432 WEATHER AND FORECASTING VOLUME 15

Regeneration of the Southerly Buster of Southeast

HELEN J. REID School of Mathematics, University of , , Australia

(Manuscript received 3 November 1999, in ®nal form 3 April 2000)

ABSTRACT The southerly buster has been successfully simulated using a numerical weather prediction (NWP) model and veri®ed (particularly the sea level pressure ®eld). This simulation was then used to study the behavior of the southerly buster in the region of the Hunter Valley, New South Wales, Australia, with the reintensi®cation of the surge. In simulating the dynamics of the southerly buster in the vicinity of the Hunter Valley, both the horizontal and vertical resolution of the NWP are important. This was found through a series of simulations of a case study of 27 February 1998. The best simulation was achieved with 20 vertical levels, a coarse nesting into the Australian Bureau of Meteorology Limited Area Prediction System model, then down to ®ner grids in progressively higher resolutions. The pressure ridge associated with the southerly buster is induced by the southerly ¯ow up the southern parts of the resulting in anticyclonic vorticity that creates a region of high pressure ahead of the main high pressure cell behind the frontal system. This same mechanism is used to explain the reintensi®cation of the surge at the northern part of the Great Dividing Range, which is characterized by a renewed peak in wind speed north of the Hunter Valley.

1. Introduction of the NWP model, HIRES, are in section 4 with thor- ough veri®cation of the simulations in section 5. The The southerly buster along the coast of New South sea level pressure (SLP) pattern is veri®ed in consid- Wales (NSW), Australia, has been the focus of many erable detail as it provides the basis for the investigation studies. In Reid and Leslie (1999, hereafter RL99) and of the pressure ridge of the southerly buster dealt with Reid (2000), a numerical weather prediction (NWP) in section 6, before the conclusions and ideas for further model capable of running at high resolution (HIRES) work are presented in section 7. was employed to simulate the southerly buster with very RL99 focussed on the arrival time and Reid (2000) good results. The region of the Hunter Valley was shown addressed the intensity of the southerly buster. This to have a signi®cant effect on the propagation of the study will endeavor to address the SLP pattern in re- southerly buster. Reid (2000) demonstrated that the lation to the propagation of the southerly buster. As a weaker surge across the Hunter Valley mouth (see Fig. result, section 5b will verify the model in more detail 1) reintensi®es to the north after a period of several in terms of SLP and consider how this is related to the hours. In many cases there is a ¯ow up the Hunter Valley southerly buster. prior to the coastal locations of similar latitude. The question of the extent of northward propagation is often posed (e.g., Baines 1980) and there have been many 2. Southerly busters studies on the development (e.g., McBride and McInnes 1993) but none on the decay. It is proposed that the Southerly busters occur during the spring and summer reintensi®cation of the southerly buster surge occurs in months in the southeast of Australia to the east of the a manner similar to that of the original development of Great Dividing Range along the coast of NSW (see Fig. the southerly buster surge in the south of . 1). They are a strong, sudden, and squally southerly Ϫ1 Section 2 provides a brief description of the southerly wind surge of at least 15 m s and have been known Ϫ1 buster phenomenon with the case study of 27 February to gust up to 35 m s (Colquhoun et al. 1985) and are 1998, introduced in section 3. Notes on the con®guration con®ned to the coastal regions, trapped against the by the Coriolis force. The depth of the surge is generally less than 1 km and therefore is below the average height of the Great Dividing Range. Corresponding author address: Dr. Helen J. Reid, Bureau of Me- The passage of the southerly buster is occasionally teorology, Level 16, 300 Elizabeth St., 2000 Sydney, Australia. accompanied by a roll cloud and precipitation is not E-mail: [email protected] usual. In addition the passage is notable for the common

᭧ 2000 American Meteorological Society

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FIG. 2. SLP chart for 0900 27 Feb 1998.

a. Synoptic situation The cold front from which the southerly buster de- veloped was over the during the afternoon of 26 February 1998. Over the next 2 days, it progressed farther east and out into the . The frontal system on this occasion was complex. A prefrontal trough became dominant as it progressed FIG. 1. A map of southeast Australia over the domain of the 30- across Victoria, although it weakened as it approached km resolution model simulation indicating the location of the Great Dividing Range (shaded region over 500 m), New South Wales, Vic- the southern coast of NSW and the front merged with toria, Queensland, the Hunter Valley, , and the Tasman Sea. the weakened trough as the system progressed up the coast of NSW during 27 February 1998. A region of low pressure existed over the northwest of the continent and a high pressure cell moved across the southeast of wind shift from northwesterly to southerly, as well as Australia directly behind the frontal system. Figure 2 for the sudden drop of temperature, which can be up to illustrates the broadscale synoptic situation in which the 15ЊC within minutes (Howells and Kuo 1988; Mass and southerly buster was embedded; however, it does not Albright 1987). Also, a signi®cant rise in the SLP occurs show the detail of the 3-hourly hand analyses from the as a ridge of high pressure follows the cold frontal sys- Sydney of®ce of the Australian Bureau of Meteorology tem. This study will focus on the origins and movement on which the above comments were based. of this ridge. A southerly buster is generated when a cold front is blocked and experiences anticyclonic deformation near b. Observational summary the Great Dividing Range (McInnes 1993). The surge Data were obtained for eight locations in NSW to of air propagates northward as a coastally trapped oro- verify the HIRES NWP model simulations. These lo- graphic jet up the east coast of Australia, the duration cations are Bellambi, Sydney, Norah Head, William- of which is generally less than 24 h, from the time the town, Taree, and Coffs Harbour on the coast, while cold front reaches the Great Dividing Range in southern Cessnock and Scone are inland in the region known as NSW to its dissipation on the north coast or southern the Hunter Valley. The locations are illustrated in Fig. coast of Queensland (Baines 1980; Howells and Kuo 3 with an observational summary in Table 1. More de- 1988; McInnes et al. 1994). tails of this case can be found in Reid (2000). The tem- perature drop associated with the southerly buster tend- 3. Case study ed to occur over an hour-long period. Pressure rises tended to continue for several hours, with the main in- This section describes the southerly buster of 27 Feb- crease over a 5-h period. ruary 1998 both synoptically and in terms of surface observations. This event was near the end of the south- 4. HIRES erly buster season, which coincides with midspring to late summer. Times are standard times for the east of To look at the detail of the southerly buster surge, a Australia (UTC ϩ 1000). NWP model that can be run at high resolution (HIRES)

