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Spatio-temporal Variation of Rainfall During Post-monsoon Season Over Bangladesh

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
SAARC Meteorological Research Centre (SMRC), E-4/C, Agargaon, Dhaka-1207, Bangladesh

Abstract/ Executive Summary:

In the present study, daily rainfall data during 1948-2012 of 30 meteorological stations of Bangladesh Meteorological Department (BMD) have been used to understand the rainfall characteristics during the postmonsoon 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 interdecadal spatial and temporal variations. Average rainfall has been found to be 3.53 mm/day with a standard deviation of ±1.52 mm/day and coefficient of variation of 42.91% during 65 years in the postmonsoon season. Average rainy days have been found to be 10 days with a standard deviation of ±3.11 days and a coefficient of variation of 31.12%. 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 and rainy days both have been found maximum in the last decade.

Keywords: Rainfall, Rainy day, Post-monsoon, Decadal variation.

Location: Department of Physics, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Start Date: 00-00-1948

End Date: 00-00-2012

Program Area: Resource Development and Management

Commodity: Rainfall

Objectives:

In this study, an attempt has been made to study the spatial and temporal variation of rainfall and rainy days over Bangladesh during post-monsoon season.

Methodology:

Daily rainfall data of 30 meteorological stations of the Bangladesh Meteorological Department (BMD) from 1948 to 2012 have been used in this study. Data have been collected from the Climate Division of BMD, Agargaon, Dhaka. It is important to note that some of the stations started its operation after 1948 and some data were not available in some stations for a few years. These data have been considered as missing and have not been used in statistical computations.

The daily rainfall data have been used to obtain monthly and seasonal mean. The station-wise rainy days have been identified (if there is any rain) and accumulated for the season. Yearly variations of the rainfall and rainy days along with the trend lines, standard deviation and coefficient of variation have been studied with the standard statistical technique. Five years of moving averages have been done for both rainfall and rainy days and their variations have also been studied.

To study the decadal variations of the studied parameters the study period has 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. From seasonal average rainfall decadal average has been made for each station for each decade during the post-monsoon season (October-November). 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 these 60 years, average rainfall; anomalies (Decade average-60 years station average) have been calculated for each decade during post-monsoon for the individual stations. Distributions of rainfall anomaly have been studied for each decade over the country during the post-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 interdecadal variation of rainfall and rainy days over the country.

Source of Information: Journal of Engineering Science 06(1&2), 2015, 99-104

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Funding Source:

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Achievement/Findings:

On the basis of the study presented in this paper, the following conclusions have been drawn: i. The distribution of rainfall during the post-monsoon season over the country has been found similar for decade1 (D1), decade4 (D4) and decade5 (D5). Rainfall has been found to increase from west to east in the western part of the country while it is found to increase towards the northeast direction in the northeastern part of the country. ii. The western part of the country is found to be a deficit in the rain and the coastal regions have found excess rain in all the decades. iii. During sixty years decade2 has been found to be a deficit in rain and decade6 is found to have an excess of rain. iv. Sixty years average rainfall have been found to increase from west to eastward direction and it increased towards the northeast and southeast coastal region through the central part of the country. v. Minimum amount of rainfall is observed in the western part of the country and the maximum in the coastal belt of Khepupara, Hatiya and Sandwip region. vi. The minimum amount of rainfall is found in decade2 (2.96 mm/day) and maximum in decade6 (4.16 mm/day) with an average of 3.57 mm/day during 1950 – 2009. vii. The decadal average rainfall during the post-monsoon season is found to have an increasing trend with 0.092 mm/day/decade with the coefficient of determination 0.158. In the last 5 decades, the increasing trend became 0.267 mm/day/decade with the coefficient of determination 0.905 or the time-variant coefficient of correlation 0.95 with a 99% level of confidence. viii. Both rainfall and rainy days have been found to have a linear increasing trend with 0.008mm/day/year and 0.056days/year respectively. Although the increasing rate of rainfall is not statistically significant the increasing rate of rainy days is highly significant with a 99.5% confidence level.

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