Weather Observations by Aircraft Reconnaissance Inside Severe Typhoon Utor

Weather observations by aircraft reconnaissance inside Severe Typhoon Utor

P. W. Chan, W. K. Wong On 13 August 2013, Severe Typhoon Utor steering of the typhoon within the large- Weather – August 2014, Vol. 69, No. 8 moved into the northern part of the South scale atmospheric flow. Consideration was and K. K. Hon China Sea and was expected to bring about also given to the time limit of the flight Hong Kong Observatory, China gale-force or stronger winds to various parts because of the fuel capacity of the aircraft of Hong Kong, according to its forecast (giving a flying time of up to about 5h). The structure and track. In order to assess the flight route is shown in Figure 1. Introduction potential impact of Severe Typhoon Utor After considering the various flying con- Since 2009, the Hong Kong Observatory on Hong Kong, a tropical cyclone recon- straints, flying in the mid-tropospheric (HKO) has been collaborating with the naissance flight was conducted in the steering flow region was conducted at Government Flying Service (GFS) of the afternoon (local time) of the day. Compared FL080, which included flying through Hong Kong Special Administration Region with similar flights in the past, some special the centre of the severe typhoon. On the Government to equip a fixed-wing aircraft, conditions applied to this particular flight, return leg, a longer flight route was taken namely a BAe Jetstream 4100, with a dedi- including: within the atmospheric boundary layer at FL020 in order to observe the wind dis- cated meteorological measuring system. (i) a flight into a severe typhoon, which tribution upstream of Hong Kong, which This system consists of a probe set up was among the strongest experienced under the wing, two global positioning by the aircraft reconnaissance flights system (GPS) antennae and other compo- conducted up to then over the northern nents stored inside the cabin (including the part of the South China Sea (no such navigation unit, signal processor and data flight into a severe typhoon had been storage). The system is capable of provid- conducted since collaboration began ing accurate meteorological measurements, with the GFS in 2009); including the three components of the (ii) a relatively long flight route at lower wind, temperature, relative humidity (RH) levels within the atmospheric boundary and pressure at a frequency as high as 20Hz. layer (FL020, i.e. flight level at a height of Details of the system have been reported by 2000ft) in order to determine the gale- Chan et al. (2011). force wind radius of the typhoon, which The meteorological measuring system is is important for assessing the chance of used primarily for collecting wind-shear and occurrence of gale-force winds in Hong turbulence data in the vicinity of Hong Kong Kong. International Airport. Starting from 2011, the Jetstream 4100 has also been used to An initial study of the meteorological fly into tropical cyclones over the north- data collected in this flight is presented ern part of the South China Sea in order in this paper. Moreover, a HKO colleague to collect valuable data in support of the photographed the clouds and the sea con- typhoon warning service in Hong Kong. In ditions at different locations in this severe particular, the aircraft data may be used to typhoon during the flight, and these valua- locate the centre as well as the high-wind ble weather photographs are also presented radii of the typhoon, such as the storm-force in the paper. wind radius (the radius from the typhoon’s eye in which storm-force winds occur) and the gale-force wind radius (the radius from Flight route the typhoon’s eye in which gale-force winds Before the flight was conducted, the flight occur), in order to assess the chance of the route was determined considering the air occurrence of high winds over the South traffic (restrictions on the flight path due to China coastal areas. In the past 3 years, a commercial jets) and the usefulness of the total of 11 missions have been flown, and meteorological data (which was mainly con- the meteorological data collected have sidered with respect to the available infor- been found to have a positive impact on the mation about the centre location and the analysis and forecasting of tropical cyclones high wind radii of the typhoon). As in previ- Figure 1. The upper panel shows the flight in the region; an example is the assimila- ous flights, data collection was at two levels, route of the aircraft with the wind fleches tion of flight data into a numerical weather in view of the need to collect data within the overlaid on a false-colour satellite image at prediction (NWP) model (see Wong et al., atmospheric boundary layer of the typhoon 1030 UTC. The lower panel shows the heights 2013, for details). as well as at a suitable level related to the of the aircraft between points A and C. 199 (a) measured onboard (the location of the aircraft at times 1015 and 1030 UTC is indicated in Figure 1); (ii) there is a number of spikes in the wind speed, for instance, between 1015 and 1030 UTC, between 1100 and 1115 UTC and between 1130 and 1145 UTC (locations at these times shown in Figure 1); during these intervals, the aircraft was crossing areas of convection, as shown on the satellite image in Figure 1. The 10m winds were estimated to be in the Aircraft reconnaissance inside Severe Typhoon Utor Typhoon Aircraft reconnaissance inside Severe (b) order of 35–40ms−1, which is lower than the estimated strength of the typhoon of 48.9ms−1 (95kn) around that time based on estimation using the Dvorak technique (Velden et al., 2006). The Dvorak technique estimates the intensity of the tropical cyclone by inspecting the pat- tern of clouds in the satellite images and making comparison with patterns from historical cyclones.