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boundary layer is treated as stability dependent with eddy diffusivities as functions of the bulk Richardson number. There is a surface heat budget with a prognostic equation for surface temperature and a modi®ed Kuo scheme for convection. Large-scale precipitation schemes as well as monthly average sea surface tem- perature, as obtained from the nesting data ®les, are used. In RL99, HIRES was run over a domain at 20- km horizontal resolution with 16 levels in the vertical. In Reid (2000), the horizontal resolution increased to 10 km and the vertical resolution increased to 20 levels, to incorporate more levels near the surface as the south- erly buster is a shallow phenomenon. This demonstrated an increase in the accuracy of the simulation with the higher resolution. However, various features of the local ¯ow were not well resolved. For example, the sea-breeze circulations during the afternoon were not clearly de- ®ned. In this study, HIRES has been run over a domain with 10- or 5-km resolution. The higher-resolution (5 km) simulations used computational routines that in- cluded an explicit treatment of moist processes (Qi et al. 2000). Also addressed was the effect of higher res- olution in the vertical, increasing the use of 20 sigma levels to 28 levels, to better resolve the middle tropo- sphere and tropopause. The following 20 levels are used: 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.75, 0.8, 0.825, 0.85, 0.875, 0.90, 0.925, 0.95, 0.975, 0.99, 0.995, 0.998, and 0.999. The following 28 levels are used: 0.10, 0.15, FIG. 3. A map of New South Wales and Victoria over the domain of the 10-km resolution model simulation indicating the locations of 0.20, 0.25, 0.30, 0.333, 0.366, 0.40, 0.433, 0.47, 0.50, the places used for veri®cation: 1, Bellambi; 2, Sydney; 3, Norah 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.825, 0.85, 0.875, Head; 4, Williamtown; 5, Cessnock; 6, Scone; 7, Taree; and 8, Coffs 0.90, 0.925, 0.95, 0.975, 0.99, 0.995, 0.998, and 0.999. Harbour. The 500-m topographical contour indicates that Cessnock Each of the simulations produced a 48-h forecast ini- and Scone are in the Hunter Valley. tialized at 2100 26 February 1998 (UTC ϩ 1000) using the operational datasets at 75-km horizontal resolution has been employed. Detailed documentation of HIRES from the Limited Area Prediction System (LAPS) model may be found in Leslie et al. (1985) and with the mod- (Puri et al. 1992) as boundary conditions. HIRES was i®cations listed in RL99, this NWP model was used to run with a coarse grid of either 50- or 30-km horizontal simulate the southerly buster. The model uses the prim- resolution, which then had the subsequent higher-res- itive equations for momentum, mass, moisture, and ther- olution simulations nested in these coarse-grid simu- mal energy with integrations carried out on the stag- lations. Table 2 lists the labels given to each simulation gered Arakawa C grid using a split semi-implicit time- for further reference in this study and the different res- differencing scheme. Sigma coordinates are used in the olutions, domain, nesting, and computational time for vertical. The surface layer is parameterized by the Mel- each simulation. The various grid dimensions are as lor±Yamada level-2.25 surface scheme. The rest of the follows: 50-km grid, 131 ϫ 80; 30-km grid, 69 ϫ 67;

TABLE 1. Observational summary from the eight speci®ed locations including the time, direction, and wind speed of the southerly buster, together with an indication of SLP (P) and temperature (T) changes, from the afternoon of 27 Feb to the morning of 28 Feb 1998. Time Direction Speed Gust P min P rise T max T drop Place (UTC ϩ 1000) (Њ) (m sϪ1) (m sϪ1) (hPa) (hPa) (ЊC) (ЊC) Bellambi 1308 180 19.0 21.6 Ð Ð 32.8 6.1 Sydney 1430 190 13.9 22.6 1014.2 6.1 35.7 11.2 Norah Head 1616 180 20.6 25.2 1014.5 6.5 24.8 2.0 Williamtown 1820 180 18.0 24.7 1014.0 Ð 33.0 Ð Cessnock 1708 180 8.2 21.1 1013.2 7.8 36.7 9.6 Scone 1844 160 16.5 20.6 1015.7 6.1 30.1 4.2 Taree 0500 170 7.2 10.8 1015.1 4.4 21.5 1.6 Coffs Harbour 1026 160 8.2 14.9 1018.2 2.8 25.2 1.9

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TABLE 2. The labeling of the seven HIRES simulations indicating the horizontal and vertical resolutions, domain, and the time taken for the model simulations.

Label Resolution (km) Self nested (km) Domain ␴ CPU time A 10 30 40Њ±29ЊS, 145Њ±155ЊE 20 1h,20min B 10 30 40Њ±29ЊS, 145Њ±155ЊE 28 3h,25min C 10 50 40Њ±29ЊS, 145Њ±155ЊE 20 1h,20min D 10 50 40Њ±29ЊS, 145Њ±155ЊE 28 3h,25min E 5 15 in 50 35.1Њ±29ЊS, 147Њ±155ЊE 20 4 days, 15 h F 5 15 in 50 35.1Њ±29ЊS, 147Њ±155ЊE 28 7 days, 15 min G 10 30 in 50 40Њ±29ЊS, 145Њ±155ЊE 20 2h,38min

