The Diurnal Cycle of Winds, Rain, and Clouds over Taiwan During the Mei-Yu, Summer, and Autumn Regimes

1. The Diurnal Cycle of Winds, Rain, and Clouds over Taiwan During the Mei-Yu, Summer, and Autumn Regimes Yi-Leng Chen and Brandon Kerns Department of Meteorology University of Hawaii Honolulu, HI 96822 USA

2. Introduction • Monsoon circulations are planetary-scale phenomena. However, monsoon rainfall over a subtropical island is frequently localized in nature, related to terrain and local winds. • Taiwan is located in the subtropics off the southeast China coast. Due to the influence of the East Asian monsoon, the island experiences a variety of environment winds, stability, and moisture throughout the annual cycle. • In this study, we would like to study the diurnal cycles over Taiwan under different seasons with different environment conditions. • How do the large-scale circulations combine with orographic effects and local diurnal winds create weather that is different from what would occur in the absence of the terrain? • Applications: QPF, validation of high resolution numerical weather prediction models, regional climate downscaling

3. Previous studies • Ramage (1952): “Diurnal variation of summer [May-August] rainfall over east China, Korea, and Japan” • Found dominant afternoon maximum, some areas secondary morning maximum. • Using TAMEX (1987) surface data: • Y.-L. Chen and Li (1995) (winds) • Yeh and Y. L. Chen (1998) (winds + rain); most areas afternoon max. • Entire island not covered • Using dense ARMTS rain gage network: • T.-C. Chen, et al. (1999); 3 years of data (May-August), afternoon + secondary early morning maximum over west coastal plain. • Using satellite • TRMM high-resolution data for June-August 1998 (Krishtawal and Kristamurti, 2001) results similar to T.-C. Chen, et al. (1999), bimodal, afternoon maximum dominant. Also, midnight maximum.

4. Data Sources • Surface data from Automatic Rainfall and Meteorological Telemetary Systems (ARMTS) (1997-2002). • 2. Twenty Five conventional Central Weather Bureau Surface Stations (1997-2002) • 3. GMS satellite data (0.05 degrees resolution) (1996-2002)

5. ARMTS data coverage Circles: rain gage Barbs: wind included Data Period: 1997-2002 ----Compute average hourly rain (>.01 mm/hr) frequency and average rain ratefor each season separately.

6. Mei-Yu Rainy Season • May 16 – June 15 • Peak seasonal rainfall W, S parts; secondary peak for N, E parts. • Southwest monsoon flow, Westerlies aloft. • Baroclinic Mei-Yu fronts • Mesoscale disturbances in the SW flow.

7. Mei-Yu (May 16-June 15) Moist Static Energy (kJ/kg) 850 mb Flow 850 mb Precipitable Water (kg/m^2) 60 From Chen and Chen (2003) 0

8. Mean (1997-2002) winds and wind steadiness at select times during the Mei-Yu. Half barbs are 0.5 m s-1, full barbs are 1.0 m s-1, and pennants are 5.0 m s-1. Terrain contour is 1000 meters. Wind steadiness is also plotted. Z

9. Island Divergence Nightime offshore flow; daytime onshore flow Island dIvergence is a measure of the Mean strength of the diurnal circulations.

10. Mei-Yu Locations of Afternoon Max Locations of Morning Max

11. Mei-Yu: Afternoon Max

12. Mei-Yu: Evening Max

13. Mei-Yu Contours: frequency of occurrence of cloud tops colder than 235 K X = rain frequency 15-25 % O = rain frequency > 25%

14. Frequencies (%) of brightness temperatures below 270 K during the Mei-Yu for 1997-2002. Terrain contour is 1000 meters.

15. Summer Rainy Season • July 16 – August 30 • Secondary Rainfall Peak for west side, lull elsewhere. • Light south-southwest winds, Easterlies aloft. • Monsoon trough between Taiwan and The Philippines (furthest north in late August) • Most unstable and moist during the year,

16. Moist Static Energy (kJ/kg) Summer Rainy Season (July 16 – August 30) 850 mb Flow Precipitable Water (kg/m^2) From Chen and Chen (2003)

17. Diurnal variations of Surface winds during the Summer

18. Summer: Afternoon Max

19. Summer: Weak Evening Max

20. Summer Contours: frequency of occurrence of cloud tops colder than 235 K X = rain frequency 15-25 % O = rain frequency > 25%

21. Frequencies (%) of brightness temperatures below 270 K during the Summer for 1997-2002. Terrain contour is 1000 meters.

22. Autumn Rainy Season • September 16 – October 15 • Rainfall peak N, E; secondary peak S, W. • The only time the east side is on the windward side under relatively moist, potentially unstable conditions. • Northeast monsoon flow; Westerlies aloft. • Baroclinic fronts. • Mesoscale systems in the NE monsoon flow

23. Autumn Rainy Season (September 16 – October 15) Moist Static Energy (kJ/kg) 850 mb Flow Precipitable Water (kg/m^2) From Chen and Chen (2003)

24. Mean (1997-2002) winds and wind steadiness at select times during the Autumn. Half barbs are 0.5 m s-1, full barbs are 1.0 m s-1, and pennants are 5.0 m s-1. Terrain contour is 1000 meters. Wind steadiness is also plotted.

25. Autumn: Afternoon Max

26. Autumn: Evening Max

27. Autumn Contours: frequency of occurrence of cloud tops colder than 235 K X = rain frequency 15-25 % O = rain frequency > 25%

28. Frequencies (%) of brightness temperatures below 270 K during the Autumn for 1997-2002. Terrain contour is 1000 meters.

29. Summary • The afternoon rainfall maxima are most pronounced in the mountainous interior areas. • The coastal areas have less pronounced diurnal cycles with maxima in the morning or evening or little diurnal cycle. • Weak Morning maxima occur along the windward coasts in the Mei-Yu and Autumn as well as inland. In summer, with relatively weak prevailing winds, they occur only inland. • The evening maxima are on the east side during the Mei-Yu and Autumn. Autumn is dominated by northeast flow while the Mei-Yu has periods of post-frontal northeasterlies. The timing is about the time of onset of downslope flow, and the terrain there is steep. Convergence between the downslope winds and northeasterlies may occur. • Summer has a relatively weak evening maximum along the west coast

30. Summary • For all three regimes, the overall strength of the island circulations, as measured by island-scale divergence, are about the same. Nevertheless, seasonal differences in diurnal variations in rainfall and cloudiness exist. • The Mei-Yu has the highest rainfall frequencies and most extensive area affected by the diurnal cycle. Much of the rainfall over the mountains is related to orographic effects as shown by a relatively low deep convective cloud frequency there. The maximum cloudiness shifts eastward in the later afternoon and early evening by the upper-level westerly winds. • In summer with undisturbed, most unstable conditions with relatively weak prevailing winds, the diurnal variations in rainfall and cloudiness are most pronounced and occur in land. The afternoon maximum in cloudiness occur over the western coastal plain while the rainfall maximum is over the mountain slopes. • In autumn, under strong, persistent northeasterly winds, afternoon rainfall maxima from deep convection occur only over a limited area of the southwestern mountain slopes, where the northeast monsoon flow is blocked by Central Mountain Range. Afternoon maxima also occur in the north and east of the island with relatively low deep convective cloud frequencies.