Categorizing of precipitation and Drought producing synoptic Circulation

Atmospheric circulation patterns are responsible for drought and flood occurrences around the globe; specifically in moderate mid latitude climates. While some circulation patterns are responsible for wet spells and floods as well، the drought events can be attributed to some other circulation patterns. Therefore، identification and distinction of dry/wet producing patterns and the geographical extent of Iran that they may affect is very crucial for many environmental planning. In the present study that aims to identify dry and wet producing atmospheric circulation patterns for Iran، the daily mean sea level pressure and 500 H. pa geopotential height corresponding to 1950-2006 period was retrieved from NCEP/NCAR and used as input variables for Principle Component Analysis coupled with Cluster Analysis. By this procedure all studied days were grouped into 18 clusters and their associated composite maps for both levels were drown and considered as the circulation patterns. To assess the relationship between the identified atmospheric circulation patterns and daily precipitation over Iran the Performance Index (PI) which is a conditional probability index was computed for 54 stations and used in the analysis. This index shows the probability of precipitation occurrence conditioned on the occurrence of a given circulation pattern. The result shows that circulation patterns of CP2، CP3، CP5، CP6، CP7، CP9 and CP10 are dry circulation patterns while CP1، CP4 and CP12 are considered as wet patterns in Iran.