15-km grid, 135 ϫ 133; 10-km grid, 101 ϫ 110; and a. Southerly wind 5-km grid, 161 ϫ 122. The time step was set at a fraction of the Courant±Friedrichs±Lewy (CFL) computation The southerly buster takes its name from the strong stability criterion. For the 30- and 50-km preliminary and sudden southerly wind and this is veri®ed before simulations the fraction was set to 0.7 and this was the SLP and temperature ®elds. There are several as- decreased to 0.5 for the 10- and 15-km simulations. The pects of the wind change to consider. The time of the explicit physics of the 5-km simulation required 0.1 of change as well as the strength are included. the CFL to complete the model simulation. Simulation G in Table 2 was included to assess wheth- er the intermediate grid (30 km) was able to produce 1) ARRIVAL TIME results comparable with the 5-km simulations but with signi®cantly less computational expense, and also to The time of arrival of the southerly buster is critically determine if the ``model shock'' of forcing real data to important for the issue of wind warnings. Table 3 in- idealized equations was smoothed. Only the 20-sigma- cludes the observed time of the southerly buster, com- level version was run, as the other simulations indicated pared to the times produced by the various simulations no signi®cant bene®t in the 28-vertical-level simula- for the event of 27 February 1998. tions. The 10-km resolution simulations nested in 30 km in LAPS (simulations A and B) provide a fair estimate of the arrival time of the southerly buster with better results 5. Model veri®cation from the 20 vertical levels (see simulation A). There is Direct comparison of simulated results to observa- signi®cant improvement in the results when the higher- tions is the most rigorous test to verify a model and is resolution simulations are nested in a 50-km run in used, as in RL99, to re¯ect the ``real time'' forecast LAPS. These simulations, C±G, yield predominantly ex- nature of the simulations. The HIRES simulations may cellent times of arrival for all locations except Scone. be treated as ``forecasts'' because only the operational Simulation C, with only 20 vertical levels, is marginally data available at the time and the boundary conditions better than simulation D with 28 sigma levels. The great- from the operational LAPS model are used. It is noted er resolution of the 5-km simulations indicates a further that simulations E and F would not be considered real improvement on the time of arrival when 28 vertical time, due to long CPU time, but for the purposes of this levels are used (see simulation F). Simulation G is the study they are veri®ed in the same manner to determine best for simulating the arrival time of the southerly bust- if the higher resolution yields a signi®cantly better result er, being excellent for the southern locations of Bellambi than coarser grids. As in RL99, the absolute error in the through to Williamtown and with prediction errors less units of the measured ®eld (not a scale or rms) is used of than 2.5 h in the Hunter Valley. However, at the when considering the skill of HIRES in simulating the northern locations of Taree and Coffs Harbour the ar- southerly buster. rival times can be considered as just satisfactory. Only

TABLE 3. A comparison between observations and the seven simulations for the arrival time of the southerly buster wind change. Times are UTC ϩ 1000 from the afternoon of 27 Feb to the morning of 28 Feb 1998. Place Obs A B C D E F G Bellambi 1308 1500 1600 1400 1400 1300 1300 1400 Sydney 1430 1700 1800 1500 1500 1300 1400 1500 Norah head 1616 1900 2000 1700 1800 1600 1600 1700 Williamtown 1820 2100 2200 2000 2100 2000 2000 1900 Cessnock 1708 2000 2100 1900 1900 1900 1900 1900 Scone 1844 2200 2300 2200 2200 2300 2200 2100 Taree 0500 0000 0100 0100 0100 0100 0200 0100 Coffs Harbour 1026 0500 0600 0700 0800 0800 0800 0700

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TABLE 4. As in Table 3 but with the time of the maximum wind strength, which can differ from the arrival time. Place Obs A B C D E F G Bellambi 1308 1600 1600 0100 0000 1500 1500 1400 Sydney 1515 1700 1800 0100 1700 1600 1600 1600 Norah Head 1700 2000 2100 1800 1900 1700 1800 1800 Williamtown 1930 2300 2300 0200 0800 0100 0000 0200 Cessnock 1708 0000 2300 0800 0600 0100 0100 2000 Scone 1850 0000 0000 0300 0900 0200 0800 2200 Taree 0500 0300 0300 0800 0900 0200 0500 0700 Coffs Harbour 1026 1000 1000 0800 0900 1000 1000 0900 simulation F is consistently better in this northern region of the locations, the simulated values tended to have and simulation E is still reasonable. maximum wind speeds lower than observed, which is important to note as maximum wind gusts are higher again. At the locations where the simulated maximum 2) TIME, DIRECTION, AND SPEED OF MAXIMUM speeds were higher than observations, these were still WIND STRENGTH signi®cantly lower (ഠ8msϪ1) than the observed wind The time of the simulated maximum wind speed is gusts, except at Taree where they were approximately shown in Table 4, together with the observed time of the same. Simulations D and F are considered to be the maximum wind. On several occasions, particularly at better representations of the wind speeds. Taree and Coffs Harbour, the time of observed maxi- mum wind is the same as the arrival time of the southerly buster. However, the simulation might have the maxi- b. Pressure mum wind occurring later than the initial change, which The ridge of high pressure that moves up the east would then coincide with the observed maximum wind coast of Australia and coincides with the passage of the speed and also the arrival time. Therefore, this simu- southerly buster is thought to play a signi®cant role in lation still provides valuable information about the the propagation of the southerly buster and therefore is southerly buster wind. Once again simulation G dem- the subject of debate (Holland and Leslie 1986; McBride onstrates the greatest ability in simulating the time of and McInnes 1993). As a result, the SLP trend has been the maximum wind speed, although it is late at Cessnock examined closely in this study. Figures 4 and 5 show and Scone and very late at Williamtown. It is noted that direct comparisons of the simulations with observations simulations E and F are better than A±D but still have at the speci®ed locations for the duration of the simu- late times for Williamtown, Cessnock, and Scone. lations. The time series data cover the 48-h period of Table 5 includes the wind directions at the time of the simulation completely, commencing at 2100 26 Feb- the simulated maximum wind speeds. Simulation C is ruary 1998. Figure 4 shows the more southern and coast- considered to be the closest to the observed wind di- al locations of Bellambi, Sydney, Norah Head, and Wil- rection. While all the simulations tended to have an liamtown. Figure 5 contains the trends for the locations easterly component from Bellambi through to Scone, it in and north of the Hunter Valley: Cessnock, Scone, was noted that at Taree and Coffs Harbour the simu- Taree and, Coffs Harbour. Bellambi is included for com- lations indicated a more westerly ¯ow than the obser- pletion although SLP is not recorded at this location. vations indicate. The most important point to note from Figs. 4 and 5 All the simulations are fairly similar in the value they is the phase of the SLP changes. There is signi®cant indicate for maximum wind speed. When these values model shock, clearly visible in the initial stages of the are compared with observed wind speeds rather than simulations. However, the southerly buster does not wind gusts (both are contained in Table 6), most of the reach the Hunter Valley until after the model shock has results are within Ϯ5msϪ1 of the observations. At ®ve stabilized and the simulated SLP follows the decrease to the observed SLP minima very well. It is noted that TABLE 5. At the times of Table 4 but for the direction of the wind (Њ). simulations A and B do not display the model shock of the other simulations. The times of the SLP minima are Place Obs A B C D E F G shown in Table 7, and for the seven locations the sim- Bellambi 180 174 173 183 181 174 176 164 ulated results are excellent compared to the observa- Sydney 190 170 174 176 163 168 168 165 tions, particularly for simulations C±G. The values of Norah Head 180 179 175 164 162 162 158 163 Williamtown 180 176 183 179 166 180 171 178 the minima are shown in Table 8. Cessnock 180 166 173 166 173 169 167 162 The time of arrival of the southerly buster has already Scone 160 137 142 146 144 137 137 139 been considered in section 5. This information is in- Taree 170 206 213 179 180 221 203 192 cluded in Table 7 for comparison with the time that the Coffs Harbour 160 195 195 211 206 187 194 198 SLP mimina occurred. The observations indicate that

