Statement of the problem: Thunderstorm is a destructive atmospheric phenomenon, which annually causes a lot of damage to various parts of human activities. Due to the accompaniment of thunderstorm with rainstorm and hail and its effective role in creating sudden floods, the analysis of the behavior of this hazard has been widely studied both in terms of agriculture and in terms of financial and life damages throughout the world. The study of thunderstorm as a hazardous atmospheric phenomenon using instability indexes in Iran has been less considered due to lack of observation stations. Convective Available Potential Energy (CAPE) and Vertical Wind Shear (VWS) are two indexes that are often used to describe and detect thunderstorm environments. This study evaluates the thunderstorms in Iran with reanalysis data using CAPE and VWS indexes.Research Methodology: Thunderstorm data in 7 different conditions at 8 times a day for 42 synoptic and upper air stations during a 37-year common period (1980-2016) was received from the Iranian Meteorological Organization. At first, frequency, trend and time of occurrence of thunderstorms in Iran were investigated during the statistical period. Then, the ERA-Interim reanalysis dataset of the European Centre for Medium-Range Weather Forecasts (ECMWF) with spatial resolution of 0.5 ° was used for the analysis of thunderstorms. To evaluate the ERA-Interim dataset, the CAPE and VWS values for the 80 selected thunderstorm events that were calculated using the RAOB software were compared with ERA data and their accuracy was confirmed. After confirming the accuracy of ERA data, the average values of CAPE and VWS indexes in 42 stations of the country were calculated based on 4, 542 thunderstorm events at 00 and 12 GMT during the study period, and the maps of these two indexes were drawn up using the IDW method. Then, using an equation, the thunderstorm severity thresholds across the country were determined using ERA data with 4, 542 thunderstorm events to distinguish between mild, severe and very severe storms. To ensure the selection of important storms, storms with CAPE values of less than 50 were removed to exclude poor environments for convection occurrence. As a result, out of 4, 542 thunderstorms, 535 events were eliminated and 4007 events remained. On this basis, a "2 x 2 contingency table" was prepared that compares thunderstorm events and forecasts. This table provides the information required to compute warning performance statistics including POD (Probability of Detection), FAR (False Alarm Ratio) and CSI (Critical Success Index). But the results of these statistics did not match well with the conditions of thunderstorm events in Iran. Therefore, the discriminant analysis was used to differentiate the intensity of thunderstorms and to discriminate mild, severe and extremely severe thunderstorms. Explain and interpret the results: The results of the study showed that thunderstorms in Iran are increasing during the statistical period with a regression slope of 0.23 events per year (8.5 events in the statistical period). The highest frequency of thunderstorms was observed in the month of May with an annual number of 111, and the lowest was observed in January with 12 events. Most thunderstorms occur around 21: 30. The highest average frequency of annual events at stations was related to the stations of Urmia, Tabriz, Khorramabad and Bushehr respectively. The proper capability of ERA data to estimate instability indexes in Iran was proved. ERA data provides a very near estimate for VWS, but estimates for the CAPE index are slightly more than observational values. The highest values of the CAPE index are observed in southern provinces, as well as in the southwest of the Caspian Sea coasts, and the highest values of the VWS index are found on the Persian Gulf coasts. When the storm severity breakdown equation for the 400 selected storm events was obtained and the "2 x 2 contingency table" was prepared, it was found that this equation was not satisfactory with respect to the POD, FAR, and CSI indexes. Hence, using the discriminant analysis, the storm severity breakdown relationships and their discriminant equations were obtained. These equations categorized 60% of the surveyed thunderstorms correctly. There is no significant difference between the mean values of CAPE and VWS in the three storm intensity groups. The role of the VWS index was higher in determining the type of storm.

The structures of thunderstorm outflows are studied using surface and sodar data at the Geophysics Station. Distinctive features associated with gust fronts are: a sudden drop in temperature (~ 3-8 °c), a sudden increase in wind speed (~10- 20 m/s), wind shift, a pressure rise (~2-3 mb), a humidity increment and rain. In this study, characteristics of 10 outflow samples are determined using sodar and surface meteorology data. Horizontal winds, maximum wind speed, propagation speed of gust fronts are computed using surface data. The charts of horizontal velocity, vertical velocity, direction and turbulence flow of wind (uv, uw, vw) are considered when the sodar was recording data. Vertical structure of horizontal wind is layered before the arrival of gust front on surface. In horizontal wind profiles, there are two maxima. When the horizontal component of wind is maximum, its vertical component wind becomes minimum, which is predictable through considering of vertical structure of gust front by continuity. Time variations of vertical component of wind speed show that Kelvin-Helmhotz instability is developed along the top of the gust front head near the surface. Estimated dimension of Kelvin-Helmholtz billows is about 10-20 km which appears to be larger than the ones observed by others. This may be due to the fact that these gust fronts occur over a sloping boundary. Contours of vertical and horizontal components of flow velocity and its horizontal direction of gust fronts show that these gust fronts have similar shapes but vary in sizes. This analysis also shows that the outflow has a complex and multi frontal structure. Further observational work is required for more detailed analysis of the gust front in vertical and horizontal extends.

