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Spatio-temporal Variation of Pre-monsoon Rainfall and Rainy Days 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

Abstract/ Executive Summary:

In the present study sixty-eight years (1948-2015) daily rainfall data of 30 meteorological stations of Bangladesh Meteorological Department (BMD) have been used to understand the rainfall characteristics during the pre-monsoon season over Bangladesh. Yearly variations of rainfall and rainy days (if there is any rainfall in a day is considered a rainy day) 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 4.66 mm/day with a standard deviation (SD) of 1.50 mm/day and coefficient of variation (CV) of 32.25% during 68 years in the pre-monsoon season. Average rainy days have been found 21.56 days with SD 5.39 days and CV 25.02%. Both rainfall and rainy days have been found to have an annual increasing trend. The amount of rainfall has been found highly correlated with rainy days with the coefficient of correlation 0.82 with a 99.5% level of significance. The maximum amount of Rainfall observed in the fourth decade and rainy days in the sixth decade.

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

Location: In Bangladesh

Start Date:

End Date:

Program Area: Knowledge Management

Commodity: Rainfall

Objectives:

Ahmed et al. (1996) and Reza et al. (2004) worked on seasonal rainfall variation over Bangladesh and found a negative correlation with ENSO and southern oscillation index (SOI) respectively. Reza et al. (2015) studied the spatial and temporal variation of rainfall and rainy days during the post-monsoon season and found an increasing trend of both rainfall and rainy days. Reza et al. (2017) also studied on winter rainfall and got similar results. In this study, an attempt has been made to study the spatial and temporal variation of rainfall and rainy days over Bangladesh during the pre-monsoon season.

Methodology:

Daily rainfall data of 30 meteorological stations of the Bangladesh Meteorological Department (BMD) from 1948 to 2015 is used in this study. Data have been collected from the Climate Division of BMD, 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 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. Statistical significance is calculated with the student’s t distribution.

To study the decadal variations of the studied parameters the study period is 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 is made for each station for each decade during pre-monsoon season (March-May). The spatial distribution of rainfall is studied for each decade. To understand the climatological variation of rainfall over the country 60 years on average (1950-2009) are made for each station. From these 60 years, average rainfall; anomalies (Decade average-60 years station average) are calculated for each decade during pre-monsoon for the individual station. Distributions of rainfall anomalies are studied for each decade over the country during the pre-monsoon season. Similar works are done with respect to 60 years of country average rainfall. Rainfall anomaly (60 years average for each station - 60 years average for all stations) is calculated. The distribution of this rainfall anomaly has also been studied during this season. The decadal average is also computed to study the interdecadal variation of rainfall and rainy days over the country.

Source of Information: Journal of Engineering Science 09(1), 2018, 21-27

Article Link (Online Journal):

Funding Source:

1.

Budget:

Total Budget (Tk.) :

Achievement/Findings:

On the basis of the study presented in this paper, the following conclusions have been drawn: i) The rainfall distribution pattern is similar in all the decades and it increased in the northeastern part of Bangladesh. The minimum amount of rainfall found in the Midwestern part of Bangladesh in D2, D5, and D6. ii) The west and northwestern part of the country is found to be a deficit of rain and the northeastern part of the country is found to have excess rain. iii) Rainfall and rainy days are found to have a similar pattern with the minimum in D2. The maximum rainfall and rainy days were 5.21 mm/day in D4 & 23.72 days in D6 and the minimum was 3.63 mm/day & 18.16 days in D2 respectively. iv) Rainy days have been found above normal distribution in D4-D6 and below normal in D1-D3 over the country. v) Average rainfall has been found 4.66 mm/day with SD 1.50 mm/day and CV 32.25% and average rainy days were 21.56 days with SD 5.39 and CV 25.02% during 68 years in the pre-monsoon season. vi) 30 years moving averaged rainfall and rainy days both have been found to have an increasing trend with 0.022 mm/day/year and 0.146 days/year with time-variant CC 0.82 and 0.97 respectively at 99.5% level of significance. vii) 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 97.5% level. viii) Rainfall is found to be highly correlated with rainy days with CC 0.82 with a 99.5% level of significance.

Knowledge Management: Journal