INTRODUCTION
1.6. Tornadoes
A tornado is an intense atmospheric vortex, violently rotating, tall, narrow column of air that occasionally extends to the ground from a cumuliform cloud. It is seen as a funnel- shaped cloud pendant from a cloud base (Fig. 1.6). The funnel does not always extend to the ground, and may be obscured by dust (Ghose, 1986). The tornadoes move in a zigzag path.
1.6.1 Form, life cycle and damage 1.6.1.1 Form
The funnels may assume various forms such as a thin rope, a cylindrical or conical shape, a thick mass of black cloud or multiple funnels. The funnel outline may be sharp- edged or very ragged, suggesting that the flows in tornadoes may be either smooth or highly turbulent. Almost all tornadoes rotate cyclonically but anticyclonic ones also exist.
1.6.1.2 Life Cycle
The tornado life cycle may broadly be divided into the following five stages (Davies Jones, 1982)
The dust whirl stage - when the first signs of circulation are visible as dust swirling upwards from the surface.
The organizing stage - characterized by overall downward descent of the funnel.
The mature stage - when the funnel reaches its greatest width and is almost vertical and when damage is most intense.
The shrinking stage - marked by decreasing funnel width, increasing funnel tilt and a narrow damage sweep.
The decay stage - when the vortex is stretched into a rope shape.
1.6.1.3 Damage
A tornado inflicts damage in various ways as in the following (1-lolford, 1 977):
By the direct impact of wind, whose force is proportional to the square of wind speed.
By twist due to unequal wind speeds within the very narrow band of the circulation. Winds are often so much greater on one side of a tree than on the other that they screw off the tops of trees.
(c) By explosion when a tornado passes directly over a building which contains air at normal atmospheric pressure. Extremely high pressure gradients exist
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across the narrow circulation, which would result in a momentary inequality of pressure between the outside and inside of a building. Roofs are sucked off and walls explode outwards. The advice given in tornado safety rules in the USA is to open doors and windows on the opposite side of the building from that of the approaching tornado, so as to facilitate the equalization of atmospheric pressure.
(d) By lift and drop in the vertical currents within the tornado. An up current of 240 km/h was indicated by a film taken in Dallas, USA, on 2 April 1957.
Sheds, cattle and human beings are carried high into the air Nandi and Mukherjee, 1966). Even vehicles such as buses with several persons inside are reported, to have been lifted a few meters above the ground (Mukherjee and Bhattacharya, 1972; Gupta and Ghosh, 1980).
1.6.2 Favourable conditions for tornado genesis
The following conditions favourable for the formation of tornadoes:
Abundant moisture in the surface layer to a depth of at least 1 km. The presence of a dry air mass at intermediate level (I to 2.5 km), providing the potential for strong downdrafts through evaporative cooling, is also favourable.
Deep conditional and convective instability i.e., large lapse rate of temperature and moisture through a great depth.
The presence of a stable layer or inversion. This acts to prevent deep convection from occurring until the potential for explosive overturning is established.
71 (iv) A mechanism to remove the stable layer. The most common mechanism is dynamic lifting with surface heating and warm air advection below the inversion. Other possible mechanism includes lifting by terrain and by gravity waves. Dynamic lifting is strongest near thin zones of low-level convergence such as fronts or dry lines (dew-point fronts) and beneath regions of pronounced upper level divergence. Typically, these features occur in combinations to provide sustained lifting through deep layers.
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(v) Moderate to strong winds that veer with height, with large values in a narrow horizontal band at altitudes above 6 km.
1.6.3 Tornadoes detection
1'ornadocs are detected mostly on the basis of radar observations. I-look shaped echoes have been associated with tornadic storms but unfortunately the correlation is far from perfect. The most promising detection devise is Doppler radar. This radar can measure the speed of precipitation target toward or away from the radar antenna, as well as the reflectivity. The tornadic vertex signature (TVS) is a small scale, high shear region associated with the tornado itself. The TVS is first detected at mid-levels and it extends both upward and downward. It reaches cloud base co-incident with funnel cloud appearance. Attempts to detect tornado from geostationary satellite have met with little success. As already mentioned, tornadoes generally occur after the collapse of the storm's overshooting top but since this signature is not unique, it cannot be used for warning purposes.
