This research is an attempt to investigate the blocking systems over the Atlantic Ocean and especially over the Mediterranean sea, which leads to surface cyclogenesis in the east of the Mediterranean Sea, and the influence of their effects over the Iranian region. At the first step of study, Mediterranean topography, local Mediterranean winds and cyclogenesis have studied; then structure and types of existing blocking systems on the synoptic weather maps have been introduced with figures.In the second step of the study, all of the blocking systems that were formed over the Atlantic Ocean were identified from 1989 to 1997, the results of which are shown in the table included in the paper. Of course this part of the work was done with the help of Prof. Colucci from USA and data are given from NCEP.Atmospheric blocking systems can lead to a stagnation of weather patterns where the patterns remain for several days or even weeks at the same location during the blocking system. This case can lead to flooding, drought, above normal temperatures, below normal temperatures and other weather extremes. Therefore, it is important to recognize a blocking pattern in its initial development. With this awareness, one can forecast up to several days in advance with a high degree of accuracy.

Blocking is one of the most protruding flow patterns and has received attention during the last decades due to its effect on seasonal weather characteristics. Moreover, finding show that Most of the blocking systems in the northern hemisphere are in all seasons and their continuity in the Atlantic and Pacific. The results of the Mann-Kandal test at the 95% confidence level of the percentage of frequency of blocking systems showed that the northern hemisphere blocking systems have been increasing in most parts of Asia-Pacific, Europe and Iran, and the trend is decreasing on the Atlantic Ocean and the American continental. Of course, these trends only increased significantly on parts of Central Asia and Central Pacific. In the monthly, the Eastern and Central Atlantic segregated from May to September and December the decreasing trend that decreasing only in June, September and December in some areas of the trend, in October, November January, February and March is an increasing, and only in October and March this trend is significant. In the Pacific, except in March, there has been an increasing trend in all months in July, August, October and September. In the Americas, except in May and August, there has been a significant decrease in the other months. In Europe, except for the months of August, September and December, in other month has an increasing trend, which has not been significant. In Asia and in the region of Iran, although it is more or less an increasing trend, it is not significant

Introduction Blocking is one of the most protruding flow patterns and has received attention during the last decades due to its effect on seasonal weather characteristics. The first qualitative conditions for the blocked atmospheric flow was introduced by Rex in 1950 telling the atmospheric jet becomes divided into two separated parts with a considerable connected mass transport, the flow to be meridional at the splitting point and downstream, and the state to continue for at least ten days with a zonal width of at least 45 degrees. The new ideas formulated by Rex became the basis for several blocking indices. Most of the indices detect typical atmospheric patterns. For shorter time series, particular criteria have been used, although, the majority of the analyses are in the 500 hPa geopotential height. In the face of the differences between the index classifications, they draw similar conclusions regarding the patterns of the Northern Hemisphere blocking climatology. However, the blocking frequency is the percentage of time steps an assured longitude. It can be as low as 5% or higher than 20%. The main aims for this wide range of frequencies in automatic recognition are modifications of the Rex criteria and the use of different parameters. Due to its simplicity, the index of Tibaldi and Molteni (TM-Index) has become a standard in automated blocking detection analyses. An important problem of the TM-Index is that it cannot effectively distinguish between blocking and cut-off low patterns, since both fulfill the TM-Index criteria. The adapted TM-Index MTM has a higher rejection rate for cut-off lows. Limits in the Rex conditions can be measured as implementations of independent filters which show synergetic effects if they are used in run...

The purpose of this study was to investigate and identify the effect of blocking systems on Iran's poisoning during the period 1975-2015 using the Blocking numerical index. The results showed that in the cold seasons most of the blockades affecting the atmosphere of Iran were in the northern Atlas. Also, the study of the severity of effective blockades affecting Iran's climate showed that the strong blocking is moderate in terms of the highest number after the blocking, indicating that Iran was more affected by moderate blocking (74. 6%) and then strong (21. 1%). The number of weak blocking is very low (4. 3%). Due to the frequency of occurrence of blocking also was found wet Iran in months. Cold, the core of the blocking in between longitude 30 ° west and 30 ° East over the Atlantic formed and as the warm season approaching, the core the land was transferred to the regions dominated by positive changes in parameters affect precipitation. Also, in Bandal's review in different seasons, it was revealed that in winter, with the retreat of the adjacent high pressure and the strength of the polar vertex, along with the occurrence of strong and long blockades, the average rainfall is more than the other seasons.

