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EARLY ONLINE RELEASE The following is a manuscript that has been peer-reviewed and accepted for publication in Atmósfera. The text will be formatted and copy-edited, and the final published version may be different from this early online release. This manuscript may be downloaded, distributed and used under the provisions of the Creative Commons Attribution Non-Commercial 4.0 International license. It may be cited using the following DOI: https://doi.org/10.20937/ATM.52840 Acceptance date: 11 July 2020 The published manuscript will replace this preliminary version at the above DOI. Atmósfera is a quarterly journal published by the Universidad Nacional Autónoma de México (UNAM) through its Centro de Ciencias de la Atmósfera in Mexico City, Mexico. ISSN 2395-8812. https://www.revistascca.unam.mx/atm
1 A rare winter thunderstorm event near Hong Kong 2 3 P.W. Chan and K.K. Hon 4 Hong Kong Observatory 5 134A Nathan Road 6 Kowloon, Hong Kong, China 7 8 Q.S. Li 9 Department of Architecture and Civil Engineering 10 City University of Hong Kong 11 Tat Chee Avenue 12 Kowloon, Hong Kong, China 13 14 Corresponding author: 15 P.W. Chan 16 Hong Kong Observatory 17 34A Nathan Road 18 Kowloon, Hong Kong, China 19 Email: pwchan@hko.gov.hk 20
21 Highlights 22 23 l A rare winter thunderstorm event is documented in this paper. 24 l The thunderstorm is well captured by weather radar and lightning location system. 25 l Origin of the occurrence of the thunderstorm is studied from dynamic and 26 thermodynamic conditions. 27 l Numerical weather prediction model has reasonable skill in capturing the thunderstorm 28 occurrence. 29 30 Graphical abstract 31
32 33 Abstract 34 35 A rare winter thunderstorm event near Hong Kong is documented in this paper. The 36 thunderstorm occurrence was confirmed by the observation of an author and the lightning 37 location information systems (both regional and global). It developed over the frontal cloud 38 band associated with the winter monsoon. Observing the upper air measurements, however, 39 the thermodynamic conditions did not seem favourable to the occurrence of the thunderstorm. 40 The lower troposphere was cool and dry in that the convective available potential energy 41 could not be determined. There was only a moister layer in the middle troposphere. The 42 weather forecaster did not anticipate the occurrence of thunder and lightning. However, 43 some features could have changed dynamically and such changes suggested a chance for 44 thunderstorm to take place. For instance, waves could be identified in the middle 45 troposphere, and a broad troughing flow could be analysed in the lower troposphere. The 46 integrated wave vapour was also of rather high level. It is hoped that the paper could serve 47 as a useful reference for forecasters in assessing the chance of thunderstorm during winter 48 over the sub-tropical latitudes in association with frontal cloud bands. 49
50 1. Introduction 51 52 Thunderstorms usually occur in Hong Kong during summer and it is rather rare during winter. 53 According to the records of weather observer at the Hong Kong Observatory headquarters, the 54 climatological number of thunderstorm days in January of each year from 1947 to 2019 was 55 0.16 (HKO, 2020). It would be useful to document the meteorological conditions when 56 thunderstorm occurs in or around Hong Kong territory in January. Such documentation aims 57 to provide useful reference to the weather forecasters in identifying the possibility of 58 thunderstorm occurrence during the season when discharging their day-to-day operational 59 forecasting duties. 60 61 A thunderstorm occurred at a distance of about 40 to 50 kilometres in southeast direction from 62 Hong Kong during the morning of 27 January 2020. As could be seen in subsequent 63 analysis, the occurrence of this thunderstorm was unexpected for one reason: it was not 64 favoured by the thermodynamic conditions of the atmosphere, namely, the various instability 65 indices of the atmosphere as given by the upper air sounding systems were not conductive for 66 thunderstorm to occur. It would be interesting to document this thunderstorm case for future 67 reference. This paper also tries to summarise the atmospheric conditions that could have 68 contributed to the occurrence of the thunderstorm. Such observation can also serve as a 69 useful reference to the weather forecasters in the future in their assessment of the chance of 70 thunderstorm near Hong Kong. 71 72 2. Data and analysis methodology 73 74 2.1 Surface-based meteorological observations 75 76 Weather radars are the major analysis tools of this paper. There are 5 microwave weather 77 surveillance radars in Hong Kong. In the present analysis, two of them would be used: the 78 long-range (512 km), S-band, dual polarization radar on the top of the mountain Tate’s Cairn 79 (about 500 m above mean sea level; Kong and Chan, 2018); and the C-band Terminal Doppler 80 Weather Radar (TDWR) at Brother’s Point (Tse et al., 2019). 