Development Mechanism of Heavy Rainfall over Gangneung Associated with Typhoon RUSA

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

2. Typhoon Rusa’s track and rainfall GangNeung • 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

3. Sokcho (SC) Gangneung (GN) Daegwallyeong (DGL) Donghae (DH) Terrain and location of stations Height (m) • 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)

4. Previous studies • 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?

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. Hourly rainfall amount at Gangneung • 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 period): hourly rainfall over 50 mm continued for 5 hours. Peak at 23 LST (14 UTC) 31 August (UTC)

9. Hourly rainfall amount at 4 stations in Youngdong area 100 DH 90 DGL GN 80 SC 70 60 Precipitation (mm) 50 40 30 20 10 0 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 3 Time (KST) SC GN DGL DH 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

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

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

12. Equivalent potential temperature COOL AIR WARM MOIST AIR

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

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

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

18. SC SC GN GN DGL DGL DH DH 06 UTC 06 UTC SC SLP and surface winds.obs. (left) and model (right) GN DH 12 UTC 12 UTC

19. SC SC SC SC SC GN GN GN GN 1130UTC 31 Aug DGL DGL DGL DGL DH DH DH DH GN C DH 09 UTC 06 UTC Terrain Sim. Rainfall 15 UTC 12 UTC GN DGL DH Radar

20. Hourly rainfall amount GN DGL

21. Gangneung 25 24 23 22 21 Temperature (°C) 20 19 18 OBS 17 CONT 16 GDAS 31/ 15 00 03 06 09 12 15 18 Time (UTC) Simulated and observed surface-air temperature at Gangneung

22. Froude Number

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 !