Meteorological Indices Approach for Prediction of Thunderstorm Probability at Two Coastal Sites in Egypt

Siting of hazardous facilities such as Nuclear power plants requires the investigation of the meteorological and climatologically of the site region to derive extreme values of the meteorological variables to be considered in the design for safety purposes. Not only normally occurring meteorological conditions but also extremely rare events are considered. The present work is devoted to predict the likelihood of the Thunderstorm which is one of the rare events of concern for plant safety. Four atmospheric stability indices such as K-indices, the Showalter-indices, total totalsindices and the SWEAT-indices have been used for that. These function of meteorological parameters such as temperature, wind speed, wind direction and dew point at levels of pressure at 500, 700 and 850 hpa to predict the thunderstorm probability. Two Egyptian coastal sities have been chosen for the investigation, one on the Mediterranean Sea and the other on Red Sea; El Sallum and Sharm El Sheikh. The results show that the probability of thunderstorm occurred within the months January and February during 1989-2003, where the maximum value of SI, K, TT and SWEAT indices at January & February months equals 29.31 & 28.20, 18.4 & 20.3 & 48.1 & 49 and 238.01 & 350.6 for Sallum city respectively, and equals 26.05 & 24.53, 24.6 & 23.9, 50.6 & 51.5 and 253.4 & 265.4 for Sharm El Sheikh city respectively, and this agrees with the pattern reviewed based on world geographic and climatic condition.


Introduction
Siting of hazardous facilities such as Nuclear Power Plants requires the investigation of the meteorological and climatological characteristics of the site region to derive extreme values of the meteorological variables to be considered in the design for safety purposes.Meteorological aspects of external hazards to be considered include extreme values of meteorological parameters as well as rarely occurring hazardous meteorological phenomena [1] [2].The important meteorological characteristics of concern include wind, precipitation, snow, temperature, storm surges [3] [4] and rare meteorological phenomena such as lightning, tornadoes, tropical cyclones.
Lightning is a visible electrical discharge most commonly produced in thunderstorms.Lightning transients exhibit extremely high voltages, currents and current rise rates.Lightning is a significant cause of interruptions or damage in almost every electrical or electronic system that is exposed to thunderstorms, [5].Damage is usually categorized as either direct or induced (indirect).The extreme electric field created under certain circumstances produces point discharges and can cause breakdown (a conductive path) in all but the most robust of insulators.Once a path has been established for the return stroke, currents of tens to hundreds of kiloamperes flow.It should be noted that lightning is an unpredictable transient phenomenon with characteristics that vary widely from flash to flash and whose measurement is difficult.The hazard assessment for lightning should result in an estimated annual frequency of exceedance for lightning strike for the planned nuclear power plant.
The present study devoted to predict the probability occurring thunderstorms across the North and East coasts of Egypt.Four atmospheric stability indexes are measured for that.These indexes are: K-indics, the Showalter, total totals and the SWEAT [6].These indexes combine important meteorological parameters of concern to thunderstorms production, to determine its relative probability.Two Egyptian coastal cities have been chosen for the investigation, one on the Northern Mediteranean Sea and the other on Red Sea; El Sallum and Sharm El Sheikh, respectively.

Location and climate of the Study Area
The investigated area comprises two coastal cities in Egypt; Sham El Sheikh and El Sallum.Sharm El Sheikh city is situated on the coastal strip along the Red Sea, which includes the smaller coastal towns of Dahab and Nuweiba as well as the mountainous interior, Saint Catherine's Monastery and Mount Sinai.Nowadays it is a major touristic hotspot and resort city in Egypt [7].And Sallum city is on the other hand situated on the Egyptian Mediterranean northwest Coast, along the border with Libya [8], as shown in Figure 1.
Table 1 and Table 2 [9] [10], show indicate mean values of the major meteoric conditions of several stations including that are concerned in the studied two cities; Sham El Sheikh and El Sallum.

Methodology
The atmospheric stability and the thunderstorm vulnerability have been examined in two coastal cities in Egypt; El Sallum at the northwest coast and Shrm El Sheikh at the Red Sea Four meteorological indexes; Showalter Index (SI) ( ] 11 [ , Total Totals Index (TT), K-index, and Severe Weather SWEAT, have been calculated and functioned for that purpose.Radio soundings [12] are used to obtain vertical profiles of thermodynamic (pressure,  temperature and humidity) and dynamic (horizontal winds) variables, which have been collected from National Ocean Atmospheric Administration (NOAA) [13] for 10 years from 1989-2003 for Sallum and Sharm Sheikh.These are used for the analysis of the four indices that are described as follow: Showalter Indices (SI) [11] (This is an indices used to determine the stability of the lower half of the troposphere.An air parcel is lifted from an initial position at 850 mb where localized low level influences are greatly reduced.It is lifted dry adiabatically to its LCL and then pseudo-adiabatically to 500 mb.The environmental temperature is then subtracted from the parcel temperature to obtain the value of the Showalter index.The risk of severe weather activity is defined as follows: The index is the difference between lifted parcel temperature (T lift ) and the environmental temperature (T env ) at 500 mb: When SI is negative, that is, the lifted temperature is warmer than the environment, the parcel is unstable.

