements at this stage to 62. These retirements included all the highly fancied maxi-yachts, such as Brindabella, Cassiopeia, Ragamuffin the Maxi and
Amazon
.
A1.2.4 The Fourth Day (Wednesday 29 December)
The wind slowly moderated to below gale force by mid afternoon. That night the low was located near 42 degrees south 158 degrees east, with a central pressure of approximately 992 hPa (Figure A4). This day saw another 3 retirements from the race.
Figure A4: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 29 December 1993.
141 A1.2.5 The Fifth Day (Thursday 30 December)
The low was beginning to drift slowly to the south to be near 45 degrees south 158 degrees east at 0000 UTC with a central pressure of 996 hPa (Figure A5). Late in the day, the winds weakened to 15 to 18 knots and veered from SE to SSW with the southward movement of the low.
Figure A5: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 30 December 1993.
Most of the fleet was experiencing more comfortable conditions but local effects, such as funnelling through Bass Strait, caused problems for yachts further back. The race leader
Ninety Seven (a brand new 14.3 metre yacht) completed the race at 1345 EDST after 4 days
54 minutes and 11 seconds at sea. This created history in that she was the shortest yacht to take line honours since 1953. The second yacht to finish was the 12.2 metre Cuckoos Nest.
142 She finished 2 hours behind Ninety Seven and in so doing took out handicap honours in the
International Measurement System (IMS) division.
Two more boats retired during the day.
A1.2.6 The Sixth Day (Friday 31 December)
The low continued to move to the south, (Figure A6) and was no longer a controlling feature of the weather over the race area. (For the next two weeks south-eastern Australia remained under westerly winds, which largely contributed to the disastrous bushfires which affected eastern New South Wales for an unprecedented 19 day period.)
Figure A6: Australian region mean sea level pressure analysis (4hPa spacing) at 1100 EDST 31 December 1993.
143
The winds became very light and proved to be very frustrating for yachts on the last leg of the race up the Derwent River to the finishing line. To add to the problems, very heavy rains had fallen on the Derwent River catchment in the previous fortnight and large amounts of water were running down the river. The river was living up to its reputation as a very fickle waterway. On Solbourne Wild Oats for example, it took us an extra four hours to reach the finish line at Hobart. In ideal conditions, the trip up the Derwent would take about two hours to complete.
A1.3 Conclusion
Not since the 1984 (now 1998) Sydney to Hobart Race had an intense low pressure system had such an effect on the race. In that year a record 106 withdrawals occurred out of a fleet of
152, when a low of central pressure 996 hPa affected the yachts during the first and second days of the race. In contrast, the 1993 race with 67 withdrawals (the second highest in the 49 year history of the race), was under the influence of a much deeper low which persisted until the fourth day of the race.
144
“The wind blows to the south
and turns to the north;
round and round it goes,
ever returning on its course.”
Ecclesiastes 1:6
145