In this paper, the updated rainfall data of 50 years for 30 selected rain gauge stations of Bangladesh have been used. The data were analyzed to investigate the variability and trends of summer monsoon (JuneSeptember) rainfall over Bangladesh. The possible teleconnection of monsoon rainfall variability with ENSO has also been investigated.Annual profile of the station mean monthly rainfall of Bangladesh shows a unimodal pattern with high rainfall between June-September (monsoon season) with highest in July and low rainfall between December -February with lowest in January. All Bangladesh mean summer monsoon rainfall is 1769.14 mm, standard deviation 209.16 mm (coefficient of variance 11.82 %) and annual country average rainfall is 2456.38 mm. Summer monsoon rainfall widely varies over the geographical areas with lowest in central-western part and highest in southeastern part with next highest in northeastern part of the country. The trend analysis shows that the trend of the country average monsoon rainfall is decreasing (-0.53 mm/year). The spatial distribution of the trend values indicates that the summer monsoon rainfall exhibits increasing trends at the rate of 5-6 mm/year in the NW region and 3-4 mm/ year in the south-central and extreme SE region. The eastern region exhibits decreasing trends of about -2 to -7 mm/year with highest (-6 to -7 mm/year) in the east-central part. The time series plot of country average summer monsoon rainfall shows the inter-annual variability in the timescales of 2-3 years and 4-6 years. The time series of 5 year moving average reveals existence of low frequency variability of timescales of 9-14 years. The time series of Bangladesh monsoon rainfall shows that there were 11 strong monsoon years and 8 weak monsoon years within the periods of 1961-2010 (50 years). The analysis of the decadal mean rainfall shows that the decades 1961-1970 and 1981-1990 were wet and the decades 1971-1980, 1991-2000 and 2001-2010 were dry. Floods in Bangladesh result from the excess rainfall occurring both inside and outside the country. Summer monsoon rainfall is characterized by active and weak (break) spells, which are associated with the fluctuation of monsoon rainfall in the time scales of 20-25 and 40-50 days. Such fluctuations are caused due to north-south movement of the monsoon trough. The fluctuations in the time scales of 4-7 and 10-14 days are associated with the formation of low pressure systems over the head Bay. The possible atmospheric teleconnections of summer monsoon rainfall with ENSO have also been investigated. It is found that there is strong impact of ENSO on the monsoon system of subcontinent scale, though the relationship is weak in case of Bangladesh. The variability of Bangladesh rainfall has been investigated with respect to that for Nepal, Bhutan and the neighbouring sub-divisional regions of India. The results show that Bangladesh rainfall has positive correlation with that of the Indian regions of Naga-Monipur-Mizo-Tripura, Variability and trends of Summer ...

A widespread thunderstorm (squall line or multicell line) occurred over Bangladesh on 11 May 2011 during 0300 UTC to 1300 UTC. A north-south oriented wellestablished squall line of 400 km length was noticed to the center of Bangladesh at 0530 UTC and the vertical extend of this system was about 16-18 km. An attempt has been made to study the impact of data assimilation in simulation of the selected thunderstorm event of 11 May 2011 using Weather Research and Forecasting (WRF) model. The WRF model was run in STORM Phase-I domain at 9 km horizontal resolution using six hourly NCEP-FNL datasets from 0000 UTC of 11 May to 0000 UTC of 12 May 2011 as initial and boundary condition for control (CNTL) run. The 0300 UTC Synop, 0000 UTC Temp and 0300 UTC Khepupara DWR reflectivity and redial velocity of 11 May 2011 were assimilated in 3DVar with the first guess of WRF model for data assimilation (DA) run. Model outputs have been analyzed for CNTL and DA to compare and/or asses the model performance. It is found that the DA enhanced in the synoptic and environmental characteristics as compared to that of CNTL. The WRF model outputs with DA help to investigate the synoptic and environmental characteristics responsible for the occurring of this unusual event. The model products showed a cyclonic circulation over Gangetic West Bengal of India at 925 hPa and over Assam and adjoining areas of India at 500 hPa. Strong upward motion was noticed over Bangladesh. Significant moisture incursion was observed over center and southeast parts of Bangladesh at 925 and 850 hPa with southerly/southwesterly flow of 10-20 ms -1 (19-39 knots). Result showed that the WRF model with DA could capture thunderstorm event of 11 May 2011 in reasonably well though there are some spatial and temporal biases in the results.