81 82 The S-band radar performs a volume scan every 6 minutes with a total of 11 elevation angles 83 ranging between 0 to 34 degrees from the horizontal. Apart from the conventional
84 reflectivity and Doppler velocity, it is able to classify the nature of hydrometeors based on the 85 dual polarization parameters. The hydrometeor classification has a radial resolution of 150 86 m and an azimuthal resolution of 1 degree. For products presented in Cartesian coordinate, 87 the finest resolution would be around 150 m x 150 m per pixel. The algorithm currently in 88 use at HKO is supplied by the radar manufacturer and based on Liu and Chandrasekhar (2000) 89 with local adaptations. On the other hand, the C-band TDWR mainly focuses on the airport 90 area in Hong Kong to provide windshear alerts. On top of dedicated sector scans covering 91 the airport area with high re-visit frequency, it performs 5-degree Plan Position Indicator (PPI) 92 scan at intervals of approximately 5 minutes with a range up to about 90 km as a backup of 93 the S-band radar for long-range measurements. However, it should be noted that these PPI 94 scans are affected by beam blockage over parts of the northern and eastern quadrants due to 95 the TDWR’s low construction height (by design) as compared to scans at the mountainous 96 terrain in other parts of Hong Kong measuring up to about 900 m. 97 98 Another important tool for lightning observation is the Lightning Location Information 99 System (LLIS). The Hong Kong Observatory collaborates with the Guangdong 100 Meteorological Bureau and the Macao Meteorological and Geophysical Bureau in operating a 101 network of lightning sensors for detection over the south China coast and adjacent waters. 102 Details of the LIIS could be found in Lee (2018). Starting from late 2016, the Observatory 103 also receives near real-time global lightning location information (GLD360) from a service 104 provider. Technical details of the GLD360 data set could be found on the manufacturer’s 105 website 106 (https://www.vaisala.com/en/products/data-subscriptions-and-reports/data-sets/gld360). 107 Both networks are able to detect cloud-ground (C-G) lightning strikes and cloud-cloud (C-C) 108 lightning strikes (GLD360 data set began providing a differentiation of C-C and C-G strikes 109 since early 2019). A comparison between the detection performance of the two data sources 110 over the south China coastal areas can be found in Chiu and Lee (2019). 111 112 In order to observe the upper air atmospheric conditions, the Observatory operates a number 113 of equipment. The first being the conventional radiosonde ascent, which is released at 114 King’s Park in the city centre. The ascent is made twice a day, namely, 00 and 12 UTC 115 (Hong Kong time, HKT, = UTC + 8 hours). It measures temperature, dew point, pressure 116 and winds of the atmosphere, providing raw data at an observation frequency up to 2 Hz. 117 Remote sensing meteorological instruments are also put into operation. Among the
118 instruments is the boundary layer type, vertically pointing radar wind profiler at the city 119 centre, which measures the three components of the wind up to about 9 km above mean sea 120 level (Chan and Yeung, 2003). There is also a ground-based microwave radiometer at 121 King’s Park, which provides temperature and dew point profiles up to 10 km above mean sea 122 level (Chan, 2009). Retrieval algorithm used by the radiometer is described and detailed in 123 Chan (2010). The radiosonde ascent and microwave radiometer data such as K-index and 124 convective available potential energy (CAPE) are taken to calculate instability indices of the 125 atmosphere. These could be useful indicators for the occurrence of thunderstorms in Hong 126 Kong (Chan and Hon, 2011). 