Using the Constant Pressure (C.P) chart, upper air anticyclonic axis position at 200 and 300 hPa levels was studied. Then we tried to identify how the monsoon rainfall is correlated with the movement of the upper air anticyclonic axis position. We found from the analysis that rainfall over Dhaka, Barisal, Chitlagong, Comilla, Dinajpur, Khulna, Mymensingh, Rajshahi, Seemangal and Syihet station occurred when the upper air anticyclonic axis of 200 and 300 hPa levels near the stations were.
When the anticyclonic axis at 200 and 300 hPa levels moved continuously from north to south or south to north, rainfall occurred over all the stations either during this duration or at the end of this duration during the monsoon season. In the 15th half of June the anomalies for the anticyclonic axis position were below the mean position and in the 2nd half the axis was above the mean position for 200 and 300 hPa levels. The correlation coefficient between the daily mean rainfall in the monsoon season and the upper air anticyclonic axis position has been investigated and we observed that the correlation coefficient between them is not significant for 200 and 300 hPa levels.
Sylhet station 47 3.4 Relationship between rainfall and anticyclonic axis position at 200 hPa level 48 3.4.1 Relationship between rainfall and axis position w.r.t. Sylhet station 63 3.5 Relationship between rainfall and anticyclonic axis position at 300 hPa level 65 3.5.1 Relationship between rainfall and axis position w.r.t.
INTRODUCTION
Rainfall
The rainfall over Bangladesh during the 1995 summer monsoon season has been studied by Ohsawa et al., (2000) with respect to the intraseasonal variation of monsoon activities. The rainfall over Bangladesh is basically dominated by the north-south oscillations of the monsoon trough. The area south of this rainfall minimum falls in the wake of monsoon depressions, which are responsible for much of the rainfall.
In the northwestern parts of the subcontinent, rainfall decreases gradually westward, from 40 cm in Rajasthan to 5 cm in Baluchistan. At the south-eastern tip of the peninsula, where it is not the rainy season, it is 100 per cent. The spatial relationships between the seasonal rainfall and the position of the Subtropical Ridge (STR) over the Indian and Western Pacific regions, the Darwin Pressure Tendency (DPT) and the Northern Hemisphere Surface Temperature (NHST) - reveal that within the Asian monsoon regime there are not only regions, which is in phase with Indian monsoon rains, but there are also regions which are out of phase (Kripalani and Kulkarni, 1998).
On the southern slopes of the Khasi-Jaintia Hills, rainfall is more than 8000 mm, while in the north, in the Brahmaputra valley, it falls to about 1200 mm. In the higher parts of the Western Ghats there are places with seasonal rainfall of 5000 mm.
Anticyclone
In the upper troposphere, strong southerly wind anomalies over India indicate a weakening of the tropical easterly jet. This movement of the Tibetan anticyclone during inactive monsoon periods has been observed in observations (Krishnamurti et al, 1989; Raman and Rao, 1981). The flickering of the Tibetan Plateau, which is at the level of the mid-troposphere, produces a high anticyclone.
The effect of this cell is to reverse the normal temperature and pressure gradients in the layers between 600 and 300 mb. At that time, the subtropical anticyclone was located in the upper troposphere over northern India, between 200 and 250 latitude, well south of the Tibetan Plateau. The streamlines spiraling toward the center of the low pressure intersect the contours at an acute angle, especially in the southern half between 700 mb and 500 mb.
We also tried to investigate the anomaly of the position of the anticyclone axis in the upper part of the air for different months of the monsoon season at the considered levels. Our study also includes the calculation of the correlation coefficient (CC) between the daily mean rainfall at different stations over Bangladesh during the monsoon season and the position of the anticyclonic axis in the upper air at these considered levels.
Monsoon trough
This is because a pre-existing near-surface disturbance with sufficient vorticity and convergence is one of the six requirements for tropical cyclogenesis. There appears to be a 15- to 25-day cycle of storm activity associated with the monsoon trough that is about half the wavelength of the Madden-Julian Oscillation, or MJO. This reflects tropical cyclone genesis in the vicinity of these features, with genesis clustering into 2-3 weeks of activity followed by 2-3 weeks of inactivity.
