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Decadal and Inter-decadal Variation of Rainfall and Rainy Days Over Bangladesh During Monsoon Season

M. K. H. Reza*
Department of Physics, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

M. M. Alam
Department of Physics, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

M. M. Rahman
Save Earth Climate Services Ltd. (SECL), Dhaka-1216, Bangladesh

Abstract/ Executive Summary:

In the present study, daily rainfall data of 30 meteorological stations of Bangladesh Meteorological Department (BMD) during 1948-2015 has been used to understand the rainfall characteristics during the monsoon season over Bangladesh. Yearly variations of rainfall and rainy days have been studied to see the long-term variability. The study period has been divided into six decades starting from 1950 to investigate the decadal and Inter-decadal spatial and temporal variations. Average rainfall has been found to be 14.25 mm/day with a standard deviation of 1.77 mm/day and a coefficient of variation of 12.43 % during 68 years in the monsoon months. Average rainy days have been found to be 77.58 days with a standard deviation of 6.56 days and a coefficient of variation of 8.46%. Both rainfall and rainy days have been found to have an annual increasing trend. Rainfall has been found to increase from west to eastward direction and it increased further from the central region to the northeast and southeast direction. Rainfall has been found maximum in the last decade and rainy days in the fifth decade.

Keywords: Rainfall, Rainy day, Monsoon, Decadal variation

Location: In Bangladesh

Start Date: 00-00-1948

End Date: 00-00-2015

Program Area: Knowledge Management

Commodity: Rainfall

Objectives:

The objective of this article is to study decadal and inter-decadal variation of rainfall and rainy days over Bangladesh during monsoon season.

Methodology:

Daily rainfall data of 30 meteorological stations of the Bangladesh Meteorological Department (BMD) from 1948 to 2015 have been used in this study. Data have been collected from the Climate Division of BMD, Agargaon, Dhaka. The BMD observation network density is low, in some cases observation sites are located about 25km apart, whereas these are about 145 km apart in some other areas. It is important to note that some of the stations started their journey after 1948 and some data were not available in some stations for a few years. These data have been considered missing and have not been used in statistical computations.

The daily rainfall data is used to obtain monthly and seasonal mean. The station-wise rainy days (if there is any rainfall in a day) is identified and accumulated for the season. Yearly variations of the rainfall and rainy days along with the trend lines, standard deviation, coefficient of variation, and time-variant coefficient of correlation are studied with the standard statistical technique.

To study the decadal variations of the rainfall, the study periods have been divided into six decades starting from 1950. 1950-1959 has been designated as Decade1 (D1), 1960-1969 as Decade2 (D2), 1970-1979 as Decade3 (D3), 1980-1989 as Decade4 (D4), 1990-1999 as Decade5 (D5) and 2000-2009 as Decade6 (D6). Hereafter different decades will be used as D1, D2, D3, D4, D5 and D6, respectively. From seasonal average rainfall decadal average has been made for each station for each decade during monsoon season (June-September). The spatial distribution of rainfall has been studied for each decade. To understand the climatological variation of rainfall over the country, 60 years average (1950-2009) has been made for each station. From this 60 years average rainfall, rainfall anomalies (Decade average – 60 years station average) have been calculated for each decade during monsoon for the individual stations. Distributions of rainfall anomaly have been studied for each decade over the country during the monsoon season. Similar works have been done with respect to 60 years of country average rainfall. Rainfall anomaly (60 years average for each station - 60 years average for all stations) has been calculated. The distribution of this rainfall anomaly has also been studied during this season. A decadal average has also been made to study the inter-decadal variation of rainfall and rainy days over the country.

Source of Information: Journal of Engineering Science 10(1), 2019, 59-66

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

On the basis of the study presented in this paper, the following conclusions have been drawn: i) In all the Decades minimum amount of rainfall is found in the Midwestern part of Bangladesh have and it has increased in the northeastern and southeastern coastal regions of Bangladesh. ii) The western and central parts of the country are found to have a deficit of rain and the northeast and southeastern parts of the country are found to have an excess of rain. iii) Minimum rainfall is found in D1 and maximum rain in D6 while minimum rainy days are in D3 and maximum in D5. The maximum rainfall and rainy days are 14.81 mm/day & 83.02 days and the minimum is 13.26 mm/day and 69.09 days respectively. iv) Average rainfall has been found 14.25 mm/day with SD 1.77 mm/day and CV 12.43% and average rainy days were 77.58 days with SD 6.56 and CV 8.46% during 68 years in the monsoon season. v) Both rainfall and rainy days are found to have an increasing trend with 0.028 mm/day/year and 0.259 days/year with a time-variant coefficient of correlation of 0.96 and 0.97 respectively at a 99.5% level of significance. vi) Variability of rainfall and rainy days shows that both the parameters have an annual increasing trend. Although the increasing rate of rainfall is not statistically significant the increasing rate of rainy days is significant with a 99.5% level. vii) Rainfall is found to be positively correlated with rainy days with the coefficient of Correlation 0.39with 99.5% level of significance.

Knowledge Management: Journal