چكيده لاتين :
Introduction
Thunderstorms of the most important, most abundant of the most severe weather hazard that each year, in addition to large quantities of crops and infrastructure development, causing human casualties are also around the globe. Between 1953 and 1957 the financial losses caused by the hurricanes in America has over 3.6 million dollars (Changnon;2003,1231).Storms of varying intensities have been identified. The average horizontal range of a few tens of kilometers of the storm, the vertical range of about 10/000 m and has a lifetime of 30 minutes (Henderson, 2006).These storms often clouds Rick, Rick rain and are shaped tower along. Thunder usually high or heated air masses in the ground or in the air fronts, especially in the cold front, caused, so the Thunder or the origin of air masses or the front.Thunderstorm climate risk as part of the nature of Iran 's North West Province, socio-economic and environmental damages each year to the people, especially farmers and ranchers are (Khoshhaldastjerdi, J. GhavidelRahimi, Yousef;2007,101).(Sari Sarraf et al.; 2007,123) of rainfall in the southern basin of Aras River barrage to approach and came to the conclusion that the main causes of showers barrage in the area are two regional instability (in summer) and in front cold (in winter) is. Meteorologists to study the atmosphere of the period of the formation of storms, the following factors have the appropriate factors or lack of conditions for hurricane formation and climate instability or the possibility of convection as well as show. High levels of cold weather hot weather, low levels and the abundance of moisture, all of unstable atmosphere. Accordingly, since the dynamic atmosphere of instability factors, the most important scientific method to the study of atmospheric instabilities is a storm.
The study area
Hamedan province with an area of approximately 19545/82 kilometers, the circuit (33 degrees 33 minutes) to (35 degrees and 38 minutes) north latitude and between the meridians (47 degrees and 45 minutes) to (49 degrees and 36 minutes.) is located east longitude. Hamedan province to the north is limited province zanjan and ghazvin,and to the south is limited province lorestan,and to the east is limited province markazi,to the West is limited provinces Kermanshah and Kurdistan.According to the latest administrative divisions, including 8 of the city, 21 cities, 20 districts, 71 rural districts and 1120 villages. The average height of about 1,800 meters of surface water. Alvand peak with an altitude of 3584 meters the highest point of Hamedan province and the lowest point is 1,600 meters with a height of land amrabad.These stations are given in Table View.
Name of station longitude latitude altitude
Hamadan Airport 48.32 34.52 1741.5
Nojeh Kaboudarahang 48.71 35.20 1679.7
1Author: 09183177169 E-mail: fa.iranpour@gmail.com
Materials and Methods
To determine the incidence of storms synoptic data from two stations Nojeh Hamadan airport and the period of 15 years (1992,2006) were used. In this study, the initial data from the Meteorological Research Center of Hamadan in the period 1992 to 2006 was obtained, After controlling the data, initial tests have been done and data on the present and past weather the storm of the code were specified time scale.Were determined by surveys conducted during the period of 15 years, 270 thunderstorms occurred in the region. Continue to identify synoptic patterns cause storms in the region, a cluster analysis was performed on 270 days with thunderstorms and storms occurred in this province were clustered. And hours were significant in terms of duration and intensity of storms and showers barrage studied were synoptic, To analyze the thunderstorm occurred during the period, first by synoptic weather map at 500 hPa surface is determined daily and thermodynamic methods, To draw them, raw data and the level of 500 hPa surface of obtaining environmental predictions Center and then by using the Grads were plotted on a map. Thermodynamics diagrams also known as April 10, 2005 and October 31, 2006, due to the occurrence of severe thunderstorms in these days, for example, the website of the University of Wyoming to study, and have been used. Thermodynamic charts (upper) and mean monthly, seasonal and yearly for both stations was calculated, Atmospheric instability to volatility indices were studied. These indicators reflect the high probability of instability and, in particular, storms in the days mentioned.
Result and discussion
Analysis of circulation patterns storm and showery precipitation
In order to identify patterns cause storms Showers and identify the days of the index, a cluster analysis on 270 days with thunderstorms in the area. And these days were divided into six clusters. Then in each cluster on a day which was more unstable, was elected as a representative day. Synoptic pattern of the day, for example, were studied. Study maps of the surface and 500 hPa long-term patterns show that during storms Showers several successive systems of the North West and from Iraq into Iran, causing storms and rain showers in the area is. At ground level from the South West of Iran and the Arabian Peninsula, low pressure Sudan to Iran. ISO time to draw maps and contour maps of the Earth 's surface and the level of 500 hPa,The raw data of the Earth 's surface and the level of 500 hPa, the environmental prediction America Center was received, And then by using the Grads were plotted on a map.
Assessment of changes in storm studied stations
Change the hours of thunderstorms in the airport station is shown in Fig. Most storm at the station between the hours of 15 and 18 respectively and 12 and 21 synoptic hours after they have had the greatest storm. Monthly changes in Days of Thunder in the airport station is such that the most frequent occurrence in April and May and the lowest frequency of its occurrence is in December In between seasons, spring is the most frequently Days of Thunder. And winter, the lowest frequency is allocated.Over half of storms reported in the spring, then fall, most of it is dedicated to, as well as the annual changes in the region studied storm, nearly has a growing trend, in 1993, the lowest of the In the entire region, and 2004 as the most prominent in terms of frequency of occurrence of this phenomenon has been reported.Nojeh station thunderstorms are most frequent occurrence is between 15 and 18 hours. Synoptic hours, followed by 12 and 21 accounted for most of it. During the months of the year, the April moon and may have the highest frequency and December and February have the lowest frequency of thunderstorms.In the between seasons, spring represents the highest frequency. Over half of storms have been reported in the spring, then fall, most of it is allocated, as well as the annual changes in the region studied storm, nearly has a growing trend, in 1993, the lowest of the In the entire region, and 2004 as the most prominent in terms of frequency of occurrence of this phenomenon has been reported.
Conclusion
Analysis and output maps and weather patterns show that the most important indicators of large-scale storms could be severe pressure gradient at the surface, atmosphere and clouds pile a dry layer of more or less similar pluvial instabilies atmospheric noted. Therefore, large-scale patterns useful in detecting not only the exact occurrence of these phenomena can be qualitatively predict the weather. However, volatility indices such as Ki, Si, Sweat and numerical thresholds for these indicators help predict storms in the area. Maximum storm event Nojeh station 534 times and Airport Station 312 time (over 15 years) experienced the lowest incidence of hurricanes. Most of these storms nature front and Synoptic and because the systems climax to the region, because the airport station number of event storms, 81 percent of the station Nojeh 71 percent of the synoptic and the front of the storm, stations, airports, 19 percent of stations Nojeh 29 percent of the local thermodynamic nature. Most storms at both stations during spring and in the afternoon and early evening of April happened, this phenomenon occurs when the active area of low pressure over southern and south with hot, humid currents, their the higher latitudes can be seen in the spring and take a combination of these conditions in parts of West, North West of active centers and the high-pressure cold latitudes of both the lower moves of different weather, significant influence on the northern regions of atmospheric fronts, West and South West of the country has been producing cumulus clouds and cumulonimbus. Which is usually accompanied by torrential rainfall has been observed on these areas. At the time of the storms at the level of 500 hPa, a deeply unstable trough is formed on the Black Sea.