Indeed, tropical cyclones can form in bursts around these features under special circumstances and follow the next cyclone to the north and west. This is in contrast to the Atlantic Ocean, where tropical cyclones mostly form from tropical waves moving off the coast of Africa. Tropical cyclone formation in the eastern Pacific shows a hybrid of these two mechanisms [http://en.wikipedia.org/wiki/Monsoon_trough].
METHODOLOGY
- Constant Pressure Chart
- Curve Fitting
- The Straight Line
- The Method of Least Squares
- The Least-Squares Line
- Mean
- Deviation
- Data collection and Analyzed stations
Equation (5) implies that y = 0 when x = 0; so the least-squares line passes through the point (X,Y), called the centroid, or center of gravity, of the data. The arithmetic mean or simply the mean, is the central value of the items in a series. It is obtained by dividing the total value of the items in a series by the number of items.
The ratio of the explained variation to the total variation is called the coefficient of determination. If there is zero explained variation (ie, the total variation is unexplained), this ratio is 0. If there is zero unexplained variation (ie, the total variation is all explained), the ratio is 1.
In other cases, the ratio lies between 0 and 1. Since the ratio is always non-negative, we can denote it by r 2. The quantity r, called the correlation coefficient or short correlation coefficient, is given by. The total variation of Y is defined as -Y)2; that is, the sum of the squares of the deviations of the values of Y from the mean of Y.
RESULTS & DISCUSSION
Distribution of rainfall in monsoon season
- Distribution of rainfall at Dhaka station
- Distribution of rainfall at Barisal Station
- Distribution of rainfall at Chittagong Station
- Distribution of rainfall at Comilla Station
- Distribution of rainfall at Dinajpur Station
- Distribution of rainfall at Khulna Station
- Distribution of rainfall at Mymensingh Station
- Distribution of rainfall at Rajshahi Station
- Distribution of rainfall at Sreemangal Station
- Distribution of rainfall at Sylhet Station
The rest of the days of the monsoon season, the axis was oscillating around the station. 1 week of September rainfall occurred over Dhaka station when the axis was close to the station.
Correlation Coefficient
August: The CC between daily mean precipitation and the position of the anticyclonic axis in the upper air at the 200 hPa level shows that the highest values are −0.2531 and −0.
Position anomaly of upper air anticyclonic axis at 200 hPa and 300 hPa level In this section we have discussed the anomaly of the position of upper air
- Position anomaly of upper air anticyclonic axis at 200 hPa level .1 Position anomaly of upper air anticyclonic axis of June
- Position anomaly of upper air anticyclonic axis of July
- Position anomaly of upper air anticyclonic axis of August
- Position anomaly of upper air anticyclonic axis of September
- Position anomaly of upper air anticyclonic axis at 300 hPa level .1 Position anomaly of upper air anticyclonic axis of June
- Position anomaly of upper air anticyclonic axis of July
- Position anomaly of upper air anticyclonic axis of August
- Position anomaly of upper air anticyclonic axis of September
In the year 2000, the movement of the axis of the upper air anticyclone oscillated around the mean position throughout the month. The position of the anticyclone axis in the upper part of the air in July 2001 was below the mean position between July 1 and 4 and July 10 and 16. In July, the axis was above the mean position, and on July 20 and 25, it was behind the mean position.
In 2000, the motion of the axis of the upper air anticyclone was oscillating about the mean position throughout the month. In 2002, the axis position of bolus anticyclone was above the average position during 1-8 August. The axis position of bolu anticyclone was above the average position during September 1-5 for the month of September of 2001.
The upper air anticyclonic axis for the month of June 2000 was below the mean position during the 1st-5th. June. In 2002 we observed that the position of the axis was below the mean position during the 1st-7th. June. The upper air anticyclonic axis for the month of July 2002 was below the mean position during the 1st-9th. July.
In 2002, the movement of the anticyclonic axis of the upper air was oscillating around the mean position during the 1st–12th. July. In 2000, the movement of the anticyclonic axis of the upper air was oscillating about the mean position almost throughout the month of August. We observed in 2000 that the motion of the upper air anticyclonic axis was also oscillating around the mean position.
In 2001, the position of the axis of the upper air anticyclone above the mean position was between 2-16. The anticyclonic axis in the upper air for the month of August 2000 also fluctuated with respect to the mean position. In 2001, the movement of the axis oscillated around the middle position for almost the entire month of September.
CONCLUSIONS
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