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TABLE 6. As for Table 5 but with wind and gust (observed only) speed (m sϪ1). Place Obs A B C D E F G Bellambi 19.0 21.6 15.8 17.7 15.2 15.0 13.0 12.8 13.8 Sydney 13.9 22.6 15.8 17.4 15.5 14.3 12.7 12.7 14.3 Norah Head 20.6 25.2 14.0 15.4 14.8 14.2 12.3 12.1 14.4 Williamtown 18.0 24.7 14.5 14.2 14.7 14.1 11.2 10.7 12.6 Cessnock 8.2 21.1 13.3 14.0 15.4 14.4 10.9 10.6 12.9 Scone 16.5 20.6 10.8 10.1 11.0 13.3 9.2 11.3 9.0 Taree 7.2 10.8 11.5 11.0 10.7 10.7 9.2 9.7 10.1 Coffs Harbour 8.2 14.9 11.5 11.1 6.5 6.2 7.2 7.1 7.8 the SLP minima occur an hour or two ahead of the much later in the simulation that the magnitude has southerly buster. At Coffs Harbour there is a greater increased to a degree similar to that of the observed lead time in the SLP minima of approximately 7 h ahead initial rise. Although the magnitudes of the pressure of the southerly buster. It should be noted that for Taree rises indicated by simulations A and B are poor, the the daily SLP minima time was contained in Table 7, gradient of the SLP rise after the passage of the south- and with reference to Fig. 5 it is clearly seen that there erly buster re¯ects the observed rise more closely than is a distinct change in gradient for the SLP rise, with the other simulations at the locations south of the Hunter constant SLP for the period from 2300 27 February 1998 Valley (Fig. 4). until 0300 28 February, which is closer to the time of Simulations C±G in Table 8 simulate the local SLP the southerly buster passage. It is interesting to note that minima very well in terms of both the value and time. at Taree, simulation E (and others) did not have a steady It is noted that at the northern locations of Scone, Taree, SLP rise but actually re¯ects the observed stable SLP and Coffs Harbour the SLP trend is better represented level before rising again at 0300 28 February 1998 (refer by all the simulations. Although the magnitude of the to Fig. 5). All the simulations are consistent with the SLP rise is not as great, there is the signature of a local small lead time of the SLP minima ahead of the south- maximum in the SLP after the passage of the southerly erly buster wind change. The observed SLP trends are buster whereas the southern locations do not display a well represented by the simulations particularly to the local maximum. In considering the time and value of north of the Hunter Valley. the minima and the subsequent trend variations, partic- The absolute minima of the observed SLP at each ularly north of the Hunter Valley (Fig. 5), simulation E location was compared to the simulated SLP at each demonstrates an excellent representation of the SLP location. These results are summarized in Table 8 and trend. illustrated in Figures 4 to 5. They indicate that HIRES was able to simulate the value of the SLP minima in c. Temperature all the simulations, particularly those nested in the 50- km simulation, the difference being generally less than The along-coast temperature gradient has also been 2 hPa. suggested as having a signi®cant role in the propagation The rise in SLP after the passage of the southerly of the southerly buster (Colquhuon 1981). However, in buster at the various locations was not well simulated this study, those details are not examined closely as the using the same time frame as that of the observations. simulations did not clearly represent the presence of a The rate of the observed rise in SLP was greatest in the local at various locations, particularly Norah initial hours after the passage of the southerly buster. Head. The recorded maximum temperatures are listed While the simulations indicate a rise in SLP coinciding in Table 9. In several cases the daily maxima occurred with the passage of the southerly buster, it is not until before the arrival of the southerly buster. For those cas-

TABLE 7. A comparison of the arrival time of the southerly buster, as in Table 3, with the time of the SLP minima for observations and the seven simulations. Wind time SLP time Place Obs A B C D E F G Obs A B C D E F G Bellambi 1308 1500 1600 1400 1400 1300 1300 1400 Ð 1400 1500 1200 1100 1300 1300 1200 Sydney 1430 1700 1800 1500 1500 1300 1400 1500 1400 1600 1600 1500 1500 1400 1400 1400 Norah Head 1616 1900 2000 1700 1800 1600 1600 1700 1500 1700 1800 1600 1600 1500 1500 1600 Williamtown 1820 2100 2200 2000 2100 2000 2000 1900 1700 1900 1900 1800 1800 1800 1700 1800 Cessnock 1708 2000 2100 1900 1900 1900 1900 1900 1600 1800 1900 1600 1800 1700 1700 1600 Scone 1844 2200 2300 2200 2200 2300 2200 2100 1700 2000 2000 1800 1800 1800 1700 1700 Taree 0500 0000 0100 0100 0100 0100 0200 0100 1600 2100 2200 2300 2300 1800 2300 1800 Coffs Harbour 1026 0500 0600 0700 0800 0800 0800 0700 0330 0300 0300 0600 0600 0500 0500 0300

