The Role of Subtropical High Pressure in the Spatial Daily Precipitation Distribution in Iran

In this research, geometric configuration and temporal rate of subtropical high pressure was studied at the level of 500 hPa. This study has been done using daily data of geopotential height at 12 GMT in NCEP/NCAR database with spatial resolution of 2.5×2.5 degree in a 55 years period including 20089 days from Dey, 11^th, 1329 to Dey, 10^th, 1384. At First, the mean geopotential height at 12 GMT is extracted for all days of solar calendar to determine geometric configuration of subtropical high pressure and its relation to the contiguous patterns. Then, an agglomerative hierarchical cluster analysis on the basis of ward linkage method has been carried out using Matlab soft­ware. According to this analysis five circulation features in the north hemisphere have been recognized. To identify temporal rate of subtropical high pressure with the selection of a suitable framework, the  mean of geopotential height at 500 hPa level has been calculated and standardized for each day of solar calendar from 1330 to 1383. Thus, a matrix of size 19724×1 was obtained which indicates positive and negative anomalies (strengthening and weakening of subtropical high pressure) in each day. Comparing the frequency of positive phase occurrence days with that of  negative phase event days in the  level under study showed that during recent 50 years the aggravation days of subtropical high pressure were definitely more than weakening days of this dynamic system. Applying contingency table method and calculating χ ² statistic during statistical period(1330-1383) showed that occurrence of precipitation in about 3.1% of studied stations(33 out of 1066) is independent of the  influence of subtropical high pressure. These stations mainly encompass south coast of Caspian sea and north west of Iran. According to this analysis, the occurrence of precipitation in 96.9% of studied stations(1033 out of 1066) are intensively under control of subtropical high pressure at the time of the predominance of this system over Iran.