1. Diurnal Variations of Tropical Convection Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe, and J. Matsumoto, 2001 : Diurnal Variations of Convective Activity and Rainfall in Tropical Asia, J. Meteor. Soc. Japan, 79, 333-352. Kubota, H., A. Numaguti, and S. Emori, 2004 : Numerical Experiments Examining the Mechanism of Diurnal Variation of Tropical Convection, J. Meteor. Soc. Japan, 82, 1245-1260.

2. Introduction The diurnal variations of convective activity and rainfall greatly influence the energy and water cycles on a daily basis. Over tropical oceans, the maximum convection appears between midnight and sunrise.

3. Procedure of data The rainfall data from 101 stations in four countries (Bangladesh, Thailand, Vietnam, Malaysia) are used. The information on convective activity is derived from the data from GMS-5. The frequency of occurrence of is used as the index of convective activity.

4. Convective activity In the afternoon or early evening over both land and sea. In the late morning over land and in the late evening over the sea.

5. 22% ~75% 12% ~75% 57/101 21/101 same phase about 7 mm/day more than 10 mm/day

7. Harmonic analysis delay 2 hours advance 3 hours

8. Climatological winds

9. Late night-early morning maxima Bangladesh

10. Thailand

11. Vietnam

12. Malaysia relatively dry season

13. Interim summary The late night-early morning maximum is found in the windward areas of mountains, in basins and valleys, and in coastal areas. These areas are the place where a low-level convergence is expected during nighttime due to local circulations such as mountain and land breezes or their interaction with a prevailing wind.

18. The westward shift seem to be caused by a westward advection of upper non- precipitating clouds such as an anvil by a strong easterly wind in the upper troposphere.

19. Conclusions Over land, the maximum of convective activity tends to occur during the afternoon- early evening or during the late night-early morning. Over the sea, the maximum primarily appears during the afternoon.

20. Conclusions These afternoon-early evening maxima are caused by the solar radiative heating on the surface during daytime. The late night-early morning maxima are caused by local circulations such as mountain and land breezes or their interactions with a prevailing wind.

21. Diurnal Variations of Tropical Convection Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe, and J. Matsumoto, 2001 : Diurnal Variations of Convective Activity and Rainfall in Tropical Asia, J. Meteor. Soc. Japan, 79, 333-352. Kubota, H., A. Numaguti, and S. Emori, 2004 : Numerical Experiments Examining the Mechanism of Diurnal Variation of Tropical Convection, J. Meteor. Soc. Japan, 82, 1245-1260.

22. Purpose They want to clarify the mechanism that drives the diurnal variation of tropical convection over oceans by previous investigators as controlling the diurnal cycle.

23. Model The 2-D Regional Atmospheric Modeling System (RAMS) is uesd. It was 400 km wide, with a 2-km horizontal resolution, and extended to 30 km vertically, with 41 levels. The model ran 15 days, and the data for the last 10 days were analyzed.

24. Initial conditions The IC consisted of a horizontally uniform monthly (Dec, 1992) average vertical distribution. The SST was fixed at the Dec 1992 average (302.5 K).

25. Standard experiment (STND)

26. The time series for the horizontal distribution of precipitation.

28. UNICLD experiment Gray and Jacobson (1977) hypothesized that differences in radiative cooling between convective regions and surrounding clear regions might intensify circulation and convection more during the night than during the day. To test this, an experiment (UNICLD) included horizontally uniform cloud water when calculating the radiative scheme was conducted.

29. UNICLD experiment

30. NOCLDR experiment Randall et al. (1991) and Xu and Randall (1995) suggested that convection is activated at night due to destabilization caused by cloud radiative cooling in the upper troposphere over the ocean. The NOCLDR (no cloud radiative cooling) experiment including a reduced cloud radiative cooling effect.

31. NOCLDR experiment

32. NOPBLR experiment Randall et al. (1991) and Xu and Randall (1995) mentioned only the high-level cloud radiative cooling effect. The cloud radiative cooling at the top of the boundary layer was removed, and imposed uniform radiative heating and cooling (NOPBLR).

33. NOPBLR experiment

36. Water vapor budget in the boundary layer E : evaporation from the sea : vertical diffusion : vertical advection P : the precipitation budget within the boundary layer Q : the large-scale advection of water vapor

39. Conclusions The differences in radiative cooling between convective regions and surrounding clear regions are not the primary mechanism forcing the diurnal variation of precipitation. The cloud radiative cooling at both high levels and the top of the boundary layer is important in driving the diurnal variation of convection.

40. Conclusions Boundary-layer cloud radiative cooling destabilizes the low-layer atmosphere, and provides moisture from the boundary layer to the free troposphere. This creates a favorable condition for shallow convection to develop to deep convection.