The available tropospheric instability gives a better indication of the occurrence of nor'westers. The vertical wind shear on the dates of nor'westers occurrence was also investigated.
SURFACE AND UPPER AIR SYNOPTIC CONDITIONS IN RELATION TO
2.4.1 .2.2 Spatial distribution of mean monthly thunderstorms Spatial distribution of mean seasonal thunderstorms Coefficient of variation of thunderstorms Coefficient of variation of monthly thunderstorms Coefficient of variation of seasonal thunderstorms Interannual variability and trends in the frequency of thunderstorms 25.
INSTABILITY OF THE TROPOSPHERE PRIOR TO THE OCCURRENCE OF
TROPOSPHERIC MOISTURE, ITS FLUXES AND THE RELATION BETWEEN MOISTURE
ENERGETICS OF THE TROPOSPHERE AND THEIR FLUXES IN RELATION TO NOR'WESTERS
CHAPTER 7 USE OF RADAR AND SATELLITE IMAGERIES IN THE STUDY OF NOR'WESTERS
AVAILABLE INSTABILITY ENERGY AND VERTICAL WIND SHEAR ASSOCIATED
SIMULATION OF SURFACE WIND AND RAINFALL FIELDS ASSOCIATED WITH
PREDICTION TECHNIQUES FOR THE NOR'WESTERS AND ASSOCIATED MAXIMUM
PUBLICATIONS
LIST OF SYMBOLS/ABBREVIATIONS
Weighted Average of Specific Humidity Zonal Dry Static Energy Westerly Dry Static Energy East Dry Static Energy Meridional Dry Static Energy South Dry Static Energy North Dry Static Energy Flow Zonal Moist Static Energy Westerly Flow Moist Static Energy East Moist Static Energy Flow Meridional moist static energy flow South moist static energy flow North moist static energy flow. Vertically integrated zonal flow of dry static energy Vertically integrated meridional flow of dry static energy Vertically integrated zonal flow of moist static energy Vertically integrated meridional flow of moist static energy.
1 CHAPTER 1
INTRODUCTION
- General
- Existing climatological aspects of nor'westers
- Classification of local severe storms
- Conditions favourable for the occurrence of nor'westers
- Tornadoes
- Form, life cycle and damage .1 Form
- Favourable conditions for tornado genesis
- Tornadoes detection
- Measurement of wind speed
- Development part
- Sequence of chapters
- Chapter 2: Climatology of thunderstorms and thunderstorm days
- Chapter 3: Surface and upper air synoptic conditions in relation to nor'westers
- Chapter 4: Instability of the troposphere prior to the occurrence of nor'westers
- Chapter 5: Tropospheric moisture, its fluxes and the relation between moisture and rainfall due to nor'westers
- Chapter 6: Energetics of the troposphere and their fluxes in relation to nor'westers
- Chapter 7: Use of radar and satellite imageries in the study of nor'westers In this chapter, the satellite and radar cloud imageries have been analyzed on the dates
- Chapter 8: Available instability energy and vertical wind shear associated with nor'westers
- Chapter 9: Simulation of surface wind and rainfall fields associated with nor'westers in Bangladesh
- Chapter 10: Techniques for the prediction of nor'westers and associated maximum wind speed and rainfall
- Conclusions
Various stability indices are calculated to determine the state of instability of the troposphere before the occurrence of nor'westers. The critical values of the indices are determined in relation to the occurrence of sunsets in the country.
CLIMATOLOGY OF THUNDERSTORMS AND THUNDERSTORM DAYS
Introduction
Variable: Monthly nor'westers/thunderstorms and thunderstorm days, which include surface codes of 17, 29 and 95-99 [17(i.e. 4 precipitation at time of observation), 29 {i.e. Thunderstorm (with or without precipitation) + hail, small hail, snow during the preceding hour but not at the time of observation), and Thunderstorm, light or moderate, without hail but with rain and/or snow at the time of observation;.
