The Study of Drought Synoptical Patterns in Chahar Mahal and Bakhtiary Province

Weather circulation patterns play a main role in the occurrence of environmental phenomena especially in middle latitude. Occurrence of dry and wet spells is in relation to synoptical system repetitions and types of weather. In this research first by using of Standard Precipitation Index (SPI)، extensive droughts in the province were identified. Then by using Principal Component Analysis (PCA) and Clustering Analysis (CA)، daily circulation patterns in synoptically scale were realized. In order to classify weather types، daily mean temperature data in 500 hPa and in mean sea level pressure were selected from the NCEP/NCAR reanalysis data with a 2. 5 spatial resolution during the period of 1950-2006. For identifying type of weather، array S and Varimax Circulation، and for classifying daily weather types K-Mean Clustering method were used. All the days were divided into 18 groups، which present the most predominant weather circulation patterns in the study region. Composite maps of mean sea level pressure and 500 hPa were drawn for every weather types. At the end، dry and wet spells were compared with frequency occurrence of circulation patterns، then rainy weather and dry weather types were identified.