عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Introduction: The main feature of Iran's rainfall is their variability; in fact, changes in rainfall are due to changes in their producing factors. The study of rainfall changes in the country showed that the spatial variations of precipitation from west to east and north to south decreased and these changes are well-coordinated with Iran's major rough nesses. The highest rainfall of the southern shores of the Caspian Sea is controlled by migratory cyclone, but about half of the annual precipitation and the most severe precipitation occur in the area of the Siberian highlands in the north of the Caspian Sea. The purpose of this study is to analyze the nature and structure of Variation in atmospheric spring rainfall in Iran. Unlike in-country studies that have been conducted more regionally, this study attempts to study the entire region of Iran.
Materials & Methods: According to the purpose of the study, two databases have been used: Environmental data: This group of data was obtained by interpolating the daily precipitation values for the statistical period from 1961 to 2010. . For this purpose, the daily precipitation data from 573 synoptic, climatological and rain gage in a 50-year period (1961-2010) is extracted. After extraction rainy days inclusive, corresponding pressure data, was extracted using the programming capabilities in the GRADS software environment and have been identified by the cluster analysis of representative days and Iranian spring weather patterns in the MATLAB software environment. After the creation of the database for the determination of the rainy day, three criteria were considered; Daily precipitation of 1 mm and above, minimum continuity of two days and at least 50% coverage (without spatial continuity).
Atmospheric data: For this study, variables such as sea level pressure, geopotential heights, Special moisture Wind Meridian and orbit, Atmospheric moisture flux, Perceptible water and vorticity were studied. After extracting rainy days inclusive, Cluster analysis has been used to identify the rainfall patterns. Lund's correlation method was used to select the representative day. Therefore, to select the representative day, the day that has the highest similarity with the maximum number of days in the group has been selected. The correlation coefficient represents the degree of assimilation of the two map patterns with each other so, the day with a greater number of days, the coefficient of correlation of 0.55 was selected as the representative day. After extracting the representative day, in order to analyze the dynamical equilibrium of them, variables such as sea level pressure, geopotential heights, and Special moisture Wind Meridian and orbit, Atmospheric moisture flux, Precipitable water and vorticity were analyzed.
Results & Discussion: The results showed that the highest amount of spring rainfall in Iran was due to the intense pressure changes the Caspian Sea East Turkey-low pressure, low pressure Saudi Arabia. While the most prevalent spring weather pattern of Iran, the Siberian-low-pressure Siberian strain of Saudi Arabia has been the low pressure of Sudan. So that by creating a strong pressure gradient on Iran and deployment Mediterranean landing deep in the atmosphere has led to instability in Iran. The status of moisture sources also indicates that the role of the Arabian Sea, the Persian Gulf, the Red Sea at 850 and 700 hPa, is clearly visible and detected in the spring humidity flux of Iran. In addition to providing the humidity of the coastal areas at the sea level, the Caspian Sea is one of the main sources of spring rainfall in the northwest and northeastern regions at 700 hPa; while the Arabian Sea plays the most role in the humidity flux towards the southern and western parts of Iran. Check the status of atmospheric vorticity showed the highest levels of instability in the atmosphere above the atmosphere, because of Westerly that has been accompanied by different pressure systems. In this regard, the maximum degree of divergence of the atmosphere occurred when the Siberian High Pressure was active and its tabs on Iran.
Conclusion: Spatial Distribution of Dynamic Matching Patterns showed that Iran's spring rainfall was influenced by five cycling patterns. The results showed that spring Inclusive occurred when severe compressive influences were caused by the influence of the high-pressure cold-water systems in the North and the warm and humid masses of the southern regions, this leads to instability and vertical ascent is air. However, divergence and negative vorticity maximum amount of time by which the Siberian high-pressure tanks have been located in Iran, and the highest concentration of convergence and ascent and unstable air occurred during the deployment of the Saudi and Persian Gulf and the intensification and intensification of it at the high altitude of the atmosphere.