This is to confirm that the thesis titled "Study of Rainfall Distribution over Bangladesh Associated with Tropical Disturbances in the Bay of Bengal" submitted by Morsheda Ferdousi has been approved by the Board of Examiners in partial fulfillment of the requirements for the Master of Philosophy in the Department of Physics of the University of Engineering and of Technology Khulna, Khulna, Bangladesh on February 22, 2009. Mahbub Alam, Department of Physics, Khulna University of Engineering and Technology and my joint supervisor Prof. Dr. Mohammad Arif Hossain, Department of Mathematics, Khulna University of Engineering & Technology, for their constant guidance, excellent cooperation, fruitful comments and suggestions and encouragement for proper and smooth completion of the research work.
Rain, one of the three means of destruction due to the tropical cyclone, is closely associated with the tropical cyclone. With this information in mind, in this thesis we have studied the rainfall distribution over Bangladesh with the movement of the tropical disturbances in the Bay of Bengal that crossed the Bangladesh coast. The main objective of this study is to identify the relationship between the amount of rainfall observed in various meteorological stations across Bangladesh and the track of the tropical disturbances affecting Bangladesh.
Isolines were drawn to identify the rainfall pattern on the Bangladesh map along with the disturbance trace. Then, these numbers are analyzed to determine the relationship between rainfall and the perturbation track.
CHAPTER ONE Introduction
INTRODUCTION
Bangladesh stands at the northern end of the Bay of Bengal, which is part of the northern Indian Ocean. The North Indian Ocean is one of the seven basins in which tropical cyclones form. Most of the tropical cyclones (about 87%) that occur in this basin originate in the Bay of Bengal, while the rest originate in the Arabian Sea.
Although only about 7% of global tropical cyclones form in the Bay of Bengal, they are the most lethal (Neumann, 1993). Analysis of the crossing of the tropical cyclones through different coastal boundaries of the Bay of Bengal in different seasons Alam et al. 2003) discussed the vulnerability of different seasons. Fletcher (1955) has given a very simple formula (Vmax = 1 7J- F) in terms of the pressure drop to estimate the wind speed.
Using the image method, Choudhury (1978) proposed that the cyclone track in the Bay of Bengal follows the rose petal equation. The main objective of cyclone forecasting is to predict the cyclone track and storm intensity.
CHAPTER TWO Literature Review
LITERATURE REVIEW
- Introduction
- The Mature Cyclone
- Pressure
- Cloud Pattern
- Wind fields
- Precipitation
- The eye of the cyclone
- Tropical Cyclone Formation
- Climatological Conditions for Tropical Cyclone Formation
- Large Scale Conditions associated with Tropical Cyclone Formation
- Pre-existing disturbance
- Curved cloud features
- Mid-level mesoscale vortex
- Interaction with upper-level disturbances
- Tropical Cyclone Tracks
- Cyclone track prediction
- Rainfall distribution around a tropical cyclone
- Rainfall Analysis and Forecasting
- Tropical Cyclone Speed
- Tropical Cyclone Intensity
- Tropical Cyclone Warnings
- Prevention
There is no uniform terminology to describe these stages across different parts of the world. An unusual pressure drop over 24 hours of 2 - 3 hPa or more occurs at the center of the vorticity concentration. Subsequent studies using improved observation systems have led to further refinement and detail in the documentation of the tropical cyclone formation process.
A necessary condition for cyclone formation is a large-scale environment with a small vertical shift of the horizontal wind. An early indication that cyclone formation has begun is the appearance of curved band features of the deep convection in the incipient disturbance. One of the environmental factors related to the mean seasonal and geographic distributions of tropical cyclone frequency is small vertical shear of the horizontal wind.
Although only about 7% of global tropical cyclones (Neumann, 1993) occur in the North Indian Ocean, they are the deadliest. That is, the intermediate period of the summer monsoon is a relatively suppressed period of tropical cyclone formation. There are no topographical features within the tropical cyclone's track that are significant enough to appreciably change rainfall.
Relatively low persistence values are also found over the recurring latitudes of the North Atlantic and the western North Pacific basin.
CHAPTER THREE Met/i odology
METHODOLOGY
- General Procedure
- Classification of Tropical Cyclones
- Classification of Rainfall
Rainfall data as well as Bay of Bengal cyclonic events are required for our study purpose. We have collected rainfall data for different cyclonic periods from the Climate Division of the Bangladesh Meteorological Department (BMD). For a given station, the database consists of the year, month and daily amount of precipitation.
Using BMD, a database of the cyclonic events in the Bay of Bengal has been created. Since we are only interested in analyzing the events through which the Bangladesh coast of the Bay of Bengal is affected, we have sorted the database to select only these events. For sorting purposes, the events that hit between 88 and 92,750 east longitude were selected first.
