The role of interaction subtropical- polar jet stream on heavy rainfall in the spring and autumn of Iran

The narrow bands of strong wind (≤30ms-1) in the upper levels of the atmosphere called jet stream. The formation of these streams in the maximum gradient zone of the atmosphere (the location of the maximum temperature and energy transition) has caused the establishment and movement of their speed nuclei at the western extremities of the equatorial fringes and on the polar front in the extraterrestrial region and the formation of western margin and polar fronts.  This winds blow from west to east, but they often shifts to the north and south of seasonal average position. In this case, jets may be deployed simultaneously in one region and occurrence the interaction between them may affect the heavy rainfall. Based on several definitions that have been presented with heavy rainfall according to different criteria and thresholds, Cumulative precipitation of more than 30 mm in 24 hours, causing severe financial losses to various urban and rural areas, is called heavy rainfall. In This article, shifting and intensity of subtropical - polar front jet stream in the mid-latitude region and that’s effects on heavy precipitation in Iran has been studied. In this study, the effects of interactions patterns subtropical-polar front jet stream on heavy spring and autumn rainfall in Iran (1988-2017) with Atmospheric circulation to environment Approach has been investigated.  Therefore after determining the maximum coefficient of variation of this jet streams, in order to determine the effective circulation patterns in these synchronies, by applying factor analysis by PCA on the data of a height of 500 hp, four main components with The explanation of 82% of data diffraction was determined.Then, by using cluster analysis method, the days in each cluster and by intra-group correlation, the representative day of each cluster was determined.After drawing different patterns of simultaneous deployment in the Surfer software, four synchronous patterns were determined in the range of maximum coefficient of variation of jet stream (27-40°N, 36-56 ° E). Finally, the effects of each of these patterns on the occurrence of heavy rainfall were determined by obtaining divergence, vertical velocity, vorticity, relative humidity, specific humidity, temperature and mean sea pressure from the ECMWF center. The results of the study show the effects of three interaction patterns on heavy rainfalls. The effects of direct and indirect jet streams interactions in the first and second patterns, with the change in the thickness of the polar fronts and the interaction of jets in the third model, has been affected on atmospheric conditions and occurrence of heavy rainfall. Despite the direct interactions of the first pattern of jet stream synchronization in the rainfall concentration, the indirect interactions effect of the second pattern on the cyclone circulation boost of the polar jet stream has led to the occurrence of extensive rainfall. Also, the mutual interactions of the jet stream in the third pattern, with the intensification of the frontogenesis process, have been effective in the occurrence of precipitation along these fronts, while the proximity of these opposite turns in the fourth jet streams concurrency pattern is not effective on rainfall, and the occurrence of heavy rainfall has been affected only from the cyclonic circulation of the subtropical jet stream.