Wintertime Supercell Thunderstorms in a Subtropical Environment: Diagnosis and Simulation 冬季超大胞雷雨之診斷與模擬

1. Wintertime Supercell Thunderstorms in a SubtropicalEnvironment: Diagnosis and Simulation 冬季超大胞雷雨之診斷與模擬 時間：2009年 9月 22日（星期二）上午 10:00–11:00 地點：中國科學院大氣物理所（北郊祁家害豁子） 陳 泰 然 學術副校長 / 臺大講座 / 大氣科學系終身特聘教授 國立臺灣大學

2. Motivation and Purpose • Case Description • Environmental Conditions • Mesoanalyses of Storm Environment • Model Description • Model Results

3. Motivation and Purpose • (1)Motivation • Three storms formed over Fujian / Guangding, intensified into isolated supercells, splitted and propagated eastward across the Taiwan Strait, made landfall over Taiwanand produced rain, hail, and property damage in Dec. 19,2002. • Supercell thunderstorms are rare in the subtropics,not reported in open literature south of 25oN over Southeat Asia / western North Pacific. • Northern storm registered a reflectivity of 72 dBZ, the strongest ever observed by any radar in Taiwan. • A cargo plane crashed with the loss of two pilots near Penghu Islands late on 19 December as it encountered a hailstorm. • 15 min of hail up to 2 cm in diameter, caused power outage and agriculture damage U.S. $ 6 M.

4. (2) Purpose • To understand the synoptic and mesoscale environment that were favorable for the initiation and intensification of supercell thunderstorms. • Using a cloud model, with real terrain, analysis data, and no initial thermal perturbation to simulate and diagnose storm structure, Kinematics, splitting process, and the variation in the mesoscale environment.

5. 2.Case Description Fujian Lungyen Doppler Radar 0.5 base reflectivity from 1455 to 2259 LST (every 6 min): (a) 1455 LST (f) 1727 LST (k) 1957 LST • Three storms were initiated about 80 km inland around 1400 LST near the peaks of local terrain with a NE-SW alignment. • Each of the three storms had lasted for about 10 h and propagated for over 550 km. (b) 1525 LST(g) 1754 LST (l) 2027 LST (c) 1556 LST (h) 1826 LST (m) 2057 LST (d) 1626 LST (i) 1856 LST (n) 2158 LST (e) 1656 LST(j) 1926 LST (o) 2259 LST 530 km 530 km 530 km

6. Three primary storms and their tracks: • The stormson 19 December evolved into three isolatedsupercells,which exhibited storm splitting and propagated eastwardfor 550 km to make landfall over Taiwan.

7. Inflow notch Inflow notch Inflow notch Hook echo Inflow notch Inflow notch Inflow notch Pendant echo Pendant echo 104 km 104 km 104 km Characteristics of super-cellular thunderstorms: (a) 1714 LST, N1 (c) 1644 LST, C1(e) 1754 LST, S1 (b) 1914 LST, N1 (d) 1926 LST, C1 (f) 1832 LST, S1 • Radar-observed features gave some indicationthat the three primary storms were indeed supercells. • Radial velocity not available from Lungyen radar.

8. Reflectivity contours from Lungyen and CWB radars (30, 40, 50 dBZ) during 1455-2300 LST • After formation, the three storms evolved into isolated supercells and each experienced multiple splits. • The right-movers were usually stronger than left-movers and traveled eastward rapidly at about 18 m s1 across the Taiwan Strait.

9. Lightning reports (19 Dec.) Observed severe weather phenomena: (b) (a) 2100-2400 LST19Dec. 3-h total rainfall (mm) Solid dot: hail • Producd swaths of rain, hail, and property damages.

10. 3. Environmental Conditions Surface analysis 0000 UTC (0800 LST) 19 Dec.2002 1200 UTC (2000 LST) 19 Dec. • Northerly flow prevailed over much of the East Asia under the control of the Siberian high.A NE-SW-oriented cold front extended from the Japan area to southern Taiwan. • The severe thunderstorms on 19 December developed around 1400 LST over southern Fujian / eastern Guangdong, behind the surface cold front in a post-frontal environment

11. (a) 925 hPa at 0000 UTC 19 Dec. 850 hPaat 0000 UTC 19 Dec. 30N 20N 925 hPa 0800 LST 19 Dec 2002 110E 120E (b) 30N • As 850 hPa front was farther north of 925 hPa front by 100-200 km, the surface-based postfrontal cold air over the area of storm development was rather shallow and confined to below 925 hPa in the morning of 19 December 2002. 20N 850 hPa 0800 LST 19 Dec 2002 110E 120E

12. 700 hPa at 0000 UTC 19 Dec. 200 hPa at 0000 UTC 19 Dec. • Strong vertical shear (700 hPa LLJ near Fujian and Taiwan) associated with the cold front and instability. • An approaching ULJ at 200 hPa also provided strong shear through deep layers, a factor beneficial to the longevity of supercell storms .

13. Low-level bulk shear = 6.4  103 s1 Shantou sounding at 0800 LST 19 Dec. • Favorable Environment : • The supercells occurred behind a winter cold front which provided a large west-southwesterly vertical wind shear of 6.4  103 s1 at 0-3 km. • Weak-to-moderate instability (CAPE = 887 J kg1) above the shallow surface cold air.