Introduction: Thunderstorms are formed by overheating the earth's surface in air masses or in the weather fronts especially cold front (Kaviani and Alijani, 1991). These storms are part of climatic destructive phenomenon that cause irreparable damage as hail, heavy rainfall and thunders to facilities of farms and houses every year. Thus, it is necessary to study this phenomenon. The climatic phenomena such as thunderstorms, as random phenomena, are not exactly predictable and can gain useful information by monitoring them, that this is possible through the laws of probability. Markov chain is a common method for modeling random phenomena (Wilks, 2006). This model is one of the statistical methods to examine the temporal relationship between the various climatic phenomena and it is the most common method to determine the frequency of climatic phenomena sequences...

Thunderstorm rainfall is considered as a very vital climatic factor because of its significant effects and often disastrous consequences upon people and the natural environment in the Northwest of Iran. Some experiences have indicated that thunderstorm activity is a characteristic feature of the humid / warm spring and summer months in this region and that rainfall from thunderstorms is a major source of moisture for most parts of the study area. In addition. Flash floods are usually associated with the type of thunderstorm that produces localized, but very intense rainfall, which damages property and even results in a loss of life.Consequently, the main purpose of this study was to analyze the temporal distribution and variation of thunderstorm rainfalls in the northwest of country for the duration of 1951 - 2002. It was found that such rains in the region have marked diurnal and seasonal variations. They are most frequent in the spring and summer and during the early afternoon and late evening. The final conclusion reveals the interplay of synoptic weather situations, physiographic effects and some environmental elements in the controlling of thunderstorm rainfalls in the study area.

To study the mechanisms that cause thunderstorms in Kerman province, we used daily data from 10 synoptic stations in the area. The thunderstorm event codes were extracted from stations as the current weather conditions (WW) every three hours to reach the goals. In the following, a single day as a representative was examined and analyzed to study the nature of the atmospheric structure. In the selected period, hail is due to the formation of a low-pressure chalk layer (500 hPa), low-pressure, and instability at the surface. Investigating the atmospheric patterns showed that the cause of this phenomenon in the study area is the strengthening and expansion of low-pressure systems in the East of the Red Sea and Saudi Arabia. These low-pressure systems have gained moisture through the southern seas (Persian Gulf and Oman Sea), causing instability and precipitation along their path. Also, cold air loss from the middle atmospheric layers causes unstable currents and hail. After examining atmospheric conditions through unstable indicators, we tried to track the storm and its formation in 24 hours. Results showed that the CAPE Index and the Total Index are effective tools for monitoring and forecasting thunderstorms in the area. on the other hand, the sweat index also helps expand and route unstable systems

