Wintertime Supercell Thunderstorms

in a Subtropical Environment: Diagnosis and Simulation

冬季超大胞雷雨之診斷與模擬時間：時間： 20102010 年 年 66 月 月 1717 日（星期四）下午 日（星期四）下午 02:0002:00 ––

04:3004:30地點：南京信息工程大學地點：南京信息工程大學陳 泰 然陳 泰 然

學術副校長 學術副校長 // 臺大講座 臺大講座 // 大氣科學系終身特聘大氣科學系終身特聘教授教授

國立臺灣大學國立臺灣大學

1. Motivation and Purpose

2. Case Description

3. Environmental Conditions

4. Mesoanalyses of Storm Environment

5. Model Description

6. Model Results

(1) Motivation Three storms formed over Fujian / Guangding, intensified

into isolated supercells, splitted and propagated eastward across the Taiwan Strait, made landfall over Taiwan and 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.

1. Motivation and Purpose

(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.

Fujian Lungyen Doppler Radar 0.5 base reflectivity from 1455 to 2259 LST (every 6 min):

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.

2. Case Description

(a) 1455 LST (f) 1727 LST (k) 1957 LST

(b) 1525 LST (g) 1754 LST (l) 2027 LST

(e) 1656 LST (j) 1926 LST (o) 2259 LST

(c) 1556 LST (h) 1826 LST (m) 2057 LST

(d) 1626 LST (i) 1856 LST (n) 2158 LST

530 km 530 km 530 km

Three primary storms and their tracks:

The storms on 19 December evolved into three isolated supercells, which exhibited storm splitting and propagated eastward for 550 km to make landfall over Taiwan.

Characteristics of super-cellular thunderstorms:

Radar-observed features gave some indication that the three primary storms were indeed supercells.

Radial velocity not available from Lungyen radar.

104 km 104 km 104 km

Inflow notch

Inflow notch Pendantecho

Inflow notch

Inflow notch

Pendant echo

Inflow notch

Hook echo

Inflow notch

(a) 1714 LST, N1 (c) 1644 LST, C1 (e) 1754 LST, S1

(b) 1914 LST, N1 (d) 1926 LST, C1 (f) 1832 LST, S1

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 s1 across the Taiwan Strait.

(a) (b)

Lightning reports (19 Dec.)2100-2400 LST 19 Dec. 3-h total rainfall (mm)

Observed severe weather phenomena:

Producd swaths of rain, hail, and property damages.

Solid dot: hail

Surface analysis1200 UTC (2000 LST) 19 Dec.

3. Environmental Conditions

0000 UTC (0800 LST) 19 Dec. 2002

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

925 hPa at 0000 UTC 19 Dec. 850 hPa at 0000 UTC 19 Dec.(a)

110E 120E

20N

30N

925 hPa0800 LST

19 Dec 2002

(b)

110E 120E

20N

30N

850 hPa0800 LST

19 Dec 2002

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.

700 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 .

200 hPa at 0000 UTC 19 Dec.

Shantou sounding at 0800 LST 19 Dec.

Low-level bulk shear = 6.4 103 s1

Favorable Environment : The supercells occurred behind a winter cold front which provided a large west-southwesterly vertical wind shear of 6.4 103 s1 at 0-3 km.

Weak-to-moderate instability (CAPE = 887 J kg1) above the shallow surface cold air.

The supercells occurred over the shallow cold air

behind a winter cold front, with a large west-

southwesterly vertical wind shear of 6.4 103 s1 at

0-3 km. This combined with weak-to-moderate

instability (CAPE = 887 J kg1) above the shallow

surface cold air to yield a favorable environment for

supercells.

Summary

(a)

(b)

(a) 0800 LST, 19 Dec

(b) 1100 LST, 19 Dec

(d)

(d) 1700 LST, 19 Dec

(c)

1400 LST, 19 Dec

(a) 0800 LST 19 Dec. 2002

110E

120E 130E

20N

30N

(b) 1400 LST 19 Dec. 2002

110E

120E 130E

20N

30N

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.

4. Mesoanalyses of Storm EnvironmentGMS-5 IR cloud imagery

Manual mesoscale surface analyses

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.

(a)

(b)

(c)

(d)

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.

115E 120E

20N

25N

0534 LST, 19 Dec 2002

20N

25N

1825 LST, 19 Dec 2002

(a)

(b)

115E 120E

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.