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TABLE 8. Comparison of observed SLP minima (hPa) with simulated SLP minima. Place Obs A B C D E F G Bellambi Ð 1013.5 1013.3 1014.2 1014.2 1015.3 1015.2 1014.1 Sydney 1014.2 1014.4 1014.3 1014.4 1014.2 1015.0 1014.7 1014.7 Norah Head 1014.5 1015.7 1015.1 1013.9 1014.1 1014.6 1014.3 1014.2 Williamtown 1014.0 1016.7 1016.2 1014.6 1014.9 1014.9 1014.9 1014.8 Cessnock 1013.2 1016.2 1015.9 1014.4 1014.6 1014.6 1014.6 1014.6 Scone 1015.7 1016.6 1016.6 1014.9 1015.5 1016.0 1015.8 1015.2 Taree 1015.1 1017.0 1016.9 1015.9 1016.2 1016.0 1016.3 1015.5 Coffs Harbour 1018.2 1017.7 1017.6 1017.3 1017.6 1017.6 1017.0 1016.9 es, the temperature at the time of the southerly buster was reasonably well simulated by simulation F,although is noted in the second line of Table 9 for that location. the simulations were 3ЊC too high. Overall simulation Table 10 contains the drop in temperature as a result of G provided the better simulation of the temperature passage of the southerly buster. The temperature ®eld trend at Norah Head. often has large errors associated with it, especially in Once again, the sea breeze was not resolved by the coastal regions where the grid box is in¯uenced by both simulations at Williamtown, although the effect of the land and sea dynamics. Also, the diurnal cycle may not sea breeze on temperatures ahead of the southerly buster be suf®cient and so the time of day of observation is was not as signi®cant as at Norah Head. However, the important in considering the accuracy of the simulation. simulated maxima at Williamtown were 5ЊC too low At Bellambi the simulations produced temperatures and after the change they were 4ЊC too high. Again, 2Њ±3ЊC lower than observed before the southerly buster simulation G was better than the other simulations. and mostly within Ϯ2ЊC of the ®nal observed temper- The observed maximum temperature at Cessnock was atures after the southerly buster. Simulation D had the 7ЊC higher than the simulations indicated. After the drop best representation at this location. in temperature when the southerly change reached Cess- The simulated temperatures were signi®cantly lower nock, the simulations were mostly within Ϯ2ЊC of the than observations of temperature at Sydney ahead of observed temperature. At Cessnock, simulation E was the southerly buster and did not re¯ect the magnitude considered to be the best simulation as simulation F did of the observed drop since the simulated temperatures not represent the ¯uctuations in temperature as well as were 3ЊC higher than observed after the southerly buster. simulation E. In Sydney, simulation C had the best representation of Farther north in the Hunter Valley at Scone, the sim- the temperature changes. ulated temperatures were 8Њ±9ЊC too low and after the The simulations at Norah Head did not resolve a sea cool change the observed temperatures were still 3ЊC breeze, which resulted in a signi®cant drop in the tem- higher than the simulations. Simulation E provided the perature from the daily maximum before the southerly best veri®cation at Scone. buster arrival. Despite this error, the daily maximum On the coast at Taree, simulation A was within 2ЊC of the observed maximum temperature. The other sim- ulations tended to be 4ЊC lower than observations with TABLE 9. The recorded temperature maxima (ЊC) for both the ob- the temperature after the southerly buster being 3ЊC low- servations and the seven simulations. In the case of the daily max- imum being greater than the temperature at the time of the southerly er than the simulations indicated. Simulation E was also buster, the temperature at the time of the southerly buster is recorded considered the better representation of the temperature in the second line for the particular location. changes at Taree. The establishment of the sea breeze at Coffs Harbour Place Obs A B C D E F G was simulated quite well with the temperature re¯ecting Bellambi 38.1 33.7 34.4 32.6 33.0 32.6 32.5 32.3 this in all simulations. Although the simulations tended 32.8 32.1 Sydney 38.1 34.0 34.4 32.2 32.2 31.9 32.3 32.1 to produce an early temperature drop associated with 35.7 33.9 31.4 31.8 Norah Head 34.4 31.4 32.0 33.0 32.9 33.0 33.0 32.8 24.8 31.9 TABLE 10. The recorded temperature drop (ЊC) associated with the Williamtown 37.0 30.3 30.2 31.7 30.8 32.0 32.1 31.6 passage of the southerly buster, both observed and simulated. 33.0 29.9 31.2 31.6 Place Obs A B C D E F G Cessnock 38.2 29.7 29.7 30.8 30.5 30.9 30.9 30.8 36.7 29.2 Bellambi 6.1 11.9 14.0 9.4 10.5 9.7 9.6 8.3 Scone 35.9 27.0 26.6 27.2 26.5 27.5 27.4 27.1 Sydney 11.2 5.3 11.1 7.8 9.1 8.0 8.1 8.3 30.1 26.1 26.2 26.3 25.5 25.7 25.0 25.8 Norah Head 2.0 8.0 8.4 9.2 9.6 8.0 7.9 9.1 Taree 33.9 31.0 29.5 29.8 29.1 29.7 29.6 29.7 Williamtown Ð 5.3 5.3 5.0 5.2 6.1 5.8 5.4 21.5 28.4 28.1 27.9 27.6 27.5 26.8 27.8 Scone 4.2 6.2 7.2 4.5 5.1 5.4 5.1 4.8 Coffs Harbour 25.2 27.3 25.8 25.1 24.8 27.5 27.4 25.1 Taree 1.6 6.0 3.5 3.3 3.9 3.0 2.1 3.1 25.6 24.9 24.3 Coffs Harbour 1.9 3.7 1.9 1.5 1.3 1.3 1.0 1.9

