Diurnal Variation of Rainfall and Rain Data in Banglade
In this season, daytime heating triggered by high temperatures and tower type clouds build-up which is mainly tall, high echo top and deep convection clouds such as cumulonimbus, cumulus, that is more vertical in nature and mostly come up during the late afternoon and early evening or night hours and form often an arc-shape squall line type and intense rainfall which usually gets the thunderstorms after the post-noon period. Generally in a day that begin with a transparent sky and high temperature from the afternoon, rain starts occurring and intense rain with squally cooled winds (sometimes along with hail) results fall in temperature, which helps to procreate a diurnal cycle of rainfall during this season. It is synonymous with heat and humidity with uncomfortable conditions throughout the day and night. Besides these, various weather systems (say troughs) bring in moisture and thermal instability during this season due to condensation which is also responsible to the afternoon’s maxima of rains.
Figure 6 showed the DV of rainfall of monsoon during study period. It is observed by BMD rain gauge measured dual RFPs at 0600 LST (PP) and 1500 LST (SP). On other hand, GSMaP found this twin peaks at 0300 LST and 1500 LST. These findings are almost similar to yearly average sketch of DV of rainfall. So it is clear, monsoon rainfall has a strong effect on the annual RF in Bangladesh. The quantity of RF which measured by GSMaP is underestimated than that of BMD rain gauge. In the monsoon season, rain can commence any time of the day, generally the preferred time of maximum RF at late night to early morning and late noon.
During this monsoon, because of a large amount of moisture exist in the lower atmosphere the atmosphere over Bangladesh is very unstable, and only a little perturbation is well enough to produce low-level clouds mainly nimbostratus and stratiform clouds which are mainly widespread or continuous layers of clouds. Depths of these clouds are less but the layers are thick and moisture laden which contain a very big amount of water droplets and result in precipitation especially rain which mainly occur after post noon session. This is why, during monsoon days’ most afternoons experience heavy RFs . In addition, the mesoscale circulations like mountain valley winds, katabatic-anabatic winds, land sea breezes, etc. can govern the rain regime and produce a diurnal cycle/variation.
Actually, the ground heat due to insolation is the dominant forcing for diurnal variation of rainfall during the monsoon period. Generally, convective cells formed over sea and moved toward the land results maximum rainfall over land in the late night to early morning. Terao et al., investigated that over Bangladesh especially in the northeastern part of country monsoon rainfall peak appears at 0030 to 0060 LST. In the lower atmosphere, the southeasterly advanced during afternoon, after that the wind direction changes to clockwise during night. This characteristic occurs the nocturnal jet, which corresponds to the midnight to early morning peak of rainfall in the northeastern part of the country through the increased wind blowing counter the southern border of the Meghalaya Mountains. Besides this, post-noon tidal water of rivers may accountable to these phenomena, because during this time huge water moisture come or grow from river water.
Figure 7 showed the DV of rainfall of post-monsoon season during study period. BMD rain gauge and GSMaP observed a SP at 0600 LST and PP at 1500 LST. So, it reveals that in post-monsoon season maximum RF occurred at afternoon and early night. The diurnal pattern of rainfall measured by both GSMaP and rain gauge is almost similar. These results are similar to the findings of Islam et al. During post-monsoon, high temperature and low pressure in the Bay of Bengal causes to bring cool, dry and intense air masses to many regions of the country. Winds spill across the Himalayas and flow to the southwest and north-easterly direction across the country , resulting in clear, sunny skies and sometimes depression or cyclone form in the Bay of Bengal resulting the evening hour with a significant amount rainfall. The mean rainfall decreases sharply from October to November and slowly from November to December. The gradients of mean monthly rainfall are maximum over the northeastern and southwestern parts of Bangladesh during evening period.
Figure 8 represented the DV of RF of winter season of the study period. The peak of the rainfall is observed at 0300 LST by both GSMaP and rain gauge. Since Bangladesh is a rainfall dominated climate country and maximum rainfall occur during monsoon season, only 2% or less rainfall of the annual occurs during winter season. In winter, the solar radiation falls obliquely which may not to conducive making a remarkable to generate the thermal properties of sea and land let alone to create a diurnal cycle of RFs.
A centre of high pressure exists over Bangladesh and northwestern part of India during the winter season. From this high pressure a current of dry and cold air flows eastward and enters Bangladesh over its northeast part by propagating clockwise, almost right-angle. This cod and dry wind is the main part of the winter monsoon formation over the South Asian countries. Wind over Bangladesh commonly has a northerly component (flowing toward south and southeast) which is not helpful creating a significant rainfall during this season.
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