Proposed New Criteria for Southwest Monsoon Onset Date in Thailand

Abstract

A study had been conducted in order to achieve a new criteria to identify the southwest monsoononset in Thailand. The elements used in this study included wind directions at the 850; 500; and 200 hPalevels, mean sea level pressure, relative humidity at the surface level, surface air temperature, and heat fluxat the surface level, recorded in May since 1996 until 2000. The Mesoscale Model version 5 (MM5) wasused to simulate the above elements. The domain in this study was located within 40°E to 120°E inlongitude and 30°S to 40°N in latitude. The grid size was set to be 50 by 50 kilometers. The simulation hadbeen processed for 11 consecutive days, with the assumed first date of the rainy season in the middle. Theresults revealed that surface air temperature in Thailand had been 30 – 31°C before the beginning of therainy season. Such value was about 30°C on the onset date and would reduce afterward. For the relativehumidity, it was found to be 80 – 85 %, 80 – 90 %, and 85 – 95 % prior to the monsoon onset, on the onsetdate, and later on, respectively. In case of the mean sea level pressure, it had been rather stable, but slightlydropped on the onset date. The location of a low pressure cells had a tendency to move toward the west. Thedifference between the low pressure cell above India and the high pressure area above Indonesia had beenin the range of 10 – 12 hPa before the onset date. It had increased to be about 14 hPa on the onset datebefore dropping down to about 10 hPa afterward. When examining sensible heat flux at the surface level inThailand, it had been in the high level before the monsoon onset and remained constant since the onset date.The majority of the wind field at the 850 hPa level over Thailand had been southerly before the monsoononset. It later became more southwesterly since the monsoon onset date. The wind field at the 500 hPa levelhad portrayed a trough over the eastern area of India prior to the monsoon onset before it shifted to be overBangladesh and the upper part of the Gulf of Bengal on the onset date and later on, respectively. For thewind field at 200 hPa level, it had been easterly since before till after the onset date. In addition, thedistribution of relative humidity in the vertical axis represented the downward movement of moist airmasses prior to the onset date also.

Keywords : MM5 / ECMWF / Monsoon / Temperature / Relative Humidity / Pressure /Sensible Heat Flux / Trough