The distribution of rainfall over the country has been studied during pre-monsoon, monsoon and post-monsoon seasons for each decade.
Introduction
It is characterized by very flat plains, which dominate most of the land and never rise more than 10 meters above sea level. The production of various types of crops, especially rice, is extremely dependent on the amount of adequate rainfall. In South Asia, the onset of the summer monsoon generally progresses to the northwest and retreats to the southeast [Mooley and Shukla, 1987].
In Bangladesh, the onset of summer monsoon begins in the south-eastern part of the country in early June, progresses towards the north-west and reaches the north-eastern part by mid-June. These values are based on observations at Bangladesh Meteorological Department (BMD) stations, and rainfall actually exceeds 5000 mm near the Shillong Plateau [ Ohsawa et al ., 1997 ]. 26461.2 mm, August 1860-July 1861) is located on the southern slope of the Shillong plateau and is about ten kilometers from the border with Bangladesh.
The seasonal rise in water levels of the major rivers flowing from the vast monsoon areas causes flooding in much of Bangladesh and about one-fourth to one-third of the country is inundated for a relatively long period of time each year during the monsoon season. .
Review of Monsoon
- Sea level pressure
- Surface winds
- Surface Temperatures
- Rainfall
- Pre-monsoon
- Monsoon
- Post Monsoon
The adjacent land area and the Arabian Sea and the Bay of Bengal are treated to the extent that they are important for understanding the monsoon characteristics over India. To summarize, the main features of the surface pressure distribution in the monsoon season are the heat low over Pakistan, the monsoon trough up to the Head of the Bay of Bengal, and the strong pressure gradient to the south. South of the outer lines occur over West Bengal and south-western countries in western countries elsewhere.
Over most of the Bay of Bengal and the rest of the Arabian Sea, the speed is about 15kt. Area south of this rainfall minimum falls in the track of monsoon depressions which are responsible for much of the rainfall. The summer low over northwest India and Pakistan intensifies in June and extends its trough into Bangladesh and adjacent north of the Bay of Bengal.
The surface wind changes to southerly direction in the southern and central districts and from southeast to east over the northern districts of the country.
Data and Methodology
The distribution of rainfall anomalies are studied for each decade over the entire country during pre-monsoon, monsoon and post-monsoon separately with the help of SURFER software. Accumulated annual rainfall has been studied across the country to observe long-term rainfall variation. The average daily rainfall over the country has also been studied during the pre-monsoon, monsoon and post-monsoon seasons.
The daily average precipitation obtained from 50 years of precipitation data for each season was averaged for all 30 stations. Precipitation anomalies (50 years average for each station - 50 years average for all stations) were then calculated for pre-monsoon, monsoon and post-monsoon. Finally, an average daily rainfall has been recorded from March to November for each decade and also for 50 years, and then the distribution of this average rainfall has been studied.
Results and Discussion
Long Term Variation of Rainfall over Bangladesh
The minimum average rainfall is found during Decade 2 and the maximum average rainfall is found during Decade 1.
Pre-monsoon 1. Average rainfall
- Decade2
- Decade3
- Deeade4
- DeeadeS
- Rainfall anomaly
The minimum average rainfall of 2 mm1 day is distributed in the northwestern part of Bangladesh and increases up to II mm1 day in the northeastern part of the country. The average maximum rainfall distribution in the southeastern part of the country is 4 mm/day in the south and 6 mm/day in the north. The distribution of mean rainfall (mm/day) during pre-monsoon over Bangladesh for Decade 4 is shown in Fig.
From the western part of the country, the average rainfall increases slowly towards the central part of the country. This distribution starts from the northwestern part of the country and extends up to the southeastern part of the country. Starting from 3 mm1day in the western part of the country, rising up to 15 mm1day in the northeastern part of the country.
In the southeastern part of the country, the average amount of precipitation increases from south to north. The distribution of average pre-monsoon rainfall (mm/day) over Bangladesh for decades is shown in the figure. In the western part of the country, the same line of average rainfall runs from south to north.
The same average rainfall line through the central region starts from the north-northwest part of the country and extends to the south-southeast part of the country. A maximum curvature is found in the 5 mm/day rainfall line passing through the central area. From 2 mm/day in the western part of the country, the average precipitation increases to 13 mm/day in the northeastern part of the country.
Average rainfall increases slowly from west to the central part of Bangladesh and increases rapidly to the northeastern region. In the southeastern region of the country, rainfall increases from south to north.
INDIA
- Deeadel
- Deeade2
- Deeade3
- Deeade4
- DecadeS
- Monsoon
- Average rainfall 1 Decade1
The rainfall anomaly (mm/day) during pre-monsoon over Bangladesh for Decadel is shown in Figure. In the southeastern part of the country, the rainfall anomaly lines run in the north-south direction. Precipitation anomalies (i.e. decreases in rainfall) increase from west to east in this region.