127 128 2.2 Satellite-based meteorological observations 129 130 To support daily forecasting operations, HKO receives real-time observation data from 131 Himawari-8, the next-generation meteorological satellite of Japan Meteorological Agency. 132 Ranging from a central wavelength of 0.47–13.3μm, the Advanced Himawari Imager on 133 Himawari-8 operates at 16 frequency bands and covers both the infrared and visible spectra 134 (Murata et al., 2015). HKO also operates satellite-based regional nowcasting system (HKO, 135 2019) based on a locally adapted version of the EUMETSAT Satellite Application Facility on 136 Support to Nowcasting and Very Short Range Forecasting (NWC SAF) software package 137 (Derrien and Le Gléau, 2005). Various satellite analysis and nowcasting products generate 138 real-time data including cloud type, cloud top height and derived winds. 139 140 2.3 Numerical weather prediction model 141 142 In Section 9 we will inspect the performance of the AAMC-WRF, which is a large-domain 143 implementation of the Weather Research and Forecast (WRF) model (Skamarock and Klemp, 144 2007) at 10-km resolution by HKO to support regional aviation weather prediction. 145 Configuration of the AAMC-WRF follows Hon (2020). In essence, the forecast domain 146 contains 1251 x 1101 grid points horizontally, spanning the region (20⁰S - 60⁰N, 45⁰E - 160⁰E) 147 using the Mercator projection, with 42 eta levels vertically. Version 3.9.1 of the WRF source 148 code is used. At the time of writing, the model follows a schedule of 4 runs per day 149 (initialised at 00, 06, 12 and 18 UTC) and up to T+48 hours. Boundary conditions are 150 provided by the latest available NCEP GFS output at 0.5-degree horizontal resolution. 151 Initial conditions are provided by 3DVAR analysis of surface and upper-air observations from
152 the Global Telecommunication System (GTS) based on first guess fields from the latest 153 available NCEP GFS output at 0.25-degree horizontal resolution. Key model configurations 154 are included: the WRF Double Moment 6-Class (WDM6) scheme for microphysics (Lim and 155 Hong, 2010); the RRTMG schemes for short- and long-wave radiation (Iacono et al., 2008); 156 the Noah land surface model (Niu et al 2011); the YSU planetary boundary layer scheme 157 (Shin et al., 2013); and the modified Tiedtke scheme for cumulus convection (Zhang et al., 158 2011). 159 160 To support various weather forecasting functions at HKO, forecasts fields from AAMC-WRF 161 are generated at 1-hour intervals. In addition to conventional surface and upper-air 162 parameters including sea level pressure, precipitation, winds, temperature, relative humidity 163 and more, simulated infrared radiances following Hon (2018) and simulated radar reflectivity 164 are also computed as routine products to facilitate direct visual comparison with remote 165 sensing observations by weather forecasters in an operational setting. 166 167 3. Analysis of the event 168 169 3.1 Synoptic conditions 170 171 The thunderstorm event occurred at about 09:20 a.m., 27 January 2020. Thunder was heard 172 at the location of the author (Chan) in the city centre. Meanwhile, the northeast monsoon 173 was affecting the mainland China to the northern part of South China Sea (Figure 1). 174 175 Meteorological satellite imagery showed that there were convective clouds just to the 176 southeast of Hong Kong (Figure 2(a)). The cloud top was analysed to be about 8000 m 177 above mean sea level (Figure 2(b)). Integrated water vapour reached 2-3 kg/m2 (Figure 2(c)). 178 This number was on the high side and showed that there was abundant of moisture favourable 179 to the development of convective clouds. 180 181 For the cloud drift winds, there was a trough over south China coastal area at around 800 to 182 1000 hPa layers (Figure 3(a)), namely, northerly winds over the coast and inland areas, and 183 westerly winds over the coastal waters. In the middle troposphere of 600 to 800 hPa 184 (Figure 3(b)), there was a southwesterly jet over the south China coastal waters. Short 185 waves could be found in the jet, which might as well trigger the occurrence of deep
