Development Mechanism of Heavy Rainfall over Gangneung Associated with

Typhoon RUSA

Tae-Young Lee, Nam-San Cho, Ji-Sun KangKun-Young Byun, Sang Hun Park

Laboratory for Atmospheric Modeling Research (LAMOR)Dept. of Atmospheric Sciences, Yonsei University, Seoul

5th International Conference on Mesoscale Meteorology and Typhoon, 31 Oct.-3 Nov. 2006, Boulder, Colorado

AMOP

Typhoon Rusa’s track and rainfall

0420 UTC 1. Sep990 hPa, 18m/ s

0630 UTC 31 Aug.960 hPa, 35m/ s

0000 UTC 23 Aug.

0000 UTC 29 Aug.950hPa, 41m/ s

• RUSA resulted in a record-breaking daily rainfall amount of 870.5 mm at Gangneung on 31 August 2002

• RUSA claimed 236 deaths & properties of about 6.1 Bill. Dollar

GangNeung

Terrain and location of stations

• Daegwallyeong - Near the mountain peak - 712.5 mm/day (31 Aug) • GangNeung - Relatively flat area near the coastline - Maximum daily precip. : 870.5 mm/day (31 Aug) (record)

Gangneung(GN)

Daegwallyeong (DGL)

Donghae(DH)

Sokcho (SC)

Height (m)

Kang and Lee (2004) suggested that direct terrain effect alone could not explain the heavy rainfall at Gangneung

Other studies related low-level convergence to the heavy rain at Gangneung, with terrain as a possible cause for the convergence

Mechanism of the rainfall at Gangneung is still not well understood

The heavy rainfall has not been successfully simulated by models. Why?

Previous studies

Goals and contents

• Goal - To explain the mechanism of the record-breaking heav

y rainfall at Gangneung associated with typhoon Rusa of 31 August 2002

• Contents - Observational analysis of the event - Preliminary numerical experiments - Analysis of results

Observational analysis

Two peaks on 31 August 2002

- The first peak at 09 LST (00 UTC) 31 August (before the

landfall of typhoon),

- The second peak (target periodtarget period): hourly rainfall over 50 mm continued for 5 hours. Peak at 23 LST (14 UTC) 31 August

Gangneung

0

10

20

30

40

50

60

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90

100

08/30/12 08/30/18 08/31/00 08/31/06 08/31/12 08/31/18 09/01/00Time idx

Hour

ly p

reci

p(m

m/h

)

OBS(886.0mm/ 36hrs)

Hourly rainfall amount at Gangneung

(UTC)

강수분석At GN, rainfall stronger than 50 mm/h

lasts 5 hours (19~23 LST)

At DGL, rainfall stronger than 30mm/h lasts 10 hours (15~24 LST)

At SC, rainfall greater than 30mm/h lasts 5 hours (22 LST 31Aug ~ 02 LST 1 Sep), as rainfall weakened at GN and DGL

Hourly rainfall amount at 4 stations in Youngdong area

GN DGL

DH

SC

0

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100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3Time (KST)

Pre

cip

itati

on (

mm

)

DHDGLGNSC

1240UTC 31 Aug

1030UTC 31 Aug 1130UTC 31 Aug

1330UTC 31 Aug 1420UTC 31 Aug

0930UTC 31 Aug

GN

DH (radar site)

Surface chart for 12 UTC 31 Aug 2002

GN

DH

SC

320

322

324

326

328

330

332

334

336

338

30/ 00 30/ 06 30/ 12 30/ 18 31/ 00 31/ 06 31/ 12 31/ 18 01/ 00

Time (UTC)

Thet

aE (

K)

Donghae (DH)Daegwallyeong (DGL)Gangneung (GN)Sokcho (SC)

COOL AIR

WA

RM

MO

IST

AIR

Equivalent potential temperature

Summary of observed features

DH

DGL

GN

NWNE

E~ESECell movement

Obs. Fr: 0.7 ~ 0.8

Band of strong echo

1130UTC 31 Aug

Possible causes for the presence of N or NW winds at Gangneung and Sokcho :

