京都大学21世紀COEプログラム 活地球圏の変動解明 アジア・オセアニアから世界への発信
Seminar Series of Active Geosphere #26: December 16, 2005
By Masahi KIGUCHI
Graduate school of Science, Kyoto University
"The rainfall phenomena during the pre-monsoon season over Thailand"
I had a seminar at faculty of Science, Chulalongkorn University on 16th December, 2005. The rainfall phenomena during the pre-monsoon period under the mid-latitude westerly regime over the Indochina Peninsula were revealed using daily rainfall data, daily OLR (Outgoing Longwave Radiation) and daily mean NCEP/NCAR reanalysis data for a period, 1979-2002. Mainly, the rainfall phenomena during the pre-monsoon period in the GAME-IOP year, 1998 were investigated. The land-atmospheric interaction over the Indochina Peninsula and the diurnal cycle of the rainfall phenomena over the Thailand were discussed.
It is shown that the phenomena of the intermittent rainfall migrate southward from the mid-latitude zone in early and middle April and early May 1998. It is indicated that the intermittent rainfall phenomena occur during the pre-monsoon period almost every year. From the distribution of daily rainfall, the intermittent rainfall phenomena occur not only at some points but also in the wide area of the research domain.
It is indicated that the troughs appear during the rainfall events during the pre-monsoon period. Furthermore, it is clarified that during the rainfall events, the trough at 300 hPa stretches from Myanmar to the eastern Bay of Bengal, i.e., to west of Thailand. In contrast, on non-rain days, the strong westerly jet appears over the northern part of the Indochina Peninsula and the trough is not observed around the Indochina Peninsula. The trough in the upper troposphere moves eastward along the southern edge of the Tibetan Plateau. When the trough passes through the Indochina Peninsula, the upward flow in the middle troposphere is strengthened. In contrast, after the onset of the monsoon, the upward flow is not coincident with the trough passages.
The moisture amount over the Indochina Peninsula increases before the occurrence of active convection. Furthermore, the moisture inflow from the east (the South China Sea) is strengthened, resulting in an increase in the moisture amount during the pre-monsoon period. In contrast, the moisture inflow from the west (the Bay of Bengal) is strengthened, resulting in a decrease in the moisture amount during the pre-monsoon period. As a result of the composite analysis of the vertically integrated moisture inflow into the study area from each direction, it is clarified that the moisture convergence is strengthened during the rainfall events. In contrast, there exists a moisture divergence on non-rain days. This result indicates that the moisture convergence when intermittent rainfall events occur during the pre-monsoon period is produced by the strengthening of the moisture inflow from the east.
The result of the analysis regarding the property of the heat quantity from the land surface during the pre-monsoon period, after the rainfall event in late March, reveals that the sensible heat does not exceed the latent heat. The latent heat after the rainfall event in late March is as continuous as that in the rainy season. It can be said that the land surface is wet from late March. It is suggested that the wet environment of land from late March have no small effect on the increase of the precipitable water over the Indochina Peninsula.
The diurnal variation over the central area of Thailand was investigated. We separate the coastal, inland and Koraht Plain (each 17, 17, and 15 stations) area from the central of Thailand. We investigate the regional property of the diurnal cycle around central Thailand using 3-hourly rainfall data and want to understand how the diurnal variation is related in the seasonal march. We divide a year into 4 seasons (dry, pre-monsoon, monsoon and post-monsoon season). The maximum time is from night time to early morning in the inland area of the year. The diurnal cycle related in seasonal march is not clear. But, in the late post-monsoon and dry season, the diurnal cycle itself is not clear. Over Koraht Plain, the maximum time is also from night time to early morning. Especially, in monsoon season, the diurnal cycle is very clear. In contrast, in pre-monsoon season (especially April), the diurnal cycle is not clear. In the coastal area, in pre-monsoon season, the maximum time of the diurnal cycle is around in day time. The maximum time of the diurnal cycle changes to midnight time when the monsoon starts.