the recognition of circulation patterns affecting the occurrence of late freezes of spring in kurdistan during 1344-1388

the aim of this research is recognizing synoptic patterns of the level 500 hp occurrence of late spring freezes over kurdistan province during 1965-2005. after determining freeze temporal-spatial threshold, 22 freezing days spreading over the region were extracted. then, level 500 hp height data located in a scope of 25-60 northern latitude and 30-60 eastern longitude was formulated as an s-mode matrix for freezing days. after that, the synoptic patterns were classified using statistical method of component analysis and ward’s clustering method. finally, the composite map of high and low level of each cluster was prepared and analyzed synoptically. the findings show that at the time of the most severe late spring freezes, mainly seven synoptic patterns prevailed in to three main groups: short waves trough, short waves ridge, high atmosphere cut-off low system synoptic. so that, downwards and upwards moves of the rear and front side of the above-mentioned direct cold and relatively dry air masses of high latitude from north and north–west of iran towards kurdistan provinces and determine temperature descending and bring about freezes.