واکاوی همدیدی - ترمودینامیکی الگوهای بارش فراگیر تابستانه

 Introduction The principal objective of the synoptic studies is to explain and justify the environmental changes of the surface based on the pressure patterns’ changes. The atmosphere circulation and its mean patterns as prominent factor, plays a very significant role in determining the climatic condition for each region. The plateau of Iran is experiencing a considerable climatic variety during the year in relation to the circulative patterns’ diversity and frequency in effective components, so that during the warm period the summertime subtropical high pressure (STHP) is considered as the main controller factor of the Asian southwest region’s climate. In such circumstances, the occurrence of extensive and intensive precipitations during the warm period and summertime low rainfall over the plateau of Iran has been taken into account as the conspicuous features of the precipitation variety in the region and also is considered as a debatable issue in the climatic studies of Iran. Considering Iran’s environmental characteristics, it seems necessary to study the summertime precipitation patterns over the country since the occurrence of summertime precipitation is also contemplated as the rare cases (apart from the southern coasts of in the Caspian Sea).    Methodology The study area consists of 22 provinces located in the northern half of the country which have been influenced by the activity of the systems under study. In this research, 27 synoptic stations were employed which cover approximately the whole northern half of the country. The daily precipitation data gathered from the country’s synoptic stations and the digital data were selected from the NCEP/NCAR reanalysis dataset for a 20-year period (1986-2005) in order to determine the synoptic patterns of the summertime severe precipitations and their influences on the summer climate of the northern half of the country. The criterion for selecting the date of the rainy systems was based on the fact that at least at two thirds of the study stations, it the occurrence of rainfall during the examined periods had been reported. The composite maps of geopotential height, relative vorticity, and streamlines were produced by using the reanalysis data of geopotential height, U and V wind components, vertical velocity, relative humidity, sea level pressure and temperature at various levels of the atmosphere, and then the precipitation patterns were analyzed. Finally the thermodynamic characteristics were investigated by generating the skew-t diagrams using RAOB software.    Discussion Since the main objective of the present research was to determine the summertime comprehensive precipitation patterns over the northern half of the country, in this section the atmospheric maps of comprehensive precipitation for 3 extracted patterns are investigated in a thermodynamics-synoptically manner.  In the 1st pattern, the formation of low centers over the Mediterranean Sea and Black Sea result in transferring the humidity onto the central Asia as well as the study area. The first pattern was identified as the prevailing pattern for the summertime precipitation occurrence in the northern half of the country, as most of the rainy systems have taken place in the form of the 1st pattern. In the 2nd pattern, the high latitude and polar region lows eastward movements and extending its tongues towards south affect the lower latitude regions so that the detached cut-off lows from these systems cause the summertime rainfalls over the area. In the 3rd pattern, a blocking system is formed between the latitudes 350-600north due to the interaction of Saudi Arabia’s high and the polar front’s lows. Through formation of the mentioned blocking system over the region and simultaneously the Arabia’s high retreating southward, its western low causes the summertime precipitation of the northern regions of the country.  The positive relative vorticity value was very considerable in all the low’s troughs of the study patterns, so that the values estimated between 1-5s-1and 9-5 s -1 showing a very suitable condition for dynamical climbing of the air. This condition was observed for all systems under study at 500 hPa level.  The relative humidity condition indicates that the rainy systems are mainly fed the humidity by the Caspian Sea, Mediterranean Sea, Black Sea and the emigrant anticyclones. But the separated lows formation along the northern border of the Caspian Sea should not be ignored which cause northern streams. The northern border of the country at 1st pattern has been experienced a 60-90 percentage of cloudiness cover of the sky during the examined periods. In the 2nd pattern, the mean relative humidity condition and the total cloud cover were observed 40-90% and 50-90%, respectively.    Conclusion By having the rainy systems investigated, three prevailing patterns were identified. The results showed that in the 1st pattern, the formation of low centers over the Mediterranean Sea and somewhat on the Black Sea over the Europe and their eastward movement cause the summertime rainfalls. In the 2nd pattern, movement and extension of the high latitudes and polar region lows toward east and extending its ridges southward as well as formation of the atmospheric cut-off lows resulted in the summertime rainfalls in the region. In the 3rd pattern, the interaction between Arabia’s high and the polar front’s lows leading to a blocking system formation in the northern latitudes and simultaneously with Arabia’s high going back southward, Its western lows cause the summertime precipitations over the northern half of the country.  The relative humidity condition indicates that the rainy systems are mainly fed the humidity by the Caspian Sea, Mediterranean Sea, Black Sea and the emigrant anticyclones. Although, the separated lows formation along the northern border of the Caspian Sea should not be ignored which cause northern streams. The northern border of the country in the 1st pattern has experienced a 60-90 percentage of the cloudiness cover of the sky during the examined periods. The thermodynamics diagrams analysis at the study patterns showed that the convective available potential energy (CAPE) plays a very significant role in strengthen the instability condition at the lower levels of the atmosphere.    Key Words: Comprehensive Precipitation, Synoptic Pattern, Skew-T Diagram, CAPE Index, Northern Half of Iran.