Weather – August 2014, Vol. 69, No. 8 69, No. Vol. – August 2014, Weather (MSLP) as determined from the station-level pressure and the altitude of the aircraft is presented in Figure 2(c). The MSLP is estimated based on the formula in the Handbook of Meteorological Instruments (Met Office, 1980), that is, the addition of the station-level pressure and a correction of pressure (which is related to the station-level pressure, height of the station above mean sea level as well as the station-level temperature). (d) There are two local minima in the MSLP time series, namely, between 1000 and 1015 UTC and between 1030 and 1045 UTC. The estimated MSLP has the lowest value of about 973.7hPa, which is higher than the estimated minimum pressure of 941hPa based on satellite analysis. The location of minimum MSLP is roughly consistent with the centre of the severe typhoon as determined from the satellite image (Figure 3). The time series of vertical velocity is provided in Figure 2(d). The correlation of the measured vertical velocity and the struc- Figure 2. Time series (UTC) of the data collected by the aircraft. (a) Red curve is the wind direction ture of the severe typhoon requires further and black curve is the wind speed at the flight level. (b) Red curve is the wind direction at the study, but preliminary investigation shows flight level and green curve is the wind speed reduced to a height of 10m above mean sea level. (c) that the vertical velocity exceeds 10ms−1 in Purple curve is the calculated mean sea-level pressure at the location of the aircraft (as calculated from the station-level pressure, height and station-level temperature of the aircraft). (d) Blue curve magnitude at times. The maximum upward –1 is the vertical wind velocity: positive means upwards and negative means downwards. velocity is about 12ms at about 1030 UTC, the largest downward velocity occurring at about 1108 UTC, reaching slightly more –1 also included flying through the centre of shown in Figure 2(a) and (b). The correc- than –15ms the typhoon: in fact, once the centre of tion of the wind speed to 10m is made fol- the typhoon was located, the aircraft flew lowing the method described in Chan et al. Determination of the position around the centre twice in order to collect (2011), namely, assuming a power law of the of the typhoon centre the valuable meteorological data in this wind speed as a function of height over the The flight route together with the wind region. lower troposphere and using a representa- data symbols are overlaid on the satel- tive power exponent for open oceans. There lite imagery in Figure 1. The eye of Severe are two points worth noting: Time series of meteorological Typhoon Utor is readily observed in the (i) in flying around the centre of the severe figure, with winds of opposing directions measurements typhoon between 1020 and 1030 UTC of surrounding the eye (the occurrence of The time series of wind direction and wind 13 August 2013, there were significant easterly winds first followed by westerly speed at the flight level and reduced to a changes in wind direction from north- winds over a short distance, as indicated 200 height of 10m above mean sea level are westerly to, eventually, southeasterly as at 1030 UTC in Figure 1). Aircraft reconnaissance inside Severe Typhoon Utor

However, another location with opposing wind directions is also observed about 50km southwest of the eye of Severe Typhoon Utor. The wind data have been double-checked by the manufacturer of the onboard meteorological measuring system based on the raw measurements from the system and they are found to be in order. Thus, this ‘secondary circulation centre’ is considered to be genuine, and it coincides with the edge of an outer rainband of Severe Typhoon Utor where overshooting cloud tops are observed from the satellite imagery (Figure 1). The existence of this secondary centre will be studied in detail and Weather – August 2014, Vol. 69, No. 8 reported in a future publication, for example, through numerical simulation of Severe Typhoon Utor with the assimilation of the aircraft reconnaissance data. Secondary circulation is not a common feature and, to the knowledge of the authors, there is no similar report in the existing literature.