186 convection. There may be gravity waves in the frontal clouds associated with the northeast 187 monsoon over southern China and south China coastal waters. 188 189 3.2 Local conditions 190 191 From the radarscope, intense radar echo (about 47 dBZ) with lightning strikes from LLIS 192 were observed at about 40-50 kilometres to the southeast of Hong Kong (Figure 4). Given 193 the proximity of the radar echoes and lightning strikes near Hong Kong, thunder would likely 194 be heard over the territory. This is consistent with the authors’ observations. 195 196 From the C-band weather radar (Figure 5), southwesterly flow of around 14 m/s was depicted 197 in the radar echo to the southeast of Hong Kong. There also appeared to have convergence 198 between outbound and inbound flow at the location of the stronger radar echo. This 199 mid-level convergence would be favourable to the development of the rain clouds. 200 201 From the hydrometeor classification by the S-band radar (Figure 6), graupels (yellow) were 202 found to be embedded in the extensive area of rain/wet snow/snow, which further supports the 203 occurrence of lightning over that region of radar echo. The presence of graupels would 204 likely lead to the occurrence of thunders and lightning due to their collisions. 205 206 Both cloud-to-cloud (i.e. intra-cloud) and cloud-to-ground lighting strikes were also registered 207 by the global lightning network at roughly the same location, with peak current of over 50 kA 208 (Figure 7). The observations of the global lightning network and the lightning strikes 209 registered by LLIS are generally consistent with each other. 210 211 The thermodynamic condition of the atmosphere could be revealed in the radiosonde 212 measurement at 00 UTC, 27 January 2020 (Figure 8(a)). The atmosphere was moist 213 between around 550 hPa and 700 hPa. Below that, the atmosphere was rather dry, and the 214 K-index recorded only 25 K. CAPE could not be calculated as there was strong temperature 215 inversion in the lower troposphere. 216 217 Microwave radiometer (Figure 8(b)) also showed that, at about 0120 UTC, 27 January 2020, 218 the troposphere was rather dry at the level of about 2200 m above mean sea level. The
219 CAPE was very small – at only about 20 J kg-1. And K-index was about 18 K. The 220 thermodynamic indicators in general did not favour the occurrence of deep convection. 221 222 The wind profiler (Figure 9) showed that, at around 01:20 UTC, 27 January 2020, there 223 appeared to be a westerly wave moving around that location at a height of about 2 to 224 3 kilometres above mean sea level. This wave was considered to be a major feature for 225 triggering the convection. 226 227 In summary, from both radar wind profiler and satellite winds, the dynamic conditions may 228 suggest the occurrence of deep convection, namely, troughing flow in the lower troposphere, 229 and westerly wave in the middle troposphere. 230 231 4 Performance of NWP model 232 233 We hereby inspect the AAMC-WRF model run initialised at 06 UTC, 26 January 2020, which 234 would have been available to the forecasting bench around local evening time the day before 235 the thunderstorm event took place. Figure 9(a) showed the simulated radar reflectivity in the 236 vicinity of Hong Kong at a height of 5 km. It could be seen that a wave-like echo patch, 237 with maximum intensity not exceeding 30 dBZ, was predicted to take place to the south of 238 Hong Kong. While the shape of the simulated echo patch bears reasonable resemblance to 239 the actual radar observations in terms of both shape and intensity, we can also take note of 240 some deficiencies. Firstly, the simulated echo patch did not extend far enough towards the 241 northeast and therefore could not cover the actual location where lightning was detected. 242 From here, we shall proceed to inspect its cause. Figure 9(b) showed the predicted cloud ice 243 content (mixing ratio in g kg-1) at the same altitude. The AAMC-WRF was able to produce a 244 southwest-northeast oriented band just to the south of Hong Kong, which corresponded to the 245 observed cloud band on satellite images. Hence the synoptic feature of the frontal cloud 246 band was correctly positioned. Figure 9(c) showed the predicted graupel content (mixing 247 ratio in g kg-1), also at an altitude of 5 km. It could be seen that the model produced a 248 concentrated region of graupel (peaking at about 0.1 g kg-1) which collocated with the 249 predicted radar reflectivity feature. While the occurrence of model graupel was in line with 250 the satellite-based analysis in Section 3 and was also a favourable ingredient for lightning, the 251 predicted spatial extent was too small to reproduce the full shape of the lightning-bearing 252 echoes closest to Hong Kong.
