Abstract

The diurnal variation of rainfall across Bangladesh is evaluated using data from the Global Satellite Mapping of Precipitation (GSMaP). GSMaP data has a temporal resolution of 1 hour and a spatial precision of 0.1 degree. GSMaP data is combined with three hourly rain gauge data from 35 sites of Bangladesh Meteorological Department (BMD). The diurnal variation of rainfall and their spatial distribution are compared in daily, monthly and yearly scales during the period of 2001-2015. It is observed that GSMaP data is well correlated with BMD rain gauge data. The correlation coefficients between GSMaP and BMD rain gauge for 3-hourly, daily, monthly and yearly rainfall are found 0.71, 0.74, 0.82 and 0.66, respectively. The GSMaP measured daily maximum rainfall over Bangladesh at 0300 LST (Local Standard Time) with a secondary maximum peak at 1500 LST, whereas, rain gauge got primary peak at 0600 LST with a secondary peak at 1500 LST. The diurnal variation of rainfall observed by GSMaP is well matched with rain gauge observation during the pre-monsoon, post-monsoon and winter season. During monsoon season, the primary peak of rainfall is measured at 1500 LST by GSMaP whereas rain gauge is found at 0600 LST. The maximum rainfall is found in the southern, western, eastern and central regions of the country during 1200-1500 LST. In the southeastern region of Bangladesh the maximum rainfall is found during 0600-0900 LST. The maximum rainfall is observed in the northwestern and northeastern regions of the country during 0300-0600 LST.

Abstract

A tornado outbreak occurred in West Bengal (WB), India, about 15–16 h before the landfall of Cyclone Yaas formed in May 2021 over the Bay of Bengal. High-resolution analysis data have been used to investigate the possible tornadoes in terms of environmental conditions connecting to the cyclone. The WB tornado is found as intense as EF2–3 on the tornado scale and is likely associated with a mini-supercell. The total shear of 37 m s-1 from 0–6 km above ground level (AGL) with strong clockwise rotation, the moderate instability (1504 J kg-1) and the energy helicity index of 2.2 are the substantial convective parameters related to the WB tornado. Moreover, the favorable environment owning intense bulk shear, a larger value of storm-relative environmental helicity in the lowest 1 km AGL and high values of significant tornado parameter (STP) urge the potentiality of multiple tornadoes spawning in multi-days accompanying the landfalling Cyclone Yaas. The right-front quadrant of the cyclone is found to be more vulnerable for developing moderate to severe tornadoes within its rainbands. The positive potential vorticity anomalies evidence the cloud-scale cyclonic circulation from surface to 400 hPa with the maximum in the mid-level.

Abstract

Although coastal and inland areas of Bangladesh exhibit distinct physiographic and climatic characteristics, spatiotemporal variation of extreme climatic events are poorly understood in these two areas. This study was an attempt to understand the trends in extreme climatic events in coastal and inland areas over the period 1968‐2018. The missing data in daily maximum and minimum temperature, and daily rainfall datasets were imputed using the multiple imputation by chained equations (MICE) technique and implementing a predictive mean matching algorithm. The imputed datasets were then tested for inhomogeneity using the penalized maximal t (PMT) and modified penalized maximal F (PMF) tests. A quantile matching (QM) algorithm was then applied to homogenize the datasets, which were then used for generating thirteen extreme temperature and nine extreme rainfall indices. The trends were assessed using the Trend Free Pre‐whitened (TFPW) Mann‐Kendall (MK) test and the magnitudes of the changes were determined using the Thiel‐Sen slope estimator. Additionally, standardized anomalies were calculated to understand the seasonal variability of temperature and rainfall over the past five decades. Results suggested that both coastal and inland areas were warming significantly but coastal areas exhibited a higher rate of warming. Although most of the extreme rainfall indices showed statistically non‐significant changes for coastal and inland stations, there is evidence of localized dryness and increased rainfall at individual stations. In particular, the drought‐prone northwestern region of the country experienced decreased rainfall, which is discordant to the results of previous studies. Findings from this study highlighted important local and regional‐scale changes in extreme climate events that were previously overlooked. The findings of this study can help undertake targeted climate change adaptation strategies to save population and resources.