• blocking of low-level NE flow by mountain range• local cooling of low level air along the east coast → pressure increase along the mountain slope → deceleration of winds toward the mountain range → ageostrophic northerly • magnitude changes of surface friction into the land

Preliminary Numerical Experiment

Experimental design• Model: WRF (V2.1.2)• Integration period: 00 UTC 31 Aug – 00 UTC 1 Sep 2002• 1-way nesting

Domain1 (D1)

Domain2 (D2)

Domain3 (D3)

Domain4 (D4)

Horizontal grid spacing

30km(191X171)

10km(190X181)

3.3km(217X199)

1.1km(271X298)

Vertical layers

33 layers (model top : 50hPa)

Cumulus Betts-Miller_Janjic No cumulus

Microphysics WSM 6 microphysics

LW radiation RRTM scheme

SW radiation Dudhia scheme

PBL Mellor-Yamada-Janjic TKE scheme

Surface Thermal diffusion scheme

Sea level pressure for 12 UTC 31 Aug 2002

CONT OBS

Simulated typhoon track and pressure distributionagree well with observation.

06 UTC

12 UTC

GN

DGL

DH

SC

GN

DGL

DH

SC

06 UTC

12 UTC

SLP and surface winds.

obs. (left) and model (right)

SC

GN

DH

06 UTC 09 UTC

12 UTC 15 UTC

Sim. Rainfall

GN

DGL

DH

SC

GN

DGL

DH

SC

GN

DGL

DH

SC

GN

DGL

DH

SC

GN

C

DH

SC

Terrain

1130UTC 31 Aug

Radar

GN

DH

DGL

Hourly rainfall amount1-hr accumulated precipitation (Gangneung)

Time (UTC)

6 7 8 9 10 11 12 13 14 15 16 17 18

Pre

cipi

tatio

n (m

m)

0

10

20

30

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100

OBS CONT

1-hr accumulated precipitation (Daegwallyeong)

Time (UTC)

6 7 8 9 10 11 12 13 14 15 16 17 18

Pre

cipi

tatio

n (m

m)

0

10

20

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100

OBS CONT

GN DGL

Simulated and observed surface-air temperature at Gangneung

31/

Gangneung

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16

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22

23

24

25

00 03 06 09 12 15 18

Time (UTC)

Tem

pera

ture

(°C

)

OBS

CONT

GDAS

Froude Number

Time Observation Simulation

06 UTC 31 0.69 1.52

12 UTC 31 0.77 3.13

Simulations with other models, initial and boundary conditions

Numerical models• Weather Research and Forecasting (WRF) model V2.1.2• PSU/NCAR Mesoscale Model (MM5) V3.7• CSU Regional Atmospheric Modeling System (RAMS) V4.4

Initial and boundary conditions from:• NCEP GDAS• KMA GDAPS

Integration periods:• 24hr (00 UTC 31 Aug – 00 UTC 1 Sep 2002)• 36hr (12 UTC 30 Aug – 00 UTC 1 Sep 2002)

All simulations failed to reproduce observations at and to the north of Gangneung

Possible causes for failure

• Errors in the initial conditions over the East Sea and coastal area

• Errors in boundary conditions (e.g., SST, surface roughness)

Summary (1)

• Heavy rainfall of 31 August 2002 at Gangneung (GN) is being examined using observation and numerical experiments.

• A band of strong rainfall develops along the mountain range as easterly associated with the typhoon Rusa blows toward the range.

• The band development is limited to GN and to its south during the heavy rainfall period at GN.

• Low-level N or NW winds prevail at GN and to its north as strong easterlies are found at higher levels.

• The presence of N - NW winds at low level to the north of GN appears to be a key factor for the heavy rainfall at GN.

Summary (2)

• Models have successfully reproduced the propagation of typhoon Rusa across the Korean peninsula, but not the heavy rain at Gangneung.

• N or NW winds at Gangneung and to its north are not simulated.

• Froude number of simulated flows toward the mountain range are significantly higher than the observation due to higher low-level temperature.

• Simulations using other models and initial conditions failed to reproduce observation at GN and in the area to its north.

• The cause of failure is being examined.

Thanks !