Numerical experiments on the late night-early morning maximum of rainfall in the northeastern Bangladesh
Aya Kataoka (1), Takehiko Satomura (1)
(1) Graduate School of Science, Kyoto University
Numerical experiments over the Bengal region were conducted by using the Pennsylvania State Univ.-NCAR mesoscale model (MM5). The experiments were performed during the two periods, 1995 Jun 14th-21st and 23rd-30th. The former period includes the active period when the rainfall amount was above normal over Bangladesh and the later includes the break period when the amount of rainfall was below normal (Ohsawa 1999). We focused on the northeastern Bangladesh where the late night-early morning maximum of rainfall was observed by using the data from rain gauges and radar in Dhaka. An east-west elongated mountainous range, Shillong Plateau, whose highest peak is about 2000 m, is located right north of this region. Bangladesh locates in the delta of Ganges, Brahmaputra and Meghna Rivers. The altitudes of the major part of Bangladesh are less than 10 m. At first, the diurnal variation of Tbb data was investigated to check the reliability of the model results. In the active period, a low Tbb area covering from the eastern Nepal to the northwestern Bangladesh appeared a few hours before midnight. The low Tbb area broadened to the eastward and southward and stayed in the northern part of the Bangladesh until early in the morning. During the day, the low Tbb region broadened further southward and covered over the Bangladesh.The model well simulated the observed diurnal variation of precipitation judged by Tbb. Especially the precipitation was strong at the southern foot of the Shillong Plateau from late night to early morning. On the other hand, the precipitation occurred all over the Bangladesh during the day. These characteristics also agreed with the results in previous observational studies using the data from rain gauges and radar (Ohsawa 1999; Islam 2003). Compared with the active period, the diurnal variation was clear neither in Tbb data nor the simulated precipitation during the break period.There were some interesting characteristics in the model results: 1) Although lower wind was southerly or southeasterly, the precipitation region looked like moving southward from late night to morning; 2) In Assam region, heavy precipitation occurred around midnight during the active period; 3) The numerical experiments without condensation processes showed that the convergence region stayed in the southern foot of Sillong Plateau from night to morning below about the altitude of 1000 m, where the divergence region appeared during the daytime. The mechanism of the late night-early morning maximum rainfall and also above interesting characteristics will be discussed.