Pre-monsoon rainfall anomaly (mm/day) over Bangladesh for Decade 2 is shown in Fig. The rainfall anomaly increases (i.e. rain excess) towards the northwest and increases negatively towards the east in the northeast region. Pre-monsoon rainfall anomaly (mm/day) over Bangladesh for Decade 3 is shown in Fig.
The precipitation anomaly in the northeastern part of the country increases negatively (ie, the amount of precipitation decreases) toward the northeast. In the southwestern part, the largest negative precipitation anomaly is -0.5 mm/day, in the northwestern part -1.1 mm/day and in the northeastern part -1.4 mm/day. The distribution of rainfall anomaly (mm/day) during pre-monsoon over Bangladesh for decade 4 is shown in Fig.
In the coastal region in the south-western part of the country, the rainfall anomaly is negative (deficit in rain). The distribution of rainfall anomaly (mm/day) during the pre-monsoon over Bangladesh for decades is shown in Fig. In most places over the country, rainfall anomalies are within -0.1 to 0.5 mm/day.
The largest precipitation anomaly was observed in the eastern part of the southeastern region with a value of 2.6 mm/day. From the central part of the country (Dhaka, Faridpur, Tangail), the rainfall increases towards the northeastern region.
BAY OF BENGAL
Decade3
The distribution of mean rainfall (mm/day) during monsoon over Bangladesh for decade 3 is shown in Fig. The amount of precipitation increases rapidly in the north-eastern region of the country from the south-west to the north-east and shows a maximum of 28 mm/day.
Decade4
DecadeS
BAY OF BENGALMonsoon (Decade3)
INDIAINDIA
BAY OF BENGALMonsoon (Decade4)
INDIA23
34;BAY OF BENGAL"
89INDIA
Deeadel
In the western and most of the central part of the country there is practically no abnormal rainfall. In the southeastern hilly region of Bangladesh, rainfall increases negatively from west to east.
Deeade2
Deeade3
Decade4
DecadeS
Post-monsoon .1 Average rainfall
- Decade2
- Decade3
- Decade4
- DecadeS
- Rainfall anomaly
- Decadel
- Decade2
- Decade3
- Decade4
- DecadeS
In the southeastern hilly region of the country, the average rainfall increases from north to south. Distribution of average rainfall (mm/day) during post-monsoon over Bangladesh for Decade3 is shown in Fig. Distribution of average rainfall (mm/day) during post-monsoon over Bangladesh for Decade4 shown in Fig.
The distribution of mean rainfall (mm/day) over Bangladesh during post-monsoon is shown in Figure. The post-monsoon rainfall anomaly (mm/day) over Bangladesh for ten years is shown in Figure. The precipitation anomaly (mm/day) during the post-monsoon monsoon over Bangladesh during Decade2 is shown in Fig.
The precipitation anomaly increases negatively to the south of this line in the southern part of the country. The precipitation anomaly increases positively (precipitation increases) to the east of this line in the southeastern hilly region of the country. Distribution of rainfall anomaly (mm/day) during post-monsoon over Bangladesh in decade3 is shown in fig.
Rainfall anomaly (mm/day) during post-monsoon over Bangladesh for Decade4 is shown in Fig. Rainfall anomaly (mm/day) during post-monsoon over Bangladesh for Decade5 shown in Fig. In the southeastern hilly region of the country rainfall anomaly increases from south to north direction.
In the southern coastal region of the country, rainfall anomalies increase positively (i.e. rainfall increases) towards the south. In the northeastern region of the country, rainfall anomalies increase positively in the north-northeastern direction. From this line, the average rainfall increases in a north-easterly direction in the north-east part of the country.
BAY OF BENGALINDIA
The distribution of average rainfall (mrn/day) in Bangladesh from March to November for DecadeS is shown in Fig. In many places in the central, eastern and western part of the country, the average rainfall is only 6 rnm/day. Rainfall also increases towards the coastal region. In the southeastern part of the country, rainfall increases from north to south.
Chapter-5
Conclusions
Finally, 50-year data analysis shows that rainfall in Bangladesh increases in the northeastern region, decreases in the western region and is almost static in the central region (Fig. 5.1). Karmakar, S., and Khatun, A., A statistical study of highest monsoon rainfall in 24 hours in Bangladesh, Proceedings of the seminar on predictability of monsoon rainfall and floods, Dhaka, June. Matsumoto, J., Synoptic characteristics of 1987 heavy monsoon rains associated with severe flooding in Bangladesh, Bull.
Bernard, and Arkin, Philip A., Spatial and interannual variation in the diurnal cycle of large-scale tropical convective cloud precipitation", Monthly Weather Review, VoLlI. SLI and NRC (Shawinigan Lavalin Inc. and Northwest Hydraulic Consultants), Surface Water Resources of the northeastern region, a report of the Northeastern Regional Water Management Project, 1993. Weare, B.C., The effect of diurnal variation of curnulus convection on large-scale low-frequency oscillation in the tropics, J.