Abstract

Large numbers of Bangladesh lightning fatalities during the pre-monsoon season have resulted in widespread speculation about causes for this annual event. The present study addresses the situation with lightning occurrence, lightning fatality, and agricultural data. Of the 1,424 lightning deaths from 2013-2017 in Bangladesh, an average of 1.73 deaths occur per day in the pre-monsoon season, 0.71 in the monsoon, and much smaller averages in the post-monsoon and winter seasons. About half of the deaths are related to agriculture. Population-weighted fatality rates are consistently large in several northeastern districts. Lightning fatalities are frequent in April and especially May during both morning and afternoon. Based on 37.2 million strokes from the Global Lightning Dataset GLD360 network over Bangladesh from 2013-2017, lightning is also found to occur most frequently in the northeast from mid-April through early June at all hours of the day. Several districts with large lightning stroke densities and lightning fatality rates are also the same as those with the greatest farming participation. One of the most common Bangladesh crops is Boro rice harvested during April and May in several districts with frequent lightning, and may contribute to the annual pre-monsoon lightning fatality maximum. As a result, there are temporal and spatial connections among lightning fatalities, lightning occurrence, and agricultural worker information.

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Satellite‐derived lightning data for 17 years (1998–2014) were used to evaluate the relation between environmental factors and lightning activity over the Bangladesh landmass. Time series convective available potential energy (CAPE) data were extracted from ERA‐40 reanalysis data while total and convective rainfalls were obtained from Tropical Rainfall Measuring Mission's monthly products. In addition, the product of CAPE and precipitation was computed and used as an additional variable. Three timescales – monthly, seasonal and annual – were utilized to determine the influence of precipitation and CAPE on lightning activity. The results indicated that CAPE stands out as an important variable at all of these timescales for predicting the occurrence of lightning. The correlation coefficient (r) between CAPE and lightning activity was found to be 0.902 (monthly), 0.703 (pre‐monsoon), 0.550 (monsoon) and 0.702 (annual), respectively. Total rain showed strong positive correlation with lightning on monthly scale (r = 0.734) and in the pre‐monsoon season (r = 0.701). However, such relationship was moderate during monsoon (r = 0.455). In contrast, convective rain showed slightly higher correlation during monsoon (r = 0.587) compared with that of pre‐monsoon season (r = 0.532). Because of strong seasonality in the data, convective rain did not exhibit strong relationship on annual scale (r = 0.227). The product variable (e.g. CAPE × precipitation) showed significant correlation on monthly (r = 0.895) and seasonal scales (r = 0.818 during pre‐monsoon and 0.686 in monsoon) but its influence appears to diminish on a longer timescale (r = 0.375). Spatial maps of correlation coefficient revealed significant positive correlation along relatively drier northern parts of Bangladesh. As lightning‐related fatality is on the rise, this study, first of its kind, is expected to inform public policy and provide information necessary for effective management of this atmospheric phenomenon to save lives and property in Bangladesh.

Abstract

Flood or flash flood monitoring and forecasting is a complicated tasks in densely inhabited and low -lying topography areas like Bangladesh. Recently, flash flood showed its hazardous and devastating affect in the northeastern part of the country especially Sumamganj, Kishoreganj, Netrokona due to heavy rainfall in those area and upper catchment area in India. Due to lack of rainfall data of upper catchment area in India, flash flood forecasting was not possible. Global Satellite Mapping of Precipitation (GSMaP) data can be used for flash flood forecasting. GSMaP is a high spatiotemporal resolution (0.1°×0.1°; 1 hourly) data and available four hour later after the observation. To validate GSMaP data, 3-hourly data of 35 rain gauge stations of Bangladesh Meteorological Department (BMD) used for the study period 2001-2015. Different threshold rainfall events are also studied. Several statistical analyses showed that GSMaP data are well correlated with rain gauge data. The correlation coefficients between GSMaP and BMD rain gauge for 3-hourly,and daily are found 0.71and 0.74 respectively. The Standard Deviation (SD) of 3 -hourly rainfall for G SMaP and rain gauge data are found 1.96 and 2.10 mm, respectively. According to BMD, there are five types of threshold rainfall events such as Very Heavy Rainfall (VHR > 88 mm in 24 hour), Heavy Rainfall (88 ≥ HR ≥ 43.5 mm in 24 hour), Moderate Heavy Rainfall (43.5 > MHR ≥ 22.5 mm in 24 hour), Moderate rainfall (22.5 > MR ≥ 10.5 mm in 24 hour) and Light rainfall (10.5 > LR ≥ 2.5 mm in 24 hour). The average yearly occurrence of VHR, HR, MHR, MR and LR events are found 4.1, 11.7, 17.9, 23.5 and 35.4 for BMD rain gauge data and 2.30, 8.03, 14.13, 22.56, and 42.58 for GSMaP data, respectively. The performance of GSMaP data to detect the events is 97%. So GSMaP data may be used to predict the flash flood, especially in areas where rain gauge data are limited.