A computer program is then developed to create tables showing the amount of precipitation from the precipitation database for all stations for the previously identified dates. The selection of dates takes into account the date on which the specific event was tracked by BMD until it crossed the coast. For individual events, a table is drawn up with different stations, for the individual days and the amount of precipitation.
For the purpose of studying the rainfall distribution over Bangladesh, these tables are used to plot the spatial distribution of rainfall using the software surfer. We selected 17 cyclonic events of varying intensities that formed in the Bay of Bengal and crossed the Bangladesh coast.
CHAPTER FOUR Results and Discussions
On the 3" day, moderate rainfall occurred at Comilla as the deep trough turned into CS in the Bay and the direction of movement of the cyclone was northwest. Moderate rain occurred at Syihet on the day when the depression formed in the Bay of Bengal and the direction of movement. On 41 hours moderate heavy rainfall occurred at Teknaf and moderate rain at Cox's Bazaar and light rain occurred at Kutubdia, Khepupara, Sitakundu, Sandwip regions and the direction of cyclone movement was towards North.
Moderate rainfall occurred in Srimongal on the day when the Bay of Bengal depression formed at 130N, 850E and the direction of cyclone movement was towards northeast. On the second day moderate rainfall occurred in Cox's Bazaar, Teknaf regions and light rainfall in Khepupara and the direction of movement of the cyclone was northward. On 19 November 1992, light rain fell over Kutubdia, Cox Bazaar, Chittagong, Rangamati districts and the direction of cyclone movement was towards north-northeast.
At 0, light rainfall occurred in the Teknaf, Cox's Bazaar regions during the day, and the cyclone's direction of movement was the same as that of the previous day. Moderate rainfall occurred at Faridpur on the first day when a depression formed in the Bay of Bengal at 10°N, 890E and the direction of movement was north-northwest. On 2 days, light rainfall occurred in Dhaka and Hatiya and the cyclone's direction of movement was towards the north.
Light rainfall occurred in Teknaf on the day when a depression formed in the south-central Bay of Bengal at 11°N, 85.50E and the direction of movement of the cyclone was towards the north. Moderate rain occurred in Cox's Bazaar, Rangamati and Dhaka on the day when a depression formed in the Bay of Bengal and the cyclone's direction of movement was north-northeast. Very heavy rainfall occurred in Rangpur on the first day when a depression formed in the Bay of Bengal and the direction of movement of the disturbances was northwards.
On the fourth day very heavy rainfall occurred in Dhaka, Cornilla, Madaripur, Feni, Khulna, Barisal, Bhola, Maijdi Court, Sandwip. Chittagong, Khepupara, Kutubdia, Cox's Bazaar and heavy rainfall occurred in Srimongal, Faridpur, Hatiya, Rangamati, Teknaf and the direction of movement of disturbance towards northeast. On the third day, the system crossed the Chittagong coast and Syihet region experienced moderate to heavy rains.
Moderate heavy rainfall occurred at Kutubdia and moderate rainfall occurred at Hatiya, Teknaf regions on 1st day as depression formed in Bay of Bengal at 1 3°N, 850E and the direction of movement of the system was westward. On the 4th day, the depression crossed the Noakhali-Chittagong coast as a deep depression and very heavy rainfall occurred at
CHAPTER FIVE
CONCLUSIONS
The study of rainfall distribution for cyclonic disturbances of the Bay of Bengal making landfall to Bangladesh has been done. The station-wise rainfall for 30 stations of Bangladesh during the life cycle of the disturbances was used. The rainfall distributions for the individual dates of the disturbances show that the maximum rainfall occurred on 5 June 1980 due to the depression at Chittagong.
The study shows that heavy and very heavy precipitation occurs in the front right quadrant of the disturbances. It may be worth noting that we only used data for the Bangladesh region, but we do not use rainfall data for the Bay of Bengal, India and Myanmar regions. Rainfall data at least over the Bay of Bengal regions is needed for better cyclone track forecasting.
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Liebmann, 1991: Interannual variability of the onset of the Australian summer monsoon: possible influence of the Indian summer monsoon and El Niño, Journal of Climate. Karmakar S., (Shrestha, ML ed.), 1998: Historical records of tropical cyclones making landfall in the coastal areas of SAARC region, The impact of tropical cyclones on the coastal areas of SAARC countries and their influence in the region , SMRC -No. Leftwich, P.W., and C.J., Neumann, 1977: Statistical guidelines for the prediction of eastern North Pacific tropical cyclone motion, part II.
Fraedrich, 1990: Reduction of tropical cyclone position errors using an optimal combination of independent forecasts, Wea. L Elsberry, 1993: Observations during TCM-92 of the role of mesoscale tropical convective systems in tropical cyclogenesis. Raper, S., 1992: Observational data on the relationship between climate change and the frequency and magnitude of severe tropical storms.