253 254 Secondly, the simulated radar reflectivity feature took place at too high an altitude. This is 255 also the reason as to why the 5-km layer was chosen in Figure 9, below which the predicted 256 reflectivity values turn out to be too low. The upper-air winds and relative humidity around 257 Hong Kong as predicted by AAMC-WRF are given in Figure 10. The top panels (left to 258 right) are 500 and 700 hPa layers. The moist patches (in shades of blue) to the south of the 259 coastline agree reasonably well with the observed cloud bands, while at 500 hPa there were 260 signs of short waves to the south of Hong Kong. However, the bottom panels (850 and 925 261 hPa; left to right) revealed that the dry continental airflow near the bottom of the troposphere 262 had penetrated too far in southward direction to the extent that the occurrence of 263 hydrometeors and hence radar reflectivity in the model were stifled. 264 265 As mentioned in Section 4, at the time of the thunderstorm event, the AAMC-WRF only made 266 use of conventional GTS observations for data assimilation. In Figures 11 and 12, we 267 present simulation results from a re-run utilising additional observation data. The re-run was 268 initialised at 00 UTC, 26 January 2020 with assimilation of Himawari-8 Atmospheric Motion 269 Vectors (AMV) on top of conventional GTS observations. Here the output at 02 UTC (i.e. 270 one hour earlier than that shown in Figures 9 and 10) are shown, which are able to better 271 match the actual timing of the thunderstorm event. By comparing Figures 9 and 11, it can 272 immediately be seen that the re-run is able to better capture the proximity of the radar echoes 273 to Hong Kong (main echo band shifted northwards from below 21.5⁰N to around 22.0⁰N) with 274 enhanced coverage of ice and graupel contents. By comparing Figures 10 and 12, we may 275 attribute the differences to enhanced moisture at 500 hPa and 700 hPa levels, especially aloft 276 Hong Kong, which provides a more favourable environment to the development and 277 sustenance of the simulated radar echoes closer to Hong Kong. Furthermore, we note from 278 Figure 13 that, in the re-run, it has become possible to reproduce simulated radar reflectivity 279 at 3-km altitude around Hong Kong. This was previously not possible in the original 280 real-time operational run (hence 5-km altitude was chosen in Figure 9). 281 282 5 Summary 283 284 A rare winter thunderstorm case in Hong Kong is documented in the paper. The convection 285 was quite intense, as shown by the peak current of the lightning strike. The occurrence of 286 thunder was also supported by weather radar observations. The thermodynamic conditions
287 of the atmosphere did not appear to support deep convection. However, troughing flow 288 could be analysed in the lower troposphere, and there appeared to be westerly waves in the 289 middle troposphere. Such dynamic conditions are favourable for the occurrence of 290 convection. From the tephigram (Figure 6(a)), the air parcel at about 1500 m may have been 291 lifted to the upper troposphere for the convection to take place, which was a kind of elevated 292 thunderstorm. In terms of mesoscale model prediction, the 10-km resolution AAMC-WRF 293 of HKO was able to give indication of a wave-like, graupel-rich radar reflectivity region to 294 the south of Hong Kong, bearing reasonable resemblance to the observed radar and satellite 295 features, since the evening before the event (i.e. lead time of 18 – 21 hours). However, the 296 predicted reflectivity region occurred too high in altitude due to deficiency of lower 297 tropospheric moisture, and did not extend far enough in northeast direction to coincide with 298 the actual lightning location due to misplacement of the high graupel content region. The 299 timing, location and vertical structure of the simulated reflectivity pattern could be improved 300 through assimilation of additional upper-air observations in the form of Himawari-8 301 Atmospheric Motion Vectors (AMV). It is hoped that this paper can serve as useful 302 document for future forecasting reference of similar rare and unseasonal convection events at 303 sub-tropical latitudes, while at the same time highlighting challenges in the analysis and 304 modeling of such events. 