Results and discussion .1 Thunderstorm frequency
- lTime of occurrence of nor'westers/thunderstorms in Bangladesh
- Mean monthly thunderstorms
- Spatial distribution of mean seasonal thunderstorms
- Co-efficient of variation of thunderstorms
- Interannual variability and trends of thunderstorm frequency
- Frequency of thunderstorm days
- Monthly mean frequency of thunderstorm days
- Spatial distribution of seasonal mean thunderstorm days
- Co-efficient of variation of thunderstorm days
The spatial distribution of average storm days has been analyzed and studied for each month of the pre-monsoon season. Thunderstorm days are minimal over southern and extreme southeastern parts of Bangladesh, Dinajpur-North Rajshahi region and Rangamati-Maijdi Court-Comilla region.
SURFACE AND UPPER AIR SYNOPTIC CONDITIONS IN RELATION TO
Introduction
The location of the tornado was very close to the intersection of the axes of the maximum low-level wind and the upper-level jet stream. Attempts have also been made to locate the place of occurrence of nor'westers by superimposing the axes of the upper flow of the air plane and the maximum winds of the lower level.
Description of the tornadoes .1 Saturia tornado
According to them, the intersection of the axes of the moist tongue and the dry zone at 0000 UTC was close to the location of the tornado's occurrence. The pressure drop in the center of the tornado was estimated to be more than 50–60 hPa compared to its surrounding area.
Analyses
- Surface synoptic and upper air conditions associated with nor'westers .1 Mean sea level pressure distribution
- Surface minimum temperature conditions
- Surface humidity conditions
- Upper air synoptic conditions
- Diurnal and spatial variation of 24-hr pressure change
- Spatial distribution of minimum temperature over Bangladesh
- Vertical distribution of wind over Dhaka
- Superposition of the axes of low-level maximum wind and the upper air jet stream of wind over Dhaka
3.21 (a-b): Superposition of the axes of the upper air jet stream and the maximum lower level wind speeds on the dates when severe storms occur. The intersection of the moist zone and dry zone axes is evident from the spatial distribution of the dew point depression at 0000 UTC on April 26, 1989 (Fig. 3.45). This shows that the LFC was much lower on the date the Saturia tornado occurred.
This enormous amount of instability energy was responsible for creating the tornado on May 13, 1996.
IN STABILITY OF TH E TRDPOSPH ERE PRIOR TO THE OCCURRENCE OF
N DR'WESTERS
Introduction
All stability indices have been calculated using the 850-hPa levels as the low level at which the data have been used before. From this point of view, the stability indices have been modified by considering the data at the boundary layer, i.e. the stability indices that 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).
Calculations are also made for stations in and around Bangladesh to study the spatial distribution of stability indices.
Data source
Not only this, the use of 850-hPa data gives few morning values that show atmospheric stability, so it becomes cumbersome to make any definitive decision regarding the occurrence of a nor'wester in Bangladesh, especially in the afternoon or early night. In this chapter, the stability indices have been investigated for a large number of northwesters to find out the critical values of various indices that are favorable for the occurrence of northwesters in Bangladesh. This chapter also deals with the calculation and analysis of various modified stability indices in relation to the occurrence of northwesters in Bangladesh.
It may be mentioned that data from all 26 stations were not available on the dates when nor'westers occurred.
Methodology of stability analyses .1 Analysis of unmodified stability indices
- Showalter Stability Index (SI)
- Lifted Index (LI)
- Dew- point Index (DPI)
- Dry Instability Index (DII)
- Vertical total Index (VT)
- Total Totals Index (TT)
- Severe Weather Threat (SWEAT) Index (SWI) The SWEAT Index as defined by Miller (1972) is given by
- Energy Index (El)
- K-Index (K!)
- Modified stability indices
- Modified Cross Total Index (MCT)
- Modified Vertical Total Index (MVT)
- Modified Total Totals Index (MTT)
- Modified Severe Weather Threat Index (MSWI)
- Modified Energy Index (MEl)
- Modified K-Index (MKI)
It may be mentioned that most of the northwesters in Bangladesh occur in the afternoon or at night. This is the difference between the 500 hPa temperature and the wet bulb temperature of the 850 hPa level, increased along a pseudoadiabat to 500 hPa (T 500). Some stability indices have been adjusted by taking into account the average low humidity and temperature.
It is found that the 850 hPa level has less moisture at 0000 UTC on most of the nor'westerly occurrence dates in Bangladesh, and the stability indices do not show sufficient tropospheric instability favorable for nor'westerly occurrence.