Determination of gale-force wind radius Figure 3. Flight route for levels below 2000m with wind fleches. The estimated gale-force wind The gale-force wind radius of the severe radius of Severe Typhoon Utor as obtained from the flight data is shown. The figure also shows typhoon is estimated based on the esti- the locations of the minimum estimated MSLP (‘x’) and the maximum estimated 10m wind speed mated 10m winds from the aircraft meas- (circle). The satellite-determined location of the typhoon centre is shown as ‘+’. urements. It refers to the distance (radius) from the centre of the tropical cyclone where winds of gale force or above occur. An indication wind strength along the flight route is given in Figure 3. The gale- force wind radius is estimated to be about 290km from the available measurements. The present analysis shows that the relatively longer flight span at FL020 was use- ful for analysing the high-wind radii of the severe typhoon. In order to check the accuracy of the estimated 10m wind speed from the aircraft data, comparison was made with the available wind-speed measurements from oil rigs and weather buoys in the region. For instance, the aircraft-estimated wind speed at 10m above mean sea level is found to Figure 4. The wind distribution of Severe Typhoon Utor as estimated from multiplatform satellites: be about 40kn from the east at about 20°N (a) 0600 UTC, 13 August 2013; (b) 1200 UTC, 13 August 2013. The colours refer to wind speeds in 114.5°E, which is consistent with the wind knots: strong wind in green, gale-force wind in yellow, storm-force wind in red. The contours refer speed measured at an oil rig (35kn from the to wind speeds in excess of 20, 35, 50, 65 and 80kn. The gale-force wind radius is given by the east) located at about 50km away. contour with the label 35. Its value was estimated from the latitude/longitude grids (each degree As a further check of the accuracy of is about 111km). the wind radii, the multiplatform satellite wind analysis (available at http://rammb. cira.colostate.edu/products/tc_realtime/) was used for comparison. From this analysis (Figure 4), the gale-force wind radius is estimated to be 290km at 0600 UTC and 350km at 1200 UTC, which are, in general, consistent with the estimation based on the aircraft data.

Turbulence intensity measurements Figure 5. Time (UTC) series of the derived cube root of eddy dissipation rate based on the flight data. The yellow line is the limit of moderate turbulence and the red line is the limit of severe Following the International Civil Aviation turbulence. Organization (2010), turbulence intensity 201 Aircraft reconnaissance inside Severe Typhoon Utor Typhoon Aircraft reconnaissance inside Severe

Figure 6. Photograph taken at the outer rainband of Severe Typhoon Utor during the flight at the location indicated by the arrow. Weather – August 2014, Vol. 69, No. 8 69, No. Vol. – August 2014, Weather

Figure 7. Similar to Figure 6 but at a location close to the eye of Severe Typhoon Utor.

is expressed in terms of the cube root of occurs in the region of an outer rainband near the centre of the severe typhoon, the eddy dissipation rate (EDR) of turbu- associated with the typhoon (between where the sea was rather calm. Compared lent kinetic energy. This is calculated from 1100 and 1115 UTC). with Figure 6, there is a broad area of clouds the power spectrum of the vertical velocity that might be associated with the eye wall as measured by the aircraft, by searching Photographs taken inside of the typhoon. Similar observations were for the inertial range (where the power made at lower altitudes of the flight (not spectrum follows the slope of −5/3). The the typhoon shown). time series of the cube root of the EDR Photographs were taken throughout the is shown in Figure 5. It is calculated from flight and some of those of interest are pre- vertical velocity following the method of sentated in this paper. Figure 6 shows the Conclusion Haverdings and Chan (2010). In general, cloud and sea condition near the outer rain- Investigation flights have been conducted the air was rather turbulent inside the band of the severe typhoon, with isolated for tropical cyclones over the northern severe typhoon, but moderate turbulence patches of cumulus and white wave heads part of the South China Sea since 2011. (EDR1/3 of 0.3m2/3s−1) and severe turbu- over the sea being observed. In general, the This paper reports the first observations, in lence (EDR1/3 of 0.5m2/3s−1) were observed seas were rougher outside the centre of the 2013, of a severe typhoon within the flight at a number of locations along the flight typhoon, especially in those regions with campaign for this region, especially infor- 202 route. In particular, the severe turbulence stronger winds. Figure 7 shows conditions mation from near the eye of this typhoon, Aircraft reconnaissance inside Severe Typhoon Utor using the data collected by the onboard References 30 years. Bull. Am. Meteorol. Soc. 87: meteorological measuring system and by 1195–1210. Chan PW, Hon KK, Foster S. 2011. Wind human observations (supported by a pho- Wong WK, Tse SM, Chan PW. 2013. data collected by a fixed-wing aircraft in tographic record). Two centres are indicated Impacts of reconnaissance flight data the vicinity of a tropical cyclone over the for the severe typhoon at the time of the on numerical simulation of tropical south China coastal waters. Meteorol. Z. cyclones over South China Sea. Meteorol. observation. The estimated maximum 10m 20: 313–321. Appl. doi: 10.1002/met.1412 wind speed is generally consistent with the Haverdings H, Chan PW. 2010. Quick satellite estimation, although the estimated Access Recorder (QAR) data analysis minimum MSLP appears to be higher. software for windshear and turbulence Correspondence to: P. W. Chan A special flight route was devised to provide studies. J. Aircr. 47: 1443–1447. an estimate of the gale-force wind radius International Civil Aviatio n [email protected] of the severe typhoon, and this estimate Organization. 2010. Meteorological © 2014 Hong Kong Observatory. Weather Service for International Air Navigation, published by John Wiley & Sons Ltd on behalf is consistent with satellite estimations. Annex 3 to the Convention on International Photographs were taken at the location of Civil Aviation, 206 pp. of Royal Meteorological Society. Weather – August 2014, Vol. 69, No. 8 an outer rainband and near the typhoon Met Office. 1980. Handbook of This is an open access article under the centre. The meteorological observations Meteorological Instruments, Volume 1 terms of the Creative Commons Attribution- from this investigation are valuable and will –Measurement of Atmospheric Pressure, NonCommercial-NoDerivs License, which be used for further study of the structure of Second Edition. HMSO: London. permits use and distribution in any medium, the severe typhoon, for example through Velden C, Harper B, Wells F et al. 2006. provided the original work is properly The Dvorak tropical cyclone intensity cited, the use is non-commercial and no numerical simulation by assimilating the estimation technique, a satellite-based flight data, with the results being reported method that has endured for over modifications or adaptations are made. in a future publication. doi:10.1002/wea.2315