305 306 307 References 308 309 Chan PW. 2009. Performance and application of a multi-wavelength, ground-based 310 microwave radiometer in intense convective weather, Meteorol. Z., 18(3), 253-265. DOI: 311 https://doi.org/10.1127/0941-2948/2009/0375 312 313 Chan PW. 2010. The Use of Neural Network Retrieval for Thermodynamic Profiles of a 314 Ground-based Microwave Radiometer, The 11th Specialist Meeting on Microwave 315 Radiometry and Remote Sensing of the Environment, 1-4 March 2010, Washington, DC, USA. 316 Available at https://www.hko.gov.hk/en/publica/reprint/files/r891.pdf 317 318 Chan PW, Hon KK. Application of ground-based, multi-channel microwave radiometer in the 319 nowcasting of intense convective weather through instability indices of the atmosphere, 320 Meteorol. Z., 20, 431-440. DOI: https://doi.org/10.1127/0941-2948/2011/0276
321 322 Chan PW, Yeung KK. 2003. Experimental Extension of the Measurement Range of a 323 Boundary Layer Wind Profiler to about 9 Km, Twelfth Symposium on Meteorological 324 Observations and Instrumentation, Long Beach, California, USA, 10-13 February 2003. 325 Available at https://www.hko.gov.hk/en/publica/reprint/files/r497.pdf 326 327 Chiu YY, Lee OSM. 2019. Fine-scale Comparison of Regional and Global Lightning Location 328 Data over the Pearl River Estuary Region, 33rd Guangdong-Hong Kong-Macao 329 Meteorological Technical Seminar, 6-8 March 2019, Hong Kong. Available at 330 https://www.hko.gov.hk/en/publica/reprint/files/r1358.pdf (English abstract only). 331 332 Derrien M, Le Gléau H. 2005. MSG/SEVIRI cloud mask and type from SAFNWC. Int. J. 333 Remote Sens., 26, 4707–4732. DOI: https://doi.org/10.1080/01431160500166128. 334 335 Hon KK. 2018. Simulated satellite imagery at sub-kilometre resolution by the Hong Kong 336 Observatory. Weather, May 2018, Vol. 73, No.5, 139-144. DOI: 337 https://doi.org/10.1002/wea.3100 338 339 Hon KK. 2020. Tropical cyclone track prediction using a large-area WRF model at the Hong 340 Kong Observatory. Tropical Cyclone Research and Review, 9(1), 67-74. DOI: 341 https://doi.org/10.1016/j.tcrr.2020.03.002. 342 343 Hong Kong Observatory. 2019. World Meteorological Organisation Regional Specialised 344 Meteorological Centre for Nowcasting. Available at https://rsmc.hko.gov.hk/nowcast/ 345 346 Hong Kong Observatory. 2020. Number of days with Thunderstorm in Hong Kong since 1947. 347 Available at https://www.hko.gov.hk/en/cis/statistic/tsday_statistic.htm 348 349 Iacono MJ, Delamere JS, Mlawer EJ, Shephard MW, Clough SA, Collins WD. 2008. 350 Radiative forcing by long–lived greenhouse gases: Calculations with the AER radiative 351 transfer models. J. Geophys. Res., 113, D13103. 352 353 Kong W, Chan YW. 2018. Rapid Scan Experiment of Doppler Weather 354
355 Radar for Monitoring Rainstorms and Tropical Cyclones in Hong Kong, 10th European 356 Conference on Radar in Meteorology and Hydrology (ERAD 2018) : 1-6 July 2018, 357 Ede-Wageningen, The Netherlands. DOI: https://doi.org/10.18174/454537. 358 359 Lim K–SS, Hong S–Y. 2010. Development of an effective double–moment cloud 360 microphysics scheme with prognostic cloud condensation nuclei (CCN) for weather and 361 climate models. Mon. Wea. Rev., 138, 1587–1612. 362 363 Liu H, Chandrasekar V. 2000. Classification of hydrometeors based on polarimetric radar 364 measurements: Development of fuzzy logic and neuro-fuzzy systems, and in situ 365 verification. J. Atmos. Oceanic Technol., 17, 140–164. DOI: 366 https://doi.org/10.1175/1520-0426(2000)0172.0.CO;2. 