Thunderstorms are regarded not only as a significant weather event but also as a key element in water and electricity cycles of the atmosphere. Generally, researchers consider the intense weather instability as a result of convection in lower levels of the atmosphere with high levels enough of humidity. Usually statistic instability, the humidity of lower levels of the atmosphere and lifting mechanisms near the ground are the main factors leading to convection. Moreover, the combination of three factors, instability, humidity and convergence in lower levels of the atmosphere plays an important role in increasing the possibility of thunderstorms. Accompanying phenomena like lightning, tornado, hail, winds, heavy precipitations (Changnon, 2001 and 1925) and hazardous atmospheric phenomena like turbulence, freezing, and wind sheering make considerable irrecoverable damages to natural and human environments, therefore recognizing the features of these phenomena have always been attracting the attention of researchers. The present study aims at recognizing statistic of thermodynamic, and synoptic features of thunderstorms of southern coasts of Iran. Referring to the archive of National Meteorological Organization, hourly data of atmospheric phenomena of 10 synoptic stations during a common twenty-year period (1995- 2014) were extracted. The data were processed in temporal scales of year, season and month. The data of upper atmosphere (radio-sound data), available in the website of Wayoming University, were applied to investigate the thermodynamic features of the occurred thunderstorms. The thermodynamic features include KI, SI, TT, LI, CAPE indices and skew- T chart in RAOB software. The days with the occurrence of thunderstorms had 5 mm or more at least in two stations that were selected to find synoptic patterns. As the samples were limited, synoptic patterns were done manually. The required maps were prepared using the data of geopotential height in 1000 - 500 hPa levels. Besides wind components u and v and sea level pressure, extracted from NCEP/ NCAR website, were mapped by GrADS software. Checking yearly frequency of thunderstorm occurrence in the southern coasts of Iran showed that the frequency of occurrence of storms in Booshehr station was more than its frequency in Hormozgan station. Moreover, the thunderstorms of Booshehr have a better chronological order as it occurs during all common years. However, except for Bandar Abbas, there is no chronological order for this phenomena. Therefore, it can be said that the occurrence of thunderstorm in the western coasts of the south of Iran has higher frequency than the central and the eastern regions, making it a potential area in this region for storm formation. The largest number of thunderstorm occurrence in seasonal scale is recorded for fall with 45% and winter with 43% respectively. Following seasonal conditions, the largest number of thunderstorm occurrence in monthly scale is recorded for cold months. In Hormozgan station, November, December, and January have more frequencies, while in Booshehr station January, February and March have more frequencies. Analyzing the applied instability indices showed that there was a slight extreme and great CAPE (more than 2500) in Bandarabas station. Besides the values of convection indices TT and KI for most of the thunderstorms suggested the possibility of convection occurrence. Instability indices LI and SI for the occurred thunderstorms reveals conditions of limited instability. Synoptic analysis shows not only the dominance of the westerly winds extending to the south of Saudi Arabia but also the location of divergent region and positive vorticity advection region in the studied region, making instability conditions raising air. The spread of the westerly winds is either due to formation of blocking system in the atmospheric middle level or their meridional blowing and cold air advection from Europe or the north of Asia to the east of Mediterranean. Statistical findings reveal that the occurrence of thunderstorms of western coasts of the Persian Gulf, have higher potential, and more frequency than the central and eastern regions. In seasonal scale, the largest number of occurrences is recorded for fall and winter respectively, while there is no substantial difference in different hours of day and night in hourly scale. As a matter of fact, they are possible to happen all the times. Synoptic analyses show that there is the dominance of two patterns of blocking systems and westerlies trough in the middle of atmosphere leading to instability and rising air in the studied region. The divergent region and positive vorticity advection region in the studied regions make instability condition and hence rising air. Based on the findings of thermodynamic indices, it can be said that convective activities and local instabilities are rarely responsible for thunderstorm occurrence in the region. Also for the occurrence of severe convective activities and relatively high instability, extreme instability and extreme severe instability is coincident with limited thunderstorm occurrences.

In this research parameterization of the precipitation process in Ogura & Takahashi (0- T) thunderstorm model was improved in microphysical processes, specially in the autoconversion process to form raindrops, in the glaciation process and in the terminal velocities of rain and hail. The rainfall intensity became much heavier with Kessler's parameterization, the second peak of the rainfall intensity disappeared with Bigg's freezing probability, and the rainfall intensity became much heavier and sharper with Lin et al's terminal velocities of rain and hail than in the O-T original model. Finally, the derived rainfall pattern based on the improved model has much similarities to the observation data.This paper expresses the basic research for studying the physical treatment in clouds. The modified 0- T model has different applications in analyzing radar observation data, estimate the potential of soil erosion, parameterization of shower in mesoscale numerical weather prediction and etc.