Unauthenticated | Downloaded 09/25/21 05:48 AM UTC AUGUST 2000 REID 439 the southerly buster, the resultant temperature was most- the southerly buster. The formation of the southerly ly within Ϯ2ЊC of the observed temperature. Simulation buster occurs in the ®rst hours of the simulation. There E, again, has a temperature trend that follows the ob- was signi®cant model shock for simulations C±G so the served trend the closest. SLP trend for this period is not appropriate in these simulations and does not accurately portray the devel- opment of the southerly buster. Figures 4 and 5 show d. Optimal simulation that simulations A and B were a closer representation Consideration of the effect of horizontal and vertical over this initial period, which corresponds to the de- resolution resulted in a total of seven simulations. The velopmental time of the southerly buster. Section 6c computational expense of numerical modeling is con- used simulation E to consider the regeneration of the siderable and the extra time involved in simply increas- southerly buster in the Hunter Valley region as this sim- ing the resolution is not always rewarded with a sig- ulation displays good timing and an excellent SLP trend ni®cant improvement in the results. For each simulation, in the region. various ®elds have been veri®ed to consider the skill of the model. 6. The reintensi®cation of the southerly buster Simulation A is the same as that used for 27 February in Reid (2000). While the results of simulation A are a. Theory good, it is noted that better results were obtained from other simulations (e.g., simulations E and G). The higher The southerly buster often develops when a cold fron- vertical resolution of simulation B did not have a sig- tal system encounters the southern parts of the Great ni®cant effect on the results. A coarser-grid nest in Dividing Range in Victoria (see Fig. 1 for location). LAPS of 50 km prior to the 10-km simulations, C and When the ¯ow approaches the barrier, poten- D, yielded results that were more accurate. tial vorticity must be conserved. This conservation is The improvement of the results from the 50-km nest- described as (Carlson 1991) ␪ ⌬␪ץ ed 10-km runs (simulations C and D) to the 50-km (␨ ϩ f ) ϭ constant ϭ (␨ ϩ f ) , (1) p ⌬pץ nested 15-km and nested 5-km runs (simulations E and F) is seen only in the ®nescale detail such as the wind strength but not the timing and is, therefore, not con- where ␨ is vertical component of relative vorticity, f is sidered to be signi®cant. Simulation G is an important the Coriolis parameter, ␪ is potential temperature, and simulation as it displayed comparable skill to simula- p is pressure. Also, ⌬p is the depth between a pair of tions E and F and involved far less computational time isentropic surfaces. (refer to Table 2). The results from the 28 sigma levels Carlson (1991) describes the air¯ow from both the (simulations B, D, F) were not necessarily an improve- west and the east over a north±south mountain barrier ment over the 20-sigma-level simulations (simulations and mentions that variations in the ¯ow would arise A, C, E), so the additional computational expense was from other mountain con®gurations. The fundamental not warranted. principle in the conservation of vorticity is described In comparison to the other simulations, simulation G by Carlson. ``The ascending (diverging) column ac- is an improvement in the arrival time and maximum quires anticyclonic vorticity and executes an anticy- wind time. The speed and direction errors for simulation clonic curve such that the trajectory forms a ridge on G are similar to those of the other simulations. The the windward side of the mountain crest.'' Continuing temperature maxima are similar to those of simulations the description of the easterly ¯ow ascending the moun- A±F with the temperature drop being slightly more ac- tain slope, Carlson stated that the anticyclonic curvature curate. In terms of temperature veri®cation, simulation that developed caused the air column to move poleward. E is better than the other simulations. With simulation As a result, the anticyclonic path continues under the G consistently better than the other simulations in the greater effect of the Coriolis force nearer to the pole overall veri®cation, it is worth bearing in mind that the and the ¯ow may fail to cross the mountain barrier, improvement in some cases is only slight. Simulation leading to the formation of an on the wind- G is the best result in terms of the SLP minima time, ward side of the mountain. Carlson cites this mechanism especially for Taree and Coffs Harbour, while simulation as a possible factor in cold air damming. C has better values for the minima. In comparing the timing of the southerly buster wind change and the SLP b. Southerly buster formation minima, there is little difference between simulations E and G. In this study the consideration of the SLP north When a south to southwesterly cold front interacts of the Hunter Valley is the focus and in this region with the Great Dividing Range, the west±east mountain simulation E has a better representation of the SLP alignment in Victoria causes upslope ¯ow. The upslope trend, particularly at Taree. ¯ow would necessarily acquire anticyclonic vorticity Simulation A has been used in section 6b, which ad- [Eq. (1)] and would continue to display an anticyclonic dresses the formation of the SLP ridge associated with trajectory due to the Coriolis effect. This would act to

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FIG. 4. SLP time series data for Bellambi, Sydney, Norah Head, and Williamtown. Compares observations (Obs) with simulations A±G. Note that at Bellambi the SLP is not recorded but is included for completion. Time is from 2100 26 Feb 1998 to 2100 28 Feb 1998 (UTC ϩ 1000). block the ¯ow over the mountain. The path executed front reached the western region of the peak of the Great by the ¯ow under the in¯uence of the Great Dividing Dividing Range in Victoria. The approximate location Range is shown schematically in Fig. 6. of the cold front is shown by the 5 m sϪ1 southerly wind A well-known feature of the southerly buster is the contour (dashed). The surge of the southerly buster has pressure ridge that extends north along the coast of already started its path up the coast by this time with southeastern Australia. In Fig. 7, the SLP contours (sol- the ridge of SLP behind it. The theory of section 6a is id) clearly show a ridge of high pressure extending applied to this case in order to determine if the pressure across Victoria ahead of the high pressure cell in the ridge of the southerly buster forms as a result of the Great Australian Bight. The time shown is 0600 27 Feb- upslope ¯ow (see Fig. 6). Considering the overall ex- ruary 1998, which is approximately 5 h after the cold cellence of simulation G, the results shown in Fig. 7