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On 14 September 2004, Dhaka (23.77°N, 90.38°E) -the capital city of Bangladeshreceived unprecedented heavy rainfall and more than two-third of the city was inundated. This is an extraordinary rainfall event. The highest amount of rainfall in 24 hours was recorded at Dhaka (341mm), which were the highest ever recorded [1][2]. Due to these heavy rainfall flash flood situation was created in Dhaka and some parts of the country. AbstractAn attempt has been made to simulate a heavy rainfall event on 14 September 2004 over Dhaka, Bangladesh using the fifth-generation PSU/NCAR Mesoscale model (MM5). This was an extraordinary rainfall event and recorded 341 mm rainfall in 24-h which was the highest ever recorded. The MM5 model was run on triple-nested domains at 45, 15, 5 km horizontal resolutions using Anthes-Kuo cumulus scheme. The model performance was evaluated by examining the different predicted parameters like mean sea level pressure, upper and lower level circulations, moisture, windshear, vorticity, convergence and rainfall. The model derived rainfall was compared with TRMM rainfall. The present results indicate that the MM5 model with the right combination of the nesting domain, horizontal resolution and cumulus scheme was able to simulate the heavy rainfall event, and associated dynamical and thermo-dynamical features reasonably well. The MM5 model suggested that the highly localized heavy rain over Dhaka was the result of an interaction of the monsoon land depression with southwest summer monsoon weather systems. The analysis shows that the depression almost remains stationary over southwest Bangladesh and zone of heavy rain was laid over Dhaka, and required moisture have been supplied from the Bay of Bengal.

Simulation of high impact rainfall events over southeastern hilly region of Bangladesh has been carried out using Fifth-Generation PSU/NCAR Mesoscale Model (MM5) conducting two historical rainfall events, namely, 21 June, 2004 and 11 July, 2004. These extraordinary rainfall events were localized over the Rangamati region and recorded 304 mm and 337 mm rainfall on 21 June, 2004 and 11 July, 2004, respectively, over Rangamati within a span of 24 h. The model performance was evaluated by examining the different predicted and derived parameters. It is found that the seasonal monsoon trough has northerly position compared to normal and pass through Bangladesh extending up to northeast India for both cases. The heat low was found to be intense (996 hPa) with strong north-south pressure gradient (12–15 hPa). The analysis of the geopotential height field at 200 hPa shows that the Tibetan high is shifted towards south by 7-8° latitudes with axis along 22–25°N for both cases. The analysis of the wind field shows that the areas of high impact rainfall exhibit strong convergence of low level monsoon circulation (~19–58 knots). The strong southwesterlies were found to exist up to 500 hPa level in both cases. The lower troposphere (925–500 hPa) was characterized by the strong vertical wind shear (~9–18 ms−1) and high relative vorticity (~20–40 × 10−5 s−1). The analysis also shows that the areas of high impact rainfall events and neighbourhoods are characterized by strong low level convergence and upper level divergence. The strong southwesterly flow causes transportation of large amount of moisture from the Bay of Bengal towards Bangladesh, especially over the areas of Rangamati and neighbourhoods. The high percentage of relative humidity extends up to the upper troposphere along a narrow vertical column. Model produced details structure of the spatial patterns of rainfall over Bangladesh reasonably well though there are some biases in the rainfall pattern. The model suggests that the highly localized high impact rainfall was the result of an interaction of the mesoscale severe convective processes with the large scale active monsoon system.