1. Development Mechanism of Heavy Rainfall over Gangneung Associated with Typhoon RUSA Tae-Young Lee, Nam-San Cho, Ji-Sun Kang Kun-Young Byun, Sang Hun Park Laboratory for Atmospheric Modeling Research (LAMOR) Dept. of Atmospheric Sciences, Yonsei University, Seoul 5 th International Conference on Mesoscale Meteorology and Typhoon, 31 Oct.-3 Nov. 2006, Boulder, Colorado AMOP

2. Typhoon Rusa’s track and rainfall 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

3. 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)

4.  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

5. Goals and contents Goal - To explain the mechanism of the record- breaking heavy rainfall at Gangneung associated with typhoon Rusa of 31 August 2002 Contents - Observational analysis of the event - Preliminary numerical experiments - Analysis of results

6. Observational analysis

8.  Two peaks on 31 August 2002 - The first peak at 09 LST (00 UTC) 31 August (before the landfall of typhoon), target period - The second peak (target period): hourly rainfall over 50 mm continued for 5 hours. Peak at 23 LST (14 UTC) 31 August Hourly rainfall amount at Gangneung (UTC)

9. 강수분석 At 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 10 20 30 40 50 60 70 80 90 100 123456789101112131415161718192021222324123 Time (KST) Precipitation (mm) DH DGL GN SC

10. 1240UTC 31 Aug 1030UTC 31 Aug 1130UTC 31 Aug 1330UTC 31 Aug 1420UTC 31 Aug 0930UTC 31 Aug GN DH (radar site)

11. Surface chart for 12 UTC 31 Aug 2002 GN DH SC

12. COOL AIR WARM MOIST AIR Equivalent potential temperature

13. Summary of observed features DH DGL GN NW NE E~ESE Cell movement Obs. Fr: 0.7 ~ 0.8 Band of strong echo 1130UTC 31 Aug

14. 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

15. Preliminary Numerical Experiment

16. 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) CumulusBetts-Miller_JanjicNo cumulus MicrophysicsWSM 6 microphysics LW radiationRRTM scheme SW radiationDudhia scheme PBLMellor-Yamada-Janjic TKE scheme SurfaceThermal diffusion scheme

17. Sea level pressure for 12 UTC 31 Aug 2002 CONT OBS Simulated typhoon track and pressure distribution agree well with observation.

18. 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

19. 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

20. Hourly rainfall amount GNDGL

21. Simulated and observed surface-air temperature at Gangneung

22. Froude Number TimeObservationSimulation 06 UTC 310.691.52 12 UTC 310.773.13

23. 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

24. 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)

25. 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.

26. 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.

27. Thanks !