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In this paper, the development of a statistical forecasting method for summer monsoon rainfall over Bangladesh is described. Predictors for Bangladesh summer monsoon (June–September) rainfall were identified from the large scale ocean–atmospheric circulation variables (i.e., sea-surface temperature, surface air temperature and sea level pressure). The predictors exhibited a significant relationship with Bangladesh summer monsoon rainfall during the period 1961–2007. After carrying out a detailed analysis of various global climate datasets; three predictors were selected. The model performance was evaluated during the period 1977–2007. The model showed better performance in their hindcast seasonal monsoon rainfall over Bangladesh. The RMSE and Heidke skill score for 31 years was 8.13 and 0.37, respectively, and the correlation between the predicted and observed rainfall was 0.74. The BIAS of the forecasts (% of long period average, LPA) was −0.85 and Hit score was 58%. The experimental forecasts for the year 2008 summer monsoon rainfall based on the model were also found to be in good agreement with the observation.

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A numerical simulation is performed to understand the features and development processes of the arc-shaped precipitation system that dominates in Bangladesh during the pre-monsoon (March–May) period. An arc-shaped precipitation system of 26 April 2002 is simulated using the Cloud Resolving Storm Simulator (CReSS) with a horizontal grid increment of 2 km. The Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model is used for downscaling. Hourly outputs of the finest domain (grid increment of 5 km) of MM5 and National Oceanic and Atmospheric Administration Reynolds weekly mean sea surface temperature data are used as the initial and boundary conditions for CReSS. Younger and more intense cells are formed in the southwestern end of the system. These cells move northeastward and merge with the system producing intense rainfalls. Simulation results indicate that low-level southwesterly or southerly wind brings warm moist air from the Bay of Bengal and helps develop new cells. The propagation speed of the system is 8 m/s, and the northeastern end moves faster than the southwestern end, creating clockwise rotation of the system. The propagation speed and the rotation of the simulated system coincide well with radar observations. The clockwise rotation of the system can be explained by the stronger (weaker) outflow and weaker (stronger) inflow in the northeastern (southwestern) end. The propagation of the system is attributable to the weak (≤7 m/s, storm relative) rear-to-front flow in the moist environment. Thus, the arc-shaped precipitation system common to the pre-monsoon period in Bangladesh develops through a balance of strong southwesterly or southerly moist inflow in the low altitudes below 2 km and relatively weak outflow in the rear of the system.

Abstract

Summer monsoon rainfall was simulated by a global 20 km-mesh atmospheric general circulation model (AGCM), focusing on the changes in the summer monsoon rainfall of Bangladesh. Calibration and validation of AGCM were performed over Bangladesh for generating summer monsoon rainfall scenarios. The model-produced summer monsoon rainfall was calibrated with a ground-based observational data in Bangladesh during the period 1979–2003. The TRMM 3B43 V6 data are also used for understanding the model performance. The AGCM output obtained through validation process made it confident to be used for near future and future summer monsoon rainfall projection in Bangladesh. In the present-day (1979–2003) climate simulations, the high-resolution AGCM produces the summer monsoon rainfall better as a spatial distribution over SAARC region in comparison with TRMM but magnitude may be different. Summer monsoon rainfall projection for Bangladesh was experimentally obtained for near future and future during the period 2015–2034 and 2075–2099, respectively. This work reveals that summer monsoon rainfall simulated by a high-resolution AGCM is not directly applicable to application purpose. However, acceptable performance was obtained in estimating summer monsoon rainfall over Bangladesh after calibration and validation. This study predicts that in near future, summer monsoon rainfall on an average may decrease about −0.5 % during the period 2015–2034 and future summer monsoon rainfall may increase about 0.4 % during the period 2075–2099.