Significance test of the coefficient of determination
Linear correlations between unmodified volatility indices such as SI, LI, DPI, DII, CT, VT, TT, KI, El and modified volatility indices such as MCT, MVT, MTT, MKI, MEL calculated on days of occurrence of 108 nor'westers are studied and the corresponding correlation coefficients are calculated. The regression equations and the coefficient of determination (R2) were obtained through scatter diagrams taking two indices at the same time. Where Y is the actual value, Y is the calculated value, and N is the number of data pairs.
Results and discussion
- Stability indices at Dhaka in relation to the occurrence of nor'westers in Bangladesh
- Showalter Stability Index (SI) and its frequency
- Lifted Index (LI) and its frequency
- Dew-point Index (DPI) and its frequency
- Dry Instability Index (DII) and its frequency
- Cross Total Index (CT) and its frequency
- Vertical Total Index (VT) and its frequency
- Energy Index (El) and its frequency
- K-Index (K!) and its frequency
- Spatial distribution of different instability indices
- Modified stability indices at Dhaka in relation to the occurrence of nor'westers in Bangladesh
- Modified Cross Total Index (MCT) and its frequency
- Modified Vertical Total Index (MVT) and its frequency
- Modified Total Totals Index (MTT) and its frequency
- Modified SWEAT Index (MSWI) and its frequency
- Modified K-Index (MKI) and its frequency
- Comparison of different stability indices with modified stability indices on the dates of occurrence of nor'westers in Bangladesh
- Correlation among different modified and unmodified instability indices The linear correlations among different instability indices have been studied in order to
- Correlation of Showalter Stability Index with other instability indices
- Correlation between Lifted Index (LI) and other instability indices
- Correlation among unmodified and modified instability indices
Therefore, DPI :!~ -3°C can be taken as the critical value for the occurrence of nor'westers in Bangladesh. 54°C can be taken as the critical value for the severe to very severe northwesters to occur in Bangladesh. 300 can be taken as the critical value for the occurrence of severe to very severe nor'westers.
48°C can be taken as the critical value for the severe to very severe northwesters to occur in Bangladesh.
TROPOSPHERIC MOISTURE, ITS
FLUXES AND THE RELATION BETWEEN MIDSURE AND RAINFALL DUE TO
ND R'WE STE RB
- Introduction
- Data used
- Methodology
- Precipitable water content of the troposphere over Dhaka
- Correlation of precipitable water content with different instability indices
- Correlation of rainfall due to nor'westers with different instability indices and precipitable water content
- Water vapour fluxes in the atmosphere
- Vertically integrated fluxes of moisture
- Significance test of the correlation co-efficients
- Results and discussion
- Precipitable water content of the troposphere and its relation with instability indices and rainfall due to nor'westers
- Fluxes of moisture
The frequency of precipitable water content of the troposphere over Dhaka on the dates of NW occurrence in Bangladesh has been calculated and the diagrams have been prepared accordingly. The spatial distribution of precipitation water on the northwesterly data was also studied. The frequency of WAq of the troposphere over Dhaka on the dates of occurrence of NW in Bangladesh has been calculated and the diagrams have been prepared accordingly.
The fluxes over Dhaka have also been calculated for the dates of non-occurrence of nor'westers.
Spatial distribution of the vertically integrated zonal and meridional fluxes of moisture
The spatial distribution of vertically integrated ZFM from the 925 to 400 hPa level over Bangladesh and surrounding areas is analyzed for several NW cases and a distribution is shown in Fig. It is found that the vertically integrated WFM dominates over Bangladesh on most dates of occurrence of nor'westers and the gradient of fluxes is steep across the country. The range of vertically integrated ZFM for the layer is about (2 — 12) x 10 kg x ms over Bangladesh in most cases.
The spatial distribution of the vertically integrated MFM from 925 to 400 hPa level over Bangladesh and surrounding areas has been analyzed for different cases of Northwestern and one distribution is shown in Fig.