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FIG. 5. As for Fig. 4 but with Cessnock, Scone, Taree, and Coffs Harbour. are taken from the simulation in which G was nested, 5msϪ1 southerly wind contour (dashed) with the vectors that is, the 30-km nested in 50-km simulation. It should showing the wind ®eld for the lowest sigma level over be noted that 0600 27 February 1998 is after the sig- the domain. The vorticity at the lowest sigma level is ni®cant period of model shock. shaded gray when the magnitude is greater than 10Ϫ5 Simulation A was used to investigate the formation sϪ1. This indicates regions of anticyclonic vorticity of the pressure ridge in Victoria, south of the Great caused by upslope ¯ow, which create a local pressure Dividing Range. In Fig. 8, the location of the signi®cant maximum. The SLP contours (solid) show the devel- region of the mountain barrier, at which the cold front oping ridge of high pressure in the region of the anti- is blocked, is indicated by the approximately perpen- cyclonic vorticity to the south of the mountain ridge. dicular black lines. A series of four frames, each an The 1014-hPa contour is in bold to accentuate the ridge. hour apart, shows the movement of the cold front. The In accordance with the theory (section 6a), a region approximate location of the cold front is shown by the of high pressure forms on the windward side of the

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FIG. 7. For the domain of 30 km (nested in 50 km) over southeast Australia, the SLP contours (solid; 1018 hPa, bold) show a ridge (ഠ37ЊS) behind the cold front and southerly buster surge, the location Ϫ1 FIG. 6. Schematic of the air¯ow at the Great Dividing Range (shad- of which is approximated by the5ms southerly wind contour ed region) in the south and at the Hunter Valley, showing the con- (dashed). tinuation of the anticyclonic path induced by the southerly upslope ¯ows. Adapted from Carlson (1991).

the coast appears to have a sudden and marked increase. Figure 9 shows the SLP pattern (solid; 1018 hPa, bold) mountain barrier. Figure 8 shows the development of at 3-hourly intervals and the approximate location of the high pressure ridge of Fig. 7, preceding that of the the frontal line with the inclusion of the5msϪ1 south- main high pressure cell, south of the mountain barrier, erly wind contour (dashed). During the time period from resulting from upslope ¯ow. The change in the shape 1800 to 2100 27 February 1998, the advance of the SLP of the frontal line indicates that the ¯ow has been ridge appears to be faster than the frontal movement. blocked. To address both these ideas, namely, the reintensi®- cation and the accelerating SLP ridge, simulation E has c. Regeneration of southerly busters been used to look more closely at the changes in the region of the Hunter Valley. As discussed in section 5b, In Reid (2000), it was found that as the southerly the depiction of the SLP trend by simulation E was well buster progressed northward, the strength and depth of in keeping with observed trends at the locations north the surge decreased. However, in some cases, there were of the Hunter Valley (Fig. 5). While not having the same signs of regeneration of the southerly buster north of magnitude necessarily, the time of the relatively sudden the Hunter Valley. The sensitivity experiments of Reid rise in SLP was well simulated. First, to consider the (2000) indicate that, ``the southerly buster is weaker and reintensi®cation of the southerly buster as described in slower due to the natural break in the mountain barrier Reid (2000), and in conjunction with the theory outlined at the Hunter Valley and that the mountains to the north in section 6a on the initial development of the southerly of the Hunter Valley act to delay the movement farther buster in Victoria at the Great Dividing Range, Fig. 10 north with indications of reintensi®cation in the region.'' is constructed in a manner similar to Fig. 8. In Fig. 10, Another point of interest has been the observations the 1019-hPa contour is in bold due to the different of a ``jump'' in SLP along the NSW coast associated simulations yielding slightly higher pressure values than with the southerly buster. The rate of the SLP rise along simulation G, on which Fig. 9 was based.

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FIG. 8. The ridge of the Great Dividing Range (straight lines) in Victoria where the cold front is blocked. From simulation A the wind vectors, the 5 m sϪ1 southerly wind (dashed), anticyclonic vorticity greater than 10Ϫ5 sϪ1 (shaded), and the SLP (solid; 1014 hPa, bold) show the formation of a ridge due to the anticyclonic vorticity.

There is a clear break in the Great Dividing Range over a barrier (see Fig. 6), rather than just trapped east at the Hunter Valley. The northern section of the moun- of the mountain barrier along the coast as is the situation tain range lies to the northeast of the southern section farther south. (refer to Fig. 1). This is effectively due north of the As expected, there is upslope ¯ow at this point that coastally trapped surge of the southerly buster. In Fig. acquires anticyclonic vorticity in a manner similar to 10, the black square represents several peaks over 1000 the original blocking and formation of the southerly m, the highest being Mount Barrington (ഠ1550 m), to buster in Victoria. Figure 10 shows this anticyclonic the north of the Hunter Valley. As a result of this moun- vorticity (shaded) as the surge moves north. In the re- tain alignment the ¯ow is once again from the south gion of the peak north of the Hunter Valley, the upslope

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FIG. 9. The same simulation as in Fig. 7 but with the SLP pattern (solid; 1018 hPa, bold) and frontal location depicted by the 5 m sϪ1 southerly wind contour (dashed). The 3-hourly frames show the rapid progression northward of the SLP ridge. The black square shows the peak of the mountain north of the Hunter Valley.

