4.1 Introduction
Pre-monsoon season over Bangladesh extending from March to May is characterized by very good convective activity all over the country. Convective activity progressively increases from March onwards as the season advances. The thunderstorms form from a cumulonimbus cloud, which incur substantial vertical development where the tops usually reach well into the upper levels of the troposphere. However, most cumulonimbus cloud tops are restricted in height by the tropopause, the boundary between the troposphere and stratosphere (Krishnamurti, 2003).
Though the thunderstorm activity may continue over the country even during the southwest monsoon season, the severity of thunderstorms is marked only in the pre-monsoon season, when they, on a number of occasions, are accompanied by squalls and hails (Srinivasan, ef aL, 1973).
A stability index is a measure of the potential instability of the atmosphere over a specified region. Since such an indicator can be calculated hours prior to the actual development of any convective activity, it acts as timely identifier of areas within an air mass, which are capable of supporting convective activity. Once these potential areas are identified, the severe weather forecaster can then take a closer and more precise examination of these areas, incorporating all available parameters, tools, and experience into making a final forecast (Thomson and Lin, 1985).
Over the years, studies of convective out-breaks have helped to isolate those altered during convective development. These careful isolations have led to the development of many stability indices that are in use by forecasters today. Since each storm outbreak may have its own varying environment, one index may prove to be more accurate over another within a particular storm setting.
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Meso convective systems (MCS) are responsible for much of the beneficial precipitation and most of the severe weather (heavy rains, high winds, hail, tornadoes and lighting) that occur in Bangladesh during the pre-monsoon season (March-May). Thunderstorms occur during this season and these thunderstorms generally proceed from northwest in most cases and that's why they are known as "nor'westers" or "Kalbaishakhis". The accurate forecasting of severe thunderstorms and tornadoes during this season remain a challenging problems for operational
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meteorologists. Newton (1963) has identified (i) potential instability, (ii) low-level moisture, (iii) sheared and veered environmental winds and (iv) some triggering mechanism(s) as favourable conditions for severe storm development in addition to the favourable synoptic conditions. It is very important for forecasters to be able to distinguish between severe and non- severe thunderstorms. Extreme instability, strong vertical wind shear and atmospheric instability with SI or Li <-6 are considered as deciding factors (McNult, 1988: Johns and Doswell, 1992).
Whereas Goliger ci al. (1997) have considered the following meteorological features necessary for tornado formation (i) a deep layer of mid-tropospheric dry air above a moist surface, (ii) steep moisture and temperature gradients, (iii) high surface temperature, (iv) low level convergence and tipper air divergence, (v) vertical wind shear and (vi) atmospheric instability.
Stability indices, determined from environmental soundings, have been used by forecasting offices to locate the area in which the severe weather is likely to occur (Faubush etal., 1951;
Showalter, 1953; Galway, 1956; Miller, 1972). These stability indices provide necessary conditions (but not sufficient) conducive to storm occurrence.
In Bangladesh, the meteorologists use some of the stability indices to forecast nor'westers. But extensive studies have not so for been made on the stability indices to determine the stability criteria except a few studies (Chowdhury and Karmakar, 1986; Das etal., 1994).
Meteorologists prepare charts of different stability indices to delineate the area of instability of the atmosphere conducive to the formation of thunderstorms. Some of the stability indices are SI, LI, DPI, DII, SWI, El, etc. [Showalter, 1953; Faubush ci al., 1951; Darkow, 1968; Miller, 1972; Galway, 1956; Chowdhury and Karmakar, 1986; Das cial., 19941. All the stability indices have been computed by using the 850-hPa levels as the low-level of which the data has been used before.
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Since the 850-hPa level sometimes remains relatively dryer and the low-level moisture plays a vital role in the formative stages of thunderstorm, 925-hPa level can be used as the low level. Not only that, the use of 850-hPa data gives some values in the morning showing the stability of the atmosphere and it becomes cumbersome to take any conclusive decision regarding the occurrence of nor'wester in Bangladesh specially in the afternoon or early night. From this point of view, the stability indices have been modified by considering the data at the boundary layer i.e. 925 hPa level.
In this chapter, the stability indices have been investigated for a large number of nor'westers in order to find out the critical values of different indices favourable for the occurrence of nor'westers in Bangladesh. The stability indices, which have been studied, are Showalter Stability Index (SI), Lifted Index (LI), Dew-point Index (DPI), Thy Instability Index (DII), Cross Total Index (CT) and Vertical Total Index (VT), Total Totals Index (TT), SWEAT Index (SWI), Energy Index (El) and K-Index (KI). Low-level data at 850-hPa
-i level has been considered to compute the stability indices such as SI, CT, VT, TT, El, SWI and K! (Faubush ci al., 1951; Showalter, 1953; Darkow, 1968; Miller, 1972; Galway, 1956;
Chowdhury and Karmakar, 1986; Das etal., 1994).
This chapter also deals with the computation and analysis of different modified stability indices with relation to the occurrence of nor'westers in Bangladesh. The modified stability indices, which have also been studied, are Showalter Stability Index (SI), Lifted Index (LI), Dew-point Index (DPI), Dry Instability Index (DII), Modified Cross Total Index (MCT), Modified Vertical Total Index (MVT), Modified Total Totals Index (MTT), Modified SWEAT Index (MSWI), Modified Energy Index (MET) and Modified K-Index (MKI) by using the data at 925 hPa level. Attempts have been made to relate different instability indices, both unmodified and modified, among them statistically. Computations have also been made for the stations in and around Bangladesh for studying the spatial distribution of the stability indices.
4.2 Data source
Real time rawinsonde data of 0000 UTC at different isobaric heights from 100010 100 hPa at Dhaka, Chittagong and different Indian stations have been collected from the Storm Warning Centre (SWC) of Bangladesh Meteorological Department. SWC received these data on real time basis through GTS-Link. For frequency analysis of different unmodified and modified
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instability indices, only the data at Dhaka has been considered and for spatial distribution all the available data of 26 stations have been considered. The stations are shown in Fig. 4.1(a). It may be mentioned that data of all the 26 stations were not available on the dates of occurrence of nor'westers.
For better understanding, a map of Bangladesh showing some greater districts is given in Fig. 4.1(b).
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