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Decadal Variation of Winter 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

M. M. Rahman
Center for Environmental and Geographic Information Services (CEGIS), Dhaka-1212, Bangladesh

Abstract/ Executive Summary:

In the present study, sixty-five years (1948-2012) daily rainfall data of 30 meteorological stations of the Bangladesh Meteorological Department (BMD) have been used to understand the rainfall characteristics during the winter season over Bangladesh. Yearly variations of rainfall and rainy days have been studied to see the long-term variability. The study period is divided into six decades starting from 1950 to investigate the decadal and interdecadal, spatial and temporal variations. The average rainfall is found to be 0.39 mm/day with a standard deviation (SD) of ±0.29 mm/day and the coefficient of variation (CV) is 75.03% during 65 years in the winter season. Average rainy days is found to be 3.30 days with SD of ±1.89 days and CV 57.26%. Both rainfall and rainy days are found to have an annual increasing trend. Rainfall is found highly correlated with rainy days with a coefficient of correlation (CC) 0.85 with a 99.5% level of confidence. Rainfall and rainy days both is found maximum in the fifth decade.

Keywords: Rainfall, Rainy day, Winter, Decadal variation.

Location: Bangladesh Meteorological Department (BMD)

Start Date: 00-00-1948

End Date: 00-00-2012

Program Area: Risk Management in Agriculture

Commodity: Rainfall

Objectives:

Alam et al. (2010) studied the temporal variation of rainfall over the southwestern part of Bangladesh and found an increasing trend of seasonal rainfall. 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 variations of rainfall and rainy days during the post-monsoon season and found an increasing trend in both rainfall and rainy days. In this study, an attempt has been made to study the decadal variation of rainfall and rainy days over Bangladesh during the winter season.

Methodology:

Daily rainfall data of 30 meteorological stations of Bangladesh Meteorological Department (BMD) from 1948 to 2012 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. This is worthy to note December rainfall has been added to the January and February rainfall of the next year for the rainfall of winter. The station-wise rainy days is 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, 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 the winter season (December-February). 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 winter for the individual station. Distributions of rainfall anomaly are studied for each decade over the country during the winter 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 08(1), 2017, 71-78

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) All decades except D3 have similar rainfall distribution patterns and it increased in the northeastern part of Bangladesh. Minimum amount of rainfall is found in the northwestern part of Bangladesh in D3, D4 and D6. ii) The northwestern part of the country is found to have deficit of rain and the northeastern part of the country is found to have excess of rain. iii) Rainfall and rainy days are found to have similar patterns with maximum in D5 and minimum in D3. The maximum rainfall and rainy days are 0.67 mm/day & 5.43 days and the minimum are 0.33 mm/day & 2.14 days respectively. iv) Rainy days are found above normal distribution in D4 and D5 and below normal in D2 and D3 over the country. v) Rainfall and rainy days both are found highly variable with CV 75.03% and 57.26%. The average rainfall is found 0.39 mm/day with SD ±0.29 mm/day and average rainy days is 3.30 days with SD ±1.89 during 65 years in the winter season. vi) 30 years moving averaged rainfall and rainy days both are found to have increasing trends with 0.003mm/day/year and 0.055 days/year with time-variant CC 0.82 and 0.97 respectively at 99.5% level of confidence. 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 but the increasing rate of rainy days is significant with 97.5% confidence level. Rainfall is found to be highly correlated with rainy days with CC 0.85 with 99.5% level of confidence.

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