Ext ended Abst ract Introduction Identifying pressure patterns during thunderstorms and Widespread precipitation is important. One of the methods of studying the climate of a region is the study of atmospheric phenomena in relation to the pressure pattern prevailing in that region. Classification of atmospheric patterns is a useful tool for managing a huge and unlimited continuity of atmospheric patterns. Classifications by identifying a number of representative patterns called moment patterns, simplify the physical reality of the atmosphere. One of the purposes of synoptic classifications is to help describe the effects of atmospheric circulation on the surface climate, which is the main task of synoptic climatology. In this paper, atmospheric pressure patterns are studied using factor analysis method. Then, by drawing synoptic maps of each pattern, the characteristics of the desired pattern were examined. Methodology The present study investigates the recognition of prevailing atmospheric patterns during precipitation and Widespread thunderstorms in northwestern Iran. To conduct this research, first rainfall and thunderstorm data were received daily from 18 synoptic stations with a common statistical period of 20 years (1993-1992) from the Meteorological Organization. Then, to determine the selected days, the days that were reported in 5 stations and more rain and thunderstorms were selected. Then 108 days were selected by principal component analysis (PCA). Six components out of the total components that explain more than 73% of the total variance were selected for the next analysis. Then, using Euclidean distance and ward’, s method, a cluster analysis on the matrix, Factor scores were performed. Then the clustering tree was drawn and the observations were divided into 5 clusters. To analyze the atmosphere in the obtained patterns, re-analyzed data were prepared with a resolution of 2. 5 * 2. 5 degrees from the National Center for Environmental Prediction and Atmospheric Research, USA (NCEP / NCAR). Using these data, synoptic maps in each pattern were drawn and analyzed. Results and discussion Investigations showed no compression patterns with Widespread thunderstorms in March. Because this month has winter features, extensive local climbs are less common this month. Therefore, Widespread thunderstorms in this month and other cold months of the year are considered a random phenomenon. It can also be said that hot and cold fronts can not create Widespread thunderstorms without local thermal rise. In other words, in summer due to lack of sufficient humidity and in cold seasons due to lack of surface heating we do not see the occurrence of Widespread thunderstorms. At the time of the occurrence of rains and Widespread thunderstorms in northwestern Iran, It is often located at the 500 hpa level of Trough in central Iraq to the eastern Mediterranean. Differences in the location, depth and extent of this trough have caused the patterns to differ from each other, resulting in differences in the occurrence of atmospheric events in the study area. In this study, pattern three had the highest number of events compared to the 5 extracted patterns. Patterns that are limited to April and the cold days of May are more widespread. Also, patterns that occur limited to June and hot days in May are less widespread. In the Deep trough and minor Trough pattern, the precipitation is mostly influenced by local moisture sources and moisture sources close to the study area (such as the Caspian Sea). Also, in these patterns, the sources of moisture in the lower levels of the atmosphere play a greater role in the occurrence of precipitation. At the time of the Deep Trough and Minor Trough patterns, it is the combination of western systems with local ascent that creates Widespread precipitation. These patterns are limited to May and June. During these months, the ground receives the energy it needs to climb locally. The integration of the local ascent with the western system strengthens it and creates Widespread thunderstorms and rainstorms. In patterns closed low, blocking, the Eastern Mediterranean Trough, the Red Sea and the Mediterranean Sea are the most important sources of moisture, respectively. In the mentioned patterns, moisture sources at the levels of 700 and 500 hPa play the most important role in creating Widespread precipitation. During the occurrence of closed low, blocking and trough patterns in the eastern Mediterranean, the surface temperature decreases and the role of western systems becomes more prominent. From pattern one to pattern five, the surface temperature decreases, respectively. In the Deep Trough and Minor Trough patterns, the study area experiences a temperature of 295 degrees Kelvin. In the closed low pattern, a temperature of 287 degrees Kelvin is observed in the northern half of the study area,But in the blocking pattern, the temperature of 287 is transferred to the more southern parts. In the Trough pattern of the eastern Mediterranean, the temperature of 283 degrees Kelvin is seen in the northern parts of the study area. In other words, from model one to five in the study area, the surface temperature decreases and from one model to another, the surface temperature decreases. As a result, with decreasing temperature, the effect of local ascent compared to the first and second patterns has decreased and it can be said that a significant part of thunderstorms in addition to local ascent, is affected by the front systems passing through northwestern Iran,But fronts alone cannot create widespread thunderstorms without integrating with local thermal rise. Concl usi on The result was that the closed low pattern had the highest and the Deep trough pattern had the lowest repetition. Deep trough and minor trough patterns have less geographical extent than other patterns. When occur patterns closed low, blocking and trough eastern Mediterranean, the study area experiences a lower surface temperature. Also, precipitation is more extensive in these patterns and the maximum vertical air flow is observed at the level of 700 and 500 hPa. Widespread thunderstorms are more likely to occur in the spring. In winter, due to the lack of surface heating, we do not see Widespread storms. In hot seasons, too, Widespread thunderstorms cannot occur due to lack of moisture. In other words, Widespread thunderstorms in northwestern Iran occur when a local ascent is combined with a dynamic ascent resulting from the passage of a low-pressure system through the study area.