¯ow induces a local pressure maximum. The mountains partially answered. The anticyclonic vorticity induced to the north of the Hunter Valley do act to block the by the upslope ¯ow, indicated by the shaded gray areas ¯ow in the same manner as in Victoria with the gen- of Fig. 10, is smaller than that experienced in southern eration of the southerly buster. Therefore, the reinten- Victoria although still greater than 10Ϫ5 sϪ1. It was ob- si®cation of the southerly buster in the region of the served that in the original formation of the southerly Hunter Valley can be partly attributed to the upslope buster, the local high pressure region induced by the ¯ow. upslope ¯ow led to the formation of a pressure ridge to In addition to the explanation of the strengthening of the south of the Great Dividing Range in Victoria. The the southerly buster due to this mechanism, the question local high pressure induced by the upslope ¯ow north of the pressure ridge jump along the coast may also be of the Hunter Valley would ``extend'' the pressure ridge

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FIG. 10. Similar to Fig. 8 but with simulation E and for the region of the Hunter Valley. The black square shows the location of the mountain peak north of the Hunter Valley. The 1019-hPa contour is bold. in a manner that would have the appearance of the ac- simulations have all been veri®ed in detail and indicate celeration of the SLP ridge from the south. It should be that the use of HIRES as a forecasting tool has great noted that the times shown in Fig. 10 are different from bene®t with many of the simulations predicting the pas- those in Fig. 9 due the use of a different simulation in sage of the southerly buster very well. The series of the construction of the ®gures. simulations indicates that both horizontal and vertical resolution are of great importance. The computational expense of the higher resolutions can be adequately 7. Conclusions overcome with the choice of nesting grids. This is noted This study examined the southerly buster of 27 Feb- particularly in the comparison of simulation E with sim- ruary 1998 and seven NWP model simulations. These ulation G. In this study, the additional explicit physics

Unauthenticated | Downloaded 09/25/21 05:48 AM UTC 446 WEATHER AND FORECASTING VOLUME 15 for moist processes employed for the 5-km resolution L. M. Leslie for guidance in the writing of this manu- simulations did improve the accuracy of the results but script; The Bureau of Meteorology, Australia, for ar- not signi®cantly enough to warrant the extra compu- chived data; Mr. R. Morison and Mr. N. Fraser for tech- tational expense. nical assistance; and those who have assisted in the The HIRES simulations have been employed to con- editing of this work. sider the origins of the southerly buster at the Great Dividing Range in Victoria. It was found that the pres- REFERENCES sure ridge associated with the southerly buster is the Baines, P. G., 1980: The dynamics of the southerly buster. Aust. result of the blocked cold front. This ridge arises from Meteor. Mag., 28, 175±200. the upslope ¯ow acquiring anticyclonic vorticity, which Carlson, T. N., 1991: Mid-Latitude Weather Systems. Harper Collins, blocks the air to the south of the mountain, and creates 507 pp. a local high pressure maximum behind the cold frontal Colquhoun, J. R., 1981: The origin, evolution and structure of some line. southerly bursters. Bureau of Meteorology Tech. Rep. 40, 57 pp. [Available from Australian Bureau of Meteorology, P.O. Box This same formation mechanism of the southerly 1289K, 3001, Australia.] buster has been found on a smaller scale to the north , D. J. Shepherd, C. E. Coulman, R. K. Smith, and K. McInnes, of the Hunter Valley. It is suggested that the peak of 1985: The southerly burster of southeastern Australia: An oro- the Great Dividing Range blocks the southerly ¯ow graphically forced cold front. Mon. Wea. Rev., 113, 2090±2107. Holland, G. J., and L. M. Leslie, 1986: Ducted coastal ridging over along the coast and acts to reintensify the southerly S.E. Australia. Quart. J. Roy. Meteor. Soc., 112, 731±748. buster. The mountains cause a local region of high pres- Howells, P. A. C., and Y.-H. Kuo, 1988: A numerical study of the sure to form, as a result of the upslope ¯ow, which mesoscale environment of a southerly buster event. Mon. Wea. extends the SLP ridge, thereby creating a northward Rev., 116, 1771±1788. Leslie, L. M., G. A. Mills, L. W. Logan, D. J. Gauntlett, G. A. Kelly, jump in the SLP ridge. M. J. Manton, J. L. McGregor, and J. M. Sardie, 1985: A high This secondary blocking of the air¯ow at the Hunter resolution primitive equations NWP model for operations and Valley, similar to the blocking caused by the Great Di- research. Aust. Meteor. Mag., 33, 11±35. viding Range in Victoria, is helpful in understanding Mass, C. F., and M. D. Albright, 1987: Coastal southerlies and along- the delay in the subsequent northward propagation of shore surges of the west coast of : Evidence of mesoscale topographically trapped response to synoptic forcing. the southerly buster to Coffs Harbour. The extent of the Mon. Wea. Rev., 115, 1707±1738. blocking was well simulated by HIRES. The application McBride, J. L., and K. L. McInnes, 1993: Australian southerly bust- of this information to forecasting is likely to result in ers. Part II: The dynamical structure of the orographically mod- greater accuracy in predicting the timing and intensity i®ed front. Mon. Wea. Rev., 121, 1921±1935. McInnes K. L., 1993: Australian southerly busters. Part III: The phys- of the ¯ow north of the Hunter Valley. ical mechanism and synoptic conditions contributing to devel- The effect of ocean and land temperatures on the opment. Mon. Wea. Rev., 121, 3261±3281. propagation of the southerly buster along the coast has , J. L. McBride, and L. M. Leslie, 1994: Cold fronts over south- not been addressed in this study and is dependent on eastern Australia: Their representation in an operational numer- ical weather prediction model. Wea. Forecasting, 9, 384±409. an improved representation of the sea breeze. It is Puri, K., N. E. Davidson, L. M. Leslie, and L. W. Logan, 1992: The thought that the inclusion of real-time sea surface tem- BMRC tropical limited area model. Aust. Meteor. Mag., 40, 81± peratures may enhance the development of a sea breeze, 104. rather than just the climatology currently used. The en- Qi, L., Y. Wang, and L. M. Leslie, 2000: Numerical simulation of a ergy lost from the southerly buster across the Sydney cut-off low over southern Australia. Meteor. Atmos. Phys., in press. Basin and Hunter Valley resulting in the decay of the Reid, H. J., 2000: Modeling coastally trapped wind surges over south- southerly buster also needs to be addressed in future eastern Australia. Part II: Intensity and depth. Wea. Forecasting, studies. 15, 174±191. , and L. M. Leslie, 1999: Modeling coastally trapped wind surges over southeastern Australia. Part I: Timing and speed of prop- Acknowledgments. The author wishes to thank Prof. agation. Wea. Forecasting, 14, 53±66.

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