14. Summary • The supercells occurred over the shallow cold air behind a winter cold front, with a large west-southwesterly vertical wind shear of 6.4  103 s1 at 0-3 km.This combined with weak-to-moderate instability (CAPE = 887 J kg1) above the shallow surface cold air to yield a favorable environment for supercells.

15. 4.Mesoanalyses of Storm Environment Manual mesoscale surface analyses GMS-5 IR cloud imagery (c) (a) (a) 0800 LST, 19 Dec 1400 LST, 19 Dec    (a) 0800 LST 19 Dec. 2002 30N 110E (b) 1100 LST, 19 Dec (b) (d) 1700 LST, 19 Dec (d) 20N 130E 120E (b) 1400 LST 19 Dec. 2002 30N 110E • Prior to storm initiation, significant daytime solar heating under cloud-free skies occurred over the mountain slopes over the area of storm initiation. • Daytime upslope winds on both sides of the mountain were observed. 20N 130E 120E

16. A    B • T exceeded 20 °C over mountains while induced upslope winds were about 3-5 m s-1. • Three storms all initialed close to the peaks of local topography.

17. (a) (b) (d) (c) • Significant daytime solar heating occurred over the mountain slopes along the coast of southeastern China, leading to development of local circulation and onshore / upslope winds, resulting in convergence and uplifting.

18. (a) 0534 LST, 19 Dec 2002 25N 20N 115E120E (b) 1825 LST, 19 Dec 2002 25N 20N QuikSCAT Oceanic Winds • Over the Strait, low-level shear intensified during the daytime of 19 Dec. due to cold air surge. • Increase in low-level shear produced conditions even more prone to supercell development, three storms became more isolated and reached peak intensity after 1730 LST. 115E120E

19. Summary • Significant daytime solar heating occurred over the mountain slopes along the coast of southeastern China, leading to development of local circulation and onshore / upslope winds, resulting in convergence and uplifting. • The storms reached their maximum strength over the Strait where low-level shear intensified during the day due to cold air surge.

20. 5. Model Description • The CReSS (Cloud Resolving Storm Simulator)model developed at the Hydrospheric Atmospheric Research Center of NU, Japan (Tsuboki and Sakakibara 2002, 2007). • This model used in this study (v.2.2) is a nonhydrostatic,fully compressible, cloud-resolving model. • This model employs a terrain-following verticalcoordinate . • An explicit bulk cold rain scheme are used without any cumulus parameterization. • Two experiments were performed:

21. Summary of CReSS-model configuration.

22. 6. Model Results Tracks of major storms simulated by CReSS: Observation: 1.5Km 0.5Km • CReSS successfully reproduced the three major storms at the correct time and location, but the southern storm decayed too early over the Taiwan Strait. In both runs, model storms travel about 15-20 to the left of the actual storms.

23. Comparison between observed and modeled storms: Storm Time of Life Distance Mean direction Size Split cell initiation span traveled and speed*40 dBZ (LST) (h) (km) (degree/m s1)(km) Observation N1 by 1425 8.6 570 260/19.615-35yes C1 by 1425 8.7 518 268/17.815-35 yes S1 by 1455 8.6 507 271/17.415-40 yes N2 1525 4.3 247 215/19.2<20 yes Others ~2-6~150-400~243/~17-1915 Model n1 1200 11.0 633243/16.015 yes Run 2 c1 1145 12.3692248/15.710-20 yes (0.5 km) s1 1245 7.0 278255/11.020 yes h1 1600 9.0391259/12.110-25 yes h2 1830 4.8237232/13.920 yes i1 1630 8.5575246/18.812 yes I217306.0 377 232/17.510-20 yes Model n1 1330 10.5 735248/18.315yes Run 1 c1 1400 6.0 313247/13.115yes (1.5 km) s1 1400 5.8324 250/17.610-20yes s2 1530 9.3 623227/18.910-25yes h1 1500 9.0 410258/15.410-15no * The 500-700 hPa environmental wind was from 243 at 18.5 m s1 (WEA). • The two experiments produced similar overall results, suggesting that the 1.5-km grid spacing is sufficient even for storm dynamics.

24. Vorticity budget analysis (1) (2) (3)

25. Storm c1 at 3984 m at 1630 LST: (a) (d) x x x       x x x    x x x    (b) (e) (c) (f) • The vorticity budget analysis indicates that mid-level updraft rotation arose mainly from the tilting effect, and was reinforced by vertical stretching at the supercell stage. • Vertical advection tendsto cancel with stretching. • Total tendency is mostly positive at updraft centersand thus tends to enhance existing .

26. (a)  • Solid dots mark 3 primary storms at 1830 LST. • Letter A~E indicate the locations of model sounding used. • Only subtle differences exist between observations and run1 results.   (b) A B   C D  E

27. Thermodynamic and shear parameters at points A~E in run1 during 1100 LST 19 to 0200 LST 20 Dec. 2002                                                                         PD PB PC PA DD DD PC PD PB PA PC DD PA PD PB DD PD PA PC PB DD PC DD PD PD PA PC PB PA PB (a) (d) (e) (b) (f) (c) • In addition to the common ingredients of sufficient shear and instability, the evolution of model storms depends heavily on the detailed low-level vertical structure of storm environment, which varies horizontally and is linked to the rapid evolution of surface cold air.

28. Summary • Better understand the storm structure, kinematics, dynamics and splitting process. • Better understand the storm evolution in relation to the variation in mesoscale environment.