Total Totals Indices (TT)
The Total Totals indices is a simple index derived from the temperature lapse rate between 850 mb and 500 mb and moisture content at 850 mpse rate between 850 mb and 500 mb and moisture content at 850 mb.It is defined as follows [14] [15]: where T 850 and Td 850 are the temperature and dew point temperature respectively at the 850 hPa level, T 500 is the temperature at the 500 hPa level.

K-Indices
From Wikipedia, the free encyclopedia, K-Indices in meteorology is a measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer.The index is derived arithmetically: The K indices is similar to Total Totals index except that it takes into account moist air at 700 mb contributing to air mass thunderstorm development.The K index is defined as follows [14] [16]: ( ) where T 700 & Td 700 are the temperature and the dew point temperature respectively at the 700 hPa level, T 500 is the temperature at the 500 hPa level.

SWEAT (Severe Weather Threat Indices) Indices
This is an United States Air Force indices that takes into account thermal instability, low level moisture content, vertical wind shear and horizontal wind speeds.The risk of severe weather activity is defined as follows [15] [17]: where f 850 is the wind speed at 850 hPa in knots, f 500 is the wind speed at 500 hPa in knots, d 850 is the wind direction at 850 hPa, d 500 is the wind direction at 500 hPa

Results and Discussion
The average values of the four indices chosen to infer the probability of occurrence of thunderstorm in El Sallum and Sharm El Sheikh cities at January and February for the years, 1989-2003 are indicated in Table 3         Comparing the values calculated for the four indexes with the indicated categories a considerable comparability exists.The probability of thunderstorm occurrence in the two examined cities, El Sallum and Sharm El Sheikh is low to moderate (in the range of 20%).Sharm El Sheikh is little more vulnerable than El Sallum to thunderstorm but still categorized with it.This results highly compatible with the indicated geographical distribution of thunderstorm in the world, Figure 8, this could validate the use of the four indexes for referring the thunderstorm potentiality for a given locality.

Conclusion
The current investigation has been conducted to satisfy one of the required information for safe siting of hazardous facilities such as Nuclear Power Plants.The thunderstorm probability of occurrence has been determined for two El Sallum and Sharm El Sheikh cities in the coasts.The four meteoric stability indexes employed for the study proved that likelihood of thunderstorm occurrence is low to moderate (in the range of 20%).An agreement exists with reviewed work on world distribution of thunderstorm reflecting the usability of these indexes as predictors in most cases.

Figure 1 .
Figure 1.Map of the study area.

Figure 2 .
Figure 2. Average value of SI indices at January month.

Figure 3 .
Figure 3. Average value of SI indices at February month.

Figure 4 .
Figure 4. Average value of TT indices at January month.SI values −2 < SI < 1 Thunderstorms possible (generally weak) SI values −6 < SI < −4 Strong or severe thunderstorms possible SI values SI < −6 Any thunderstorms likely to be strong or severe TT values 44 -45 Isolated moderate thunderstorms TT values 46 -47 Scattered moderate/few heavy thunderstorms

Figure 5 .
Figure 5. Average value of TT indices at February month.

Figure 6 .
Figure 6.Average value of SWEAT indices at January month.

Figure 7 .
Figure 7. Average value of SWEAT indices at February month.

Table 1 .
Mean monthly and annual wind speed (m/s) and its direction (at a height 10 m).Feb. Mar.Apr.May.Jun.Jul.Aug. Sep.Oct. Nov. Dec.

Table 2 .
Summary of the mean annual climatic data for the Selected Weather Stations in Egypt.

Table 4 .
and The indicated values of El Sallum city vary in the range from 17.8 to 23.40 and 16.19 to 20.3 & −22.34 to −3.22 and −20.52 to −2.54 & 20.15 to 31.5 and 20.53 to 33.06 and 79.54 to 113.02 & 77.05 to 113.02 for SI & K & TT and SWEAT indices respectively.On the other hand, Sharm El Sheikh city shows values varying in the range from 16.5 to 19.53 and 15.62 to 18.51 & −14.57 to −1.77 and −15.88 to 2.65 & 28.42 to 35.27 and 27.53 to 36.79 and 81.28 to 112.65 & 79.98 to 125.89 respectively.The columnar diagrams, Figures 2-7 illustrate the variability and comparability of the indicated values.The range of variation is narrower in ElSallum than Sharm ElSheikh indicating a more homogeneous and stable meteoric condition.The following categories of SI, TT, K and SW describe the probability of thunderstorms hazard and the risk associated with it[17] SI values > 3 No significant activity SI values 1 < SI < 3 Showers possible with other source of lift SI values −3 < SI < −2 Thunderstorms more probable (possibly strong)

Table 3 .
Average value for different indics at different year for Sallum city.