1.6.4 Measurement of wind speed
There are many methods to estimate wind speed in tornadoes, which are of great interest to engineers to design vital structures. Some of the methods are described below:
Ground Marks: Marks caused by suction on the ground often resemble cycloid. They consist of short pieces of stubble laid in cycloidal rows. They are indicative of suction vortices rotating around the axis of the tornado, bccausd the combination of straight line and circular motion yields a cycloidal path. The path of a suction vortex depends on the translation velocity of the parent tornado, the radial distance between the suction vortex and tornado axis, and the speed at which the suction vortex is moving around the tornado axis. The first two can be estimated from radar data and the observed size of cyclonical loops. Analysis of suction marks reveals speed of 80 to 95 m/s (Fujita el al., 1970). However there are many difficulties in the estimation of wind speed by this method because in many cases there are many suction vortices present simultaneously.
Direct passage over instruments: Anemometers are often blown away in tornadic conditions; in cases where they survive there is always the question of whether they received the storm's full force. The highest value ever recorded by an instrument was 67 m/s.
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(iii) Remote sensing: The most important tool for measuring wind speed is the meteorological Doppler radar. This specialized radar measures the Doppler frequency shift caused by the component of radar target movement parallel to the radar beam.
1.7 .Objectives of the study 1.7.1 Analytical part
'K 1.7.1.1 Distribution of nor'westers/thunderstorms and thunderstorm days over Bangladesh
This includes the spatial and temporal distribution of mean monthly and seasonal nor'westers/thunderstorms and thunderstorm over Bangladesh. This statistical analytic overview will be helpful to identify the climatological risk area for nor'westers / thunderstorms and thunderstorm days over the country.
1.7.1.2 Surface synoptic conditions and upper air circulation
The surface synoptic conditions and upper air circulation prior to the occurrence of nor'westers and tornadoes have been studied. Charts containing synoptic conditions and distributions of some of the meteorological parameters have been prepared to evaluate the areas favourable for the occurrence of nor'westers in Bangladesh.
1.7.1.3 Instability of the troposphere
Different stability indices have been computed to determine the instability condition of the troposphere prior to the occurrence of nor'westers. The critical values of the indices will be determined in relation to the occurrence of nor'westers over the country. Their spatial distributions of the different instability indices over Bangladesh and adjoining Indian States have been studied for the delineation of the area of maximum instability prior to the occurrence of nor'westers in Bangladesh
1.7.1.4 Water vapour, precipitable water and water vapour fluxes associated nor'westers
The rawinsonde observations have been utilized to compute the water vapour, its fluxes and precipitable water content of the atmosphere prior to the occurrence of nor'wcsters.
The spatial and temporal distribution of these parameters will be further studied and an attempt will be made to correlate the precipitable water with actual rainfall due to
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nor'westers. Their spatial and vertical distributions of precipitable water, specilic humidity and their fluxes over Bangladesh and adjoining Indian States have been studied for the delineation of the ares of maximum moisture content and influx of moisture prior to the occurrence of nor'westers in Bangladesh
1.7.1.5 Tropospheric energy and its fluxes
The changes of the different energy components, total energy and their fluxes before the occurrence of nor'westers are studied extensively. This explains the physical processes which are responsible for the formation of severe thunderstorms / nor'westers.
1.7.1.6 Dynamics of the troposphere
The divergence / convergence, vertical wind shear, vertical velocity etc. have been computed by using radar imageries and rawinsonde data. The findings will cast in-depth look into the dynamic evolution of nor'westers and their movement. The low-level maximum wind and upper level jet stream data have been utilized to locate the probable area of occurrence of nor'westers in Bangladesh as well.
1.7.1.7 Tropospheric conditions associated with nor'westers in Bangladesh
Tropospheric conditions associated with nor'westers in Bangladesh have been studied extensively for some cases.
1.7.2 Development part
1.7.2.1 Peak gusts and rainfall associated with nor'westers
Some parameters obtained from the analysis of thermodynamic diagram and meteorological parameters obtained from the rawinsonde data are used to calculate the peak gusts associated with nor'westers. Multiple regression equations have been developed among the peak gusts associated with nor'westers and various instability indexes as well as other meteorological parameters. Similarly, simple and multiple regression equations have also been developed for the prediction of 24 hours rainfall associated with nor'westers.
1.7.2.2 Instability of the troposphere
Different stability indices, as obtained in section 1.7.1.4, have been modified by taking into consideration of the 925 hPa in lieu of 850 hPa level. It is because of the fact that the low-level moisture plays an important role in making weather. The critical values of the indices have been determined in relation to the occurrence of nor'westers over the country.
The prediction of nor'westers does not depend on only one or two favourable meteorological parameters, but on a number of parameters and their temporal and spatial
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distributions. The present study aims at the determination of the favourable conditions and criteria of different meteorological parameters for the prediction of nor'westers in Bangladesh.