This research attempts to investigate the role of the blocking systems in surface cyclogenesis over eastern Mediterranean sea; but particular attention is focused on the flood of Iran during march 2000 over western states of Iran as: Hamadan, Ilam, Lorestan, Kermanshah, Kurdestan, Zanjan, Ghazvin.The data used for this research includes upper level and mean sea level pressure charts every 12 hours during 15 to 27 March 2000 from NCEP center, also The data of temperature, pressure, humidity studied every 3 hours and data of daily precipitation totals studied from 19 important Iran synoptic stations during 1 to 30 march 2000.Results shows that: the trough in eastern flank of Atlantic Ocean blocking system was the main reason of March 2000 flood over Iran; During blocking system activity in east of Mediterranean sea, the splitting of jet stream has occurred near the second ridge over Iran and it has intensified low pressure systems the near west of Iran; The development of Sudan low pressure system has caused enhance the intensity the eastern trough of blocking system and lead to flood over west of Iran.

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.

According to the complexity of planetary-scale mechanisms affecting extreme snowfall events, the necessity of recognizing and studying the effective components of these climatic mechanisms are presented. Due to the role of the polar vortex of northern hemisphere in tropospheric variability in winter and also, in variability of the structure of the teleconnection patterns on intensity and weakness of the polar vortex, revealing the mechanism of impact of the two mentioned components on synoptic patterns leading to extreme snowfall events in Iran was considered in this study. According to the environmental approach in this study, first the snowfall days were identified and the atmospheric circulation patterns related to these days were investigated. For this purpose, daily data on rainfall and winter temperature (December-March) of fifty Iranian synchronous stations in the statistical period 1951-2016 were received from the Meteorological Organization of Iran. Atmospheric data and teleconnection indices were also obtained from the NCEP / NCAR. Finally, five periods were identified, during which snowfall was recorded in at least six stations and their average snowfall reached more than 20 mm. Of these event periods, which were in January and February, three periods related to January were examined in this study, which occurred in 1972, 1988 and 2008. In the second stage, atmospheric patterns were drawn in combination and for the average of the event periods, in three scales of hemisphere, synoptic and expanse of Iran, in the GrADS system Then, the amount and status of the teleconnection indices including: NAM, AO, NAO and PNA were analyzed in the period of extreme snowfall events. The emphasis of this study is on the effect of mid-tropospheric conditions in terms of altitude, temperature and altitude on snowfall events in Iran, while the patterns of pressure and temperature of the earth's surface and the humidity of the lower troposphere have also been considered. In hemispheric-scale combined patterns, the emphasis has been on showing the status of the tropospheric polar vortex in relation to teleconnection indices. In the third step, by doing a combined analysis, the effect of polar vortex on the change in teleconnection indices and their effectiveness on the formation of synoptic patterns leading to extreme snowfall events in Iran was revealed. According to the research findings, the NAM index, the strength of the center of the polar vortex and the AO index have shown the shape of the vortex in terms of its concentration or elongation. The PNA index has also been associated with the strength of northwestern Europe and the North Pacific anticyclones, which have contributed to the formation of blocking systems due to the northward deflection of the extending trough of polar vortex from their axial direction. The NAO index also represents the emergence of systems in the Euro-Atlantic section. The results showed that the necessary dynamic conditions for all three events have provided the emergence of blocking patterns due to weakening and distortion of the polar vortex. These events are often associated with disconnection of jet stream in the Eastern Hemisphere, especially in the Euro-Atlantic area. Hence, NAO and PNA indices, directly and NAM and AO indices indirectly affected the conditions required for extremesnowfall events in Iran.

Pervasive cold days are one of the abnormal continental events that have a lot of effects on environment and human activity. These events are created as a result of formation of complex atmospheric patterns. In these patterns, knowledge of how these atmospheric systems interact can be very useful in prediction of the similar conditions and reduction the contingency damages. On this basis in this present research, it has been tried to analyze the earth's temperature conditions and dynamic and synoptic conditions of different levels of atmosphere on 21 January 1964 as pervasive cold day in Iran by using data from minimum temperature gained in different parts of the country and also the atmospheric data from 1960 to 2010. In this day 98. 4 % of the area has been affected by the cold. The results of synoptic and dynamic analysis of atmospheric systems that creating the cold of this day showed that created a blocking system in west and center of Europe and establishment a low height system in north of Russia and the opposite movements of these two systems to each other has caused that cold polar air from northern Scandinavia have moved to Iran. It has also been recognized that height factor has performed an important role in cold extremity in this day across the country.