Chakrit Chotamonsak and Jiemjai Kreasuwun

pp. 257 - 267

Abstract

      A numerical weather simulation of the depression 23W moving over the Gulf of Thailand during October 19-26, 2003 expressed in terms of meteorological variables including pressure, humidity, geopotential height, wind velocity, divergence, vertical velocity, vorticity and rain was performed by the fifth generation of PSN/NCAR mesoscale model, MM5 version 3.7. Favorable conditions for the storm development and storm center during its passage were indicated from the model simulation. The favorable weather conditions of the storm evolution were found to be high relative humidity of 90-100% and less than 1003 mb mean sea-level pressure. The central pressure of the storm decreased at the rate of at least 6 mb/24 hrs with the lowest value of 987 mb and the corresponding lowest geopotential height of 1326 m on October 22, 2003.
        Cyclonic flows were detected around the areas of negative horizontal divergence of -19.81 x 10^-4 s^-1 with gusts over 26 ms^-1 and the positive vorticities of 128.4-234.1 x 10^-4 s^-1 along with moist air updrafts of 10-130 m/s. Continuous updrafts in an unstable atmosphere with adequate moisture supply enhanced the condensation process, and the cloud and rain
formation, resulting in heavy rainfall in southern Thailand. Simulated rains are comparable with reported rains of 197-200 mm/24 hr at Prachuab Khiri Khan Province during October 24-25, 2003.