High temporal and spatial resolution observations of meso-scale features of pre- and mature summer monsoon cloud systems over Bangladesh |
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| Authors: | Toru Terao Md Nazrul Islam Fumie Murata Taiichi Hayashi |
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| Institution: | (1) Faculty of Education, Kagawa University, 1-1, Saiwai-cho, Takamatsu Kagawa, 760-8522, Japan;(2) Department of Physics, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh;(3) Faculty of Science, Kochi University, Kochi, Japan;(4) Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan |
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| Abstract: | Meso-scale characteristics of disturbances that bring about atmospheric disasters in pre- and mature monsoon seasons in Bangladesh
are analyzed. Several types of meteorological instruments capable of observations with high temporal and spatial resolutions
were introduced for the first time in this area to capture the meso-scale structure of rainfall systems. We installed an automatic
weather station (AWS) and several automatic raingauges (ARGs) and utilized the weather radar of Bangladesh Meteorological
Department (BMD). From the radar image in the summer of 2001 (16–18 July), a striking feature of the systematic diurnal variation
in this area was elucidated. In these 3 days, the diurnal evolutions of convective activity were remarkably similar to each
other, implying that this pattern can be understood as a typical response of local cloud systems to the diurnal variation
of insolation under some summer monsoon situations. The ARG data show the difference in characteristics of rainfall between
pre- and mature monsoon seasons. The short intense downpour tends to occur more frequently in the pre-monsoon season than
in the mature monsoon season. The pre-monsoon rainfall also has clear diurnal variation with a peak that is more strongly
concentrated in time. In the northern part the rainfall peak is found in between midnight and early morning, while it is observed
in the daytime in central to western parts of the country. Two disaster cases caused by meso-scale disturbances are analyzed.
Although they occurred in the same season, the structures of the cloud systems were largely different from each other. The
disturbance brought about tornadoes on 14 April 2004, consisting of many spherical cloud systems of approximately 20 km size.
On the other hand, another one that caused the tragic river water transport accident on 23 May 2004 had meso-scale rain band
structure. The latter case was captured by the AWS located at Dhaka. Sudden changes in temperature, wind and pressure were
observed clearly, showing the typical structure of convective rain bands. |
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| Keywords: | Bangladesh Severe storms Rainfall intensity Diurnal variation Pre-monsoon |
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