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.

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 vertical coordinate .

An explicit bulk cold rain scheme are used without any cumulus parameterization.

Two experiments were performed:

Grid size

Vertical levels

Initial timeIntegration

lengthRemarks

Run1 1.5 km 650800 LST 19

Dec. 200224h

JMA regional analyses as IC / LBCS

Run2 0.5 km 631100 LST 19

Dec. 200214h

Run1 as IC / LBCS without nudging

Run 1 Run 2Domain and basic setupProjection Lambert Conformal, center at 118E, secant at 20N and 50NGrid size (km) 1.5 1.5 100-475 m 0.5 0.5 100-475 mGrid number 648 450 65 1584 900 63Domain size (km) 972 675 20.8 792 450 19.85Topography and SST Real at 0.00833, and observed at 1 resolutionInitial and lateral boundary

conditions (IC/LBCs)JMA regional analyses

(20 20 km, 20 levels, 6 h)CReSS Run 1 outputs

(30 min)Initial thermal perturbation NoneInitial time 0800 LST 1100 LST Integration length 24 h 14 hOutput frequency 15 min 15 or 5 minModel physicsAdvection, diffusion 4th-order in H/V, 4th-order in H/VCloud microphysics Bulk cold rain scheme (6 species)Cumulus parameterization NonePBL parameterization 1.5-order closure with TKE predictionSurface processes Energy and momentum fluxes, and shortwave and longwave radiation

Soil model 41 levels, every 5 cm to 2 m deepNumerical methodsTime steps (t, ) 2 s, 1 s 1.5 s, 0.5 sIntegration method Filtered leapfrog for t (HE-VE), and leapfrog and Crank-Nicolson for

(HE-VI)Number of PEs 18 108

Summary of CReSS-model configuration.

Tracks of major storms simulated by CReSS:

1.5 Km

0.5 Km

Observation:6. Model Results

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.

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 s1) (km)

Model n1 1200 11.0 633 243/16.0 15 yesRun 2 c1 1145 12.3 692 248/15.7 10-20 yes(0.5 km) s1 1245 7.0 278 255/11.0 20 yes

h1 1600 9.0 391 259/12.1 10-25 yes h2 1830 4.8 237 232/13.9 20 yes i1 1630 8.5 575 246/18.8 12 yes

Model n1 1330 10.5 735 248/18.3 15 yesRun 1 c1 1400 6.0 313 247/13.1 15 yes(1.5 km) s1 1400 5.8 324 250/17.6 10-20 yes

s2 1530 9.3 623 227/18.9 10-25 yes h1 1500 9.0 410 258/15.4 10-15 no

* The 500-700 hPa environmental wind was from 243 at 18.5 m s1 (WEA).

Observation N1 by 1425 8.6 570 260/19.6 15-35 yes

C1 by 1425 8.7 518 268/17.8 15-35 yes

S1 by 1455 8.6 507 271/17.4 15-40 yes

N2 1525 4.3 247 215/19.2 <20 yesOthers ~2-6 ~150-400 ~243/~17-19 15

I2 1730 6.0 377 232/17.5 10-20 yes

The two experiments produced similar overall results, suggesting that the 1.5-km grid spacing is sufficient even for storm dynamics.

TppR

(Bk)

F(Bk)V)(ωzω

wω]c)[(vtω

Fzw

ζ)w(ωzζ

wζ]c)[(vtζ

ωHHH

HHHSR

H

ζHHHHSR

(1)

(2)

(3)

Vorticity budget analysis

(a)

(b)

(d)

(e)

(c) (f)

x

x

x

x

x

x

x

x

x

Storm c1 at 3984 m at 1630 LST:

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 tends to cancel with stretching.

Total tendency is mostly positive at updraft centers and thus tends to enhance existing .

(b)

A

B

C

D

E

(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.

(a)

PC

PA

PB

DD

PD

(b)

PC PAPB

DD

PD

(c)

PCPA

PB

DD

PD

(d)

(e)

(f)

PC

PAPB

DD PD

PC PA

PB

DD

PD

PC PA

PBDD

PD

Thermodynamic and shear parameters at points A~E in run1 during 1100 LST 19 to 0200 LST 20 Dec. 2002

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.

Summary

Better understand the storm structure,

kinematics, dynamics and splitting process.

Better understand the storm evolution in relation

to the variation in mesoscale environment.