367 368 Lee OSM. 2018. Latest Development of the Lightning Location Network over the Pearl River 369 Estuary and Data Analysis Related to Tropical Cyclone Monitoring, WMO Technical 370 Conference on Meteorological and Environmental Instruments and Methods of Observation 371 (CIMO TECO 2018), 8–11 October 2018, Amsterdam, the Netherlands. Available at 372 https://www.hko.gov.hk/en/publica/reprint/files/r1351.pdf 373 374 Murata H, Takahashi M, Kosaka Y. 2015. VIS and IR bands of Himawari‐8/AHI compatible 375 with those of MTSAT‐2/Imager, Technical Report No. 60. Meteorological Satellite Centre, 376 Japan Meteorological Agency, Tokyo, Japan. 377 378 Shin HH, Hong S, Noh Y, Dudhia J. 2013. Derivation of Turbulent Kinetic Energy from a 379 First-Order Nonlocal Planetary Boundary Layer Parameterization. J. Atmos. Sci., 70, 1795– 380 1805. DOI: https://doi.org/10.1175/JAS-D-12-0150.1 381 382 Skamarock WC, Klemp JB. 2007. A time-split nonhydrostatic atmospheric model for weather 383 research and forecasting applications. J. Comp. Phys. 227 (2008) 3465-3485 384 385 Tse SM, Hagio M, Maeda Y. 2019. Windshear detection by Terminal Doppler Weather Radar 386 during Tropical Cyclone Mujigae in 2015. Meteorol Appl. 2019; 26: 620– 631. DOI: 387 https://doi.org/10.1002/met.1789 388
389 Zhang C, Wang Y, Hamilton K. 2011. Improved Representation of Boundary Layer Clouds 390 over the Southeast Pacific in ARW-WRF Using a Modified Tiedtke Cumulus 391 Parameterization Scheme. Mon. Wea. Rev., 139, 3489–3513. DOI: 392 https://doi.org/10.1175/MWR-D-10-05091.1 393
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397 Figure 1. Surface isobaric chart during the morning (00 UTC, or 8 a.m. local time) of
398 27 January 2020.
399400 (a) 401 (b) 402 (c) 403 Figure 2. Meteorological satellite imageries: (a) the false colour image, (b) the cloud top 404 height, and (c) the integrated water vapour amount. The scales are given on the right side of 405 (b) and (c). 406
407 (a) 408 409 (b) 410 411 Figure 3. The satellite derived winds for lower troposphere (a) and middle troposphere (b). 412
413 414 Figure 4. The figure shows the radar picture (from the S-band radar) and the associated 415 lightning strikes (cloud-to-ground, with legend of the timing on the right). 416 417 418 419 420 Figure 5. Doppler velocity given by the Terminal Doppler Weather Radar at the time of the 421 thunderstorm. 422
423 424 Figure 6. Hydrometeor classification for the radar echo from the S-band radar at the time of 425 the thunderstorm. 426 427
428 429 430 431 Figure 7. Lighting locations as given by the global lightning location system. Descriptions 432 of the shape and colour of the symbols are shown on the right. 433 434
435 (a) 436 437 438 (b) 439 440 Figure 8. The tephigram at Hong Kong at 00 UTC, 27 January 2020 (a). (b) is the humidity 441 data from the microwave radiometer (colour scale) and the time series of the various 442 instability indices. Grey colour column means missing data from the wind. 443
444 445 Figure 9. Wind profiler data. The colour scale is the wind speed in knots. 446
(a)
447
(b) (c)
448
449 Figure 10. Forecast meteorological fields at 5-km altitude by the AAMC-WRF, including
450 (a) simulated radar reflectivity (domain of Figure 4 is given by a rectangle with broken line);
451 (b) cloud ice content; and (c) graupel content. These are taken from the real-time
452 operational run initialised at 26 Jan 2020, 06 UTC.
453
454455 456 457 Figure 11. Forecast upper-air winds and humidity fields by the AAMC-WRF at 500, 700 (top 458 row; left to right), 850 and 925 (bottom row; left to right) hPa isobaric levels. These are 459 taken from the real-time operational run initialised at 26 Jan 2020, 06 UTC. 460
461 462 463 464 Figure 12. Same as Figure 9 but using the output from a re-run initialised at 00 UTC, 465 26 Jan 2020 and valid at 02 UTC, 27 January 2020. 466
467 468 469 470 Fig. 13. Same as Figure 11 but using the output from a re-run initialised at 00 UTC, 26 Jan 471 2020 and valid at 02 UTC, 27 January 2020 . 472
473 474 Figure 14. Simulated radar reflectivity at 3-km altitude from a re-run initialised at 00 UTC, 475 26 Jan 2020 and valid at 02 UTC, 27 January 2020. In the real-time operational run 476 (Figure 9 and 10), no simulated reflectivity could be seen at the altitude of 3-km.
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