Abstract

Arc shaped precipitation systems are classified to know their seasonal and regional variation in Bangladesh. In this study, six-year (2000-2005) radar data are used from the Bangladesh Meteorological Department. Arc shaped precipitation systems are classified as symmetric type precipitation system (STPS), asymmetric type precipitation system (ATPS), combination of symmetric and asymmetric type precipitation system (CSATPS) and unclassified type precipitation system (UTPS). The occurrence frequency of STPS, ATPS, CSATPS and UTPS is found 23%, 43%, 21% and 13%, respectively. Seasonal analysis showed that ATPS (STPS) is dominant in the pre-monsoon (monsoon) period. Regional analysis of the arc shaped precipitation systems indicate that at the mature stage of their life cycle, STPS, ATPS, and CSATPS is dominated in southwest (northwest), northeast (southeast) and northwest (northwest) quadrants during the pre-monsoon (monsoon) period. The maximum occurrence frequency of UTPS is found in the northeast and the southwest quadrants during the pre-monsoon season and in the southwest quadrant during the monsoon season.

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Meteorological drought events occur in Bangladesh are diagnosed using monthly rainfall and mean air temperature from the surface observations and Regional Climate Model (RegCM) by calculating Standardized Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI) for the period 1961–1990. The historical records of drought event obtained from the Bangladesh Bureau of Statistics and International Disaster Database are used to verify the SPI and PDSI detected events. The SPI and monthly PDSI are obtained for 27 station data across Bangladesh as well as for two subregions over the country. Result based on the observed data shows that regional information is better in drought diagnosis compared to the point information. The regional analysis is able to detect about 80 % of the drought events occurred during the study period. Frequency of moderate drought is higher for all over the country. The SPI calculated from RegCM rainfall shows that the detection of moderate drought events is 10, 7, and 21 % overestimated for 1-, 3-, and 6-month length, respectively, compared to using of observed data. For extreme drought cases, detection is overestimated (underestimated) by 25 % (79 %) for 1-month (6-month) length. The PDSI results for model and observed data are nearly same to SPI calculations. Model monthly PDSI result is overestimated (underestimated) by 29 % (50 %) for moderate (severe) drought events with reference to the observed PDSI. Hence, RegCM output may be useful to detect 3–6-month (monthly to seasonal) length moderate drought events over a heavy rainfall region likely Bangladesh.

Abstract

Regional and seasonal variations of precipitation systems are studied to understand the monsoonal rainfall in Bangladesh using Bangladesh Meteorological Department S-band weather radar Plan Position Indicator scans data and National Centers for Environmental Prediction reanalysis data. Precipitation systems are divided into arc-, line- and scattered-type according to their shape. The arc-type systems are dominant in the all four quadrants with maximum occurrence in the northeastern quadrant during the pre-monsoon period whereas the scattered-type systems are dominant in the southeast, northeast and southwest quadrants during the monsoon period. The arc-type systems are dominant in the northwest quadrant during the monsoon period. In the pre-monsoon period, the arc-type systems may be developed through the interaction of different air masses and the orographic effects in the presence of strong vertical wind shear between the 925 and 500 hPa levels and dry mid level. In the monsoon period, the scattered-type systems may be developed due to plenty of moisture supplied by low-level monsoon winds from the Bay of Bengal and the orographic effects in the presence of little or no remarkable vertical wind shear between the 925 and 500 hPa levels.

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The drought events are diagnosed from the monthly rainfall data of Bangladesh Meteorological Department over Bangladesh using Standardized Precipitation Index (SPI) during 1961-1990. The historical records of drought event obtained from Bangladesh Bureau of Statistical and International Disaster Database are used to verify the SPI results. SPI is calculated at 27 locations over Bangladesh. To see the drought phenomena in regional scale, the SPI is also calculated at 4 sub-regions over the country. It is found that regional information is better in drought diagnosis compared to point information. The regional analysis is able to detect about 87 per cent (13 out of 15) of the drought events occurred during the study period. This study disclosed that the frequency of moderate drought is higher for all over the country. The central, the northern and the southwestern regions are the most severe drought prone area over Bangladesh.