UK Citizen Rainfall Network: a pilot study

Samuel Michael profitable cases by a variety of industries, 4000 registered open rain gauges, which 1 its vast potential for scientific research has can provide hourly, daily and monthly Illingworth, Catherine also recently begun to be exploited. From measurements, thus making the British rain- Louise Muller,2 Rosemarie a scientific perspective, crowd-sourcing fall network one of the densest in the world. 3 2 projects are often referred to as citizen However, even with this extensive network Graves and Lee Chapman science, which is defined by Wiggins and there are still many areas across the UK that 1Centre for Atmospheric Science, Crowston (2011) as a form of research col- do not provide measurements of rainfall. School of Earth, Atmospheric and laboration involving members of the public There is also a historical tendency for the Environmental Sciences, University in scientific research projects to address real- rainfall measurements to be somewhat of Manchester world problems. intermittent: 12 100 rainfall-monitoring sta- 2School of Geography, Earth and With its history of public involvement, tions were reported at some point between Environmental Sciences, University weather monitoring lends itself readily to 1961 and 2000, with only around 40% of of Birmingham citizen-science-style activities, with recent these stations found to be active at any one examples including: real-time tempera- time (Hollis and Perry, 2004). 3Earth Observation Science, Physics ture monitoring using smartphone battery Citizen science provides a feasible and and Astronomy, University of Leicester temperatures (Overeem et al., 2013), the low-cost solution to increasing the number use of social media broadcasts to obtain of British rainfall-monitoring stations, with Introduction accurate snowfall data (e.g. http://snow- the potential coverage of these measure- Crowd sourcing as a potential method to core.uwaterloo.ca/snowtweets/index.html; ments extended to cover all areas of settle- collect large amounts of data in a relatively Muller, 2013), smartphone weather apps ment across the British Isles. Such a voluntary inexpensive manner while also educating (e.g. MetWit: https://metwit.com/; Weather rainfall network already exists in coun- the general public, has received much press Signal: http://weathersignal.com/) and the tries outside the UK, with the Community recently in both the scientific and public uploading of amateur weather station data Collaborative Rain, Hail and Snow Network domain. It was first defined by Jeff Howe (e.g. Met Office WOW: http://wow.metoffice. (CoCoRaHS) in the USA being a particularly (Brabham, 2008, and references therein) as gov.uk/). Such activities also build on the effective example (Cifelli et al., 2005; http:// the act of a company or institution taking a number of urban meteorological networks, www.cocorahs.org/). function once performed by employees and with a range of research or operational The CoCoRaHS is a unique, non-profit, outsourcing it to an undefined (and gener- objectives, that now exist within the UK community-based network of volunteers ally large) network of people in the form of (Muller et al., 2013a). who work together to measure and map an open call and, although this method has Currently, there are over 200 automatic precipitation, using low-cost measurement been used to great effect in a number of weather stations across the UK, as well as tools and utilising an interactive website to 203