ENERGETICS OF THE TROPOSPHERE AND
THEIR FLUXES IN RELATION TO NOR'WESTERS
Introduction
But no extensive work has been done so far to study the changes in thermodynamic properties before the onset of nor'westers in Bangladesh during the pre-monsoon season. But no study has been done on the tropospheric fluxes of the energy components associated with nor'westers over this region. In this chapter, efforts have been made to study the sensible heat (SH), latent heat energy (LH), potential energy (PE), dry static energy (DSE) and moist static energy (MSE) of the troposphere and the change of them over Dhaka. before the appearance of some nor'westers over Dhaka as well as Bangladesh with the aim of discovering some criteria for their appearance.
Attempts have also been made to study the flow of energy components in the troposphere over Dhaka as well as its surrounding areas of India before the occurrence of several nor'westers in Bangladesh with a view to find out some criteria for their occurrence.
Data used
- Energetics of the troposphere
- Fluxes of energy components
- Vertically integrated fluxes of energy components
Using the thermodynamic and hydrodynamic equations, temperature, humidity, geopotential height in geopotential meters, and velocity fields can be combined to obtain the equation for the total energy of the troposphere. The zonal fluxes of the energy components are the products of the energy components and the zonal wind component. The integration is done by taking the layer-averaged values of the DSE and MSE fluxes from the 1000 to 100 hPa level over Dhaka and for the spatial distribution from the 925 to 100 hPa level.
925 hPa is taken here because the surface pressure becomes less than the 1000 hPa level at most stations during the pre-monsoon season.
Results and discussion
- Sensible heat content (SH) of the troposphere
- SH of the troposphere over Dhaka at 0000 UTC during the pre-monsoon season
- Comparison of the SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of nor'westers
- Comparison of the SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the weekly normal
- Latent heat (LH) content of the troposphere
- LH of the troposphere over Dhaka at 0000 UTC during the pre-monsoon season
- Comparison of the LH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of nor'westers
- Comparison of the LH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the weekly normal latent heat
- Comparison of the LH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the monthly normal
- Potential energy (PE) of a parcel in the troposphere
- PE of a parcel of air in the troposphere over Dhaka at 0000 UTC during the pre-monsoon season
- Comparison of the PE of a parcel of air in troposphere over Dhaka at 0006 UTC on the dates of occurrence and non-occurrence of nor'westers
- Comparison of the PE of a parcel of air in troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the monthly
- Dry static energy and moist static energy contents of the troposphere
- Comparison of DSE and MSE components on the dates of occurrence of nor'westers with the normals
- Comparison of DSE and MSE components on the dates of occurrence of nor'westers at Dhaka with the previous weekly normal
- Comparison of the DSE and MSE components of the troposphere over Dhaka between the dates of occurrence and non-occurrence of
- Comparison of the DSE and MSE components of the troposphere over Dhaka on the dates of occurrence of nor'westers in Bangladesh
- Maximum and minimum curves of DSE and MSE components of the troposphere over Dhaka on the dates of occurrence of nor'westers
- Distribution of moist static energy at 850 and 500 hPa
- Zonal fluxes of dry static energy (ZFDSE) over Dhaka
- Meridional fluxes of dry static energy (MFDSE) over Dhaka
- Vertically integrated zonal and meridional fluxes of DSE over Dhaka The ZFDSE and MFDSE, computed at different isobaric levels over Dhaka at 0000
- Zonal fluxes of moist static energy (ZFMSE) over Dhaka
- Vertically integrated zonal and meridional fluxes of MSE over Dhaka The ZFMSE and MFMSE, computed at different isobaric levels over Dhaka at 0000
- Spatial distribution of the vertically integrated zonal and meridional fluxes of MSE
The SH of the troposphere over Dhaka at 0000 UTC on the dates of appearance and non-appearance of nor'westers is compared to each other. The tropospheric SH over Dhaka at 0000 UTC on the dates of nor'westers occurrence is compared with the weekly normal. The tropospheric SH over Dhaka at 0000 UTC on the dates of occurrence of nor'westers is compared with the monthly normal.
The LH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of norwesterlies were compared with each other. Tropospheric LH over Dhaka at 0000 UTC on the dates of nor'westers occurrence was compared with the monthly normal. 6.25 (a-b): Vertical profiles of minimum and maximum (a) DSE and (b) MSE at 0000 UTC over Dhaka on dates of nor'wester occurrence.