Abstract

Characteristics of precipitation systems in and around Bangladesh are analysed using 6 years of radar data from the Bangladesh Meteorological Department (BMD). Precipitation systems are classified as arc, line, or scattered types according to their shape; then as small, medium, or large according to their length; and as stationary, slow, or fast according to their propagation speed. Arc‐, line‐, and scattered‐type systems are found 230 (29%), 117 (15%), and 442 (56%) times, respectively, from April to September during the analysis period (2000–2005). These arc‐, line‐, and scattered‐type systems have average speeds of 11.0, 7.1, and 5.8 m/s, horizontal lengths of 185, 184, and 268 km, and approximate lifetimes of 4.3, 4.0, and 4.8 h, respectively. Scattered‐type systems dominate in the monsoon period (June, July, August, and September), while arc‐type systems dominate in the pre‐monsoon period (April and May). Line‐type systems occur with nearly equal frequency in both periods. The monsoon systems are large and stationary or slow moving. In contrast, pre‐monsoon systems are small and fast moving. Pre‐monsoon systems propagate southeastward whereas monsoon systems propagate to the northeast, northwest, or southeast. A large number of systems occur in the southern, eastern, and northern parts of Bangladesh. Of the 442 scattered‐type systems, 244 are scattered‐type systems having wide areal coverage (SWAC), which extends out of radar range, and speeds too small to calculate. About 97% of SWACs develop during the monsoon period and contribute greatly to monsoon rainfall in this region.

Abstract

Providing Regional Climates for Impacts Studies (PRECIS) is a regional climate model, which is used for the simulation of regional‐scale climatology at high resolution (i.e. 50‐km horizontal resolution). The calibration of rainfall and temperature simulated by PRECIS is performed in Bangladesh with the surface observational data from the Bangladesh Meteorological Department (BMD) for the period 1961–1990. The Climate Research Unit (CRU) data is also used for understanding the performance of the model. The results for the period 1961–1990 are used as a reference to find the variation of PRECIS‐projected rainfall and temperature in 2071, in and around Bangladesh, as an example. Analyses are performed using the following two methods: (1) grid‐to‐grid and (2) point‐to‐point analyses. It is found that grid‐to‐grid analysis provides overestimation of PRECIS in Bangladesh because of downscaling of observed data when gridded from asymmetric low‐density data network of BMD. On the other hand, model data extracted at observational sites provide better performance of PRECIS. The model overestimates rainfall in dry and pre‐monsoon periods, whereas it underestimates it in the monsoon period. Overall, PRECIS is found to be able to estimate about 92% of surface rainfall. Model performance in estimating rainfall increases substantially with the increase in the length of time series of datasets. Systematic cold bias is found in simulating the annual scale of the surface temperature. In the annual scale, the model underestimates temperature of about 0.61 °C that varies within a range of + 1.45 °C to − 3.89 °C in different months. This analysis reveals that rainfall and temperature will be increased in Bangladesh in 2071. On the basis of the analyses, look‐up tables for rainfall and temperature were prepared in a bid to calibrate PRECIS simulation results for Bangladesh. The look‐up tables proposed in this analysis can be employed in the application of the projected rainfall and temperature in different sectors of the country. These look‐up tables are useful only for the calibration of PRECIS simulation results for future climate projection for Bangladesh.

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Internal dosimetry deals with the measurement of the radiation dose absorbed internally by an organ after the application of isotopes for diagnosis and treatment. In the present study radiation absorbed dose has been calculated for ^99m Tc-pertechnetate, ^99mTc-DTPA, ^99mTc-hepatate and  ¹³¹I (NaI) which are used frequently for functioning test of disordered organs and therapeutic treatment of thyroid in Bangladesh. In these cases a small amount of isotopes are accumulated in other soft tissues like brain, gonads etc. Brain tissues are soft and cannot be regenerated if it is damaged. So, to ensure the safety of brain, the internal radiation absorbed dose has been calculated from direct measurement by using planer image of gamma cameras. International Commission for Radiological Protection (ICRP) and National Commission for Radiation Protection (NCRP) have laid emphasis on direct measurement because it provides more accurate result than that of other methods which are based on few mathematical assumption, bio-kinetic modeling and extrapolation of animal data to human etc. Finally these results have been compared with the data of ICRP publication 71.  

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Measurements are performed for a cobalt beam to determine the extent of leakage and contribution of scattered radiation to have an idea for estimation of dose to points in the body outside the primary beam. It may be several centimeters outside the treatment field and may be as much as 36% of the central axis dose. Doses outside the field were calculated upto distances where until 5% of the central axis dose was obtained. The scattered and leakage radiation show a strong dependence on field size and distance to the beam axis and is predominant only at short distances.

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