Climatology of low level clouds occurrence over Iran (1981-2010)

This paper is aimed at climatological study of low level clouds in four seasons and accordance to the long-term pattern for operational cloud seeding over Iran. Clouds are observed in different shapes and heights in the atmosphere. Regarding the classification of clouds there are various ideas. Based on the Howard classification 10 types of clouds are classified in 4 groups based on the height of cloud base from surface includes low with height of less than 6500 ft, middle with height of 6500-16000 ft, high with 16000-18000 ft, and vertical development. Cloud climatology is main part of weather modification program for cloud seeding site selection. In this study, frequency of occurrences of low level cloud including Stratocumulus (Sc.), Stratus (St), Cumulus (Cu) occurrence have been collected from 44 synoptic stations of Iranian Meteorological Organization (IRIMO) for 30 years (1981-2010) for day time including 06, 09, 12, and 15 UTC and CFSR[1] data with 0.5 degree resolution have been used for deriving of long term seasonal pattern. Seasonal mean of geopotential height, temperature, horizontal wind, vertical wind at 500 hPa, surface pressure and precipitable water (PW) in (500-1000) hPa layer have been derived. Investigations have been carried out based on location of province and watershed basins. The most important investigating results show that low level clouds occurrence with less than %30, have not been observed in selected stations in cold season including autumn and winter . The Northwest, North, and Northeast provinces and basins have high (>%70) low level clouds occurrence in all months which have suitable condition for operational cloud seeding with respect to this index. Low clouds occurrences in warm season including spring and summer show that southern part of Iran have low (<%30) to medium (%30-%70) but were high (>%70) for other stations which detail information of Southern provinces are as following; provinces including Hormozgan , Bushehr, Sistan-Baluchestan and Khuzestan, Kerman, Fars , and South Khorasan are between %30 -%70. Maximum of low cloud occurrence belongs to spring, autumn, winter, and summer respectively. Maximum of cloudiness belongs to March and April in spring. The percent of low level cloud occurrence in southern basins in most months were low (<%30) which achieved very low score for operational cloud seeding with respect to this index. Long term atmospheric pattern shows suitable condition for amplification of cloudiness in clod seasons which accompanied with falling of cold weather Siberian high pressure and amplification of the warm Southern flow provide suitable condition for enhancement of humidity specially for west part of country. Formation of thermal low pressure in summer season, coverage of level cloud and precipitable water reduces over region. In cold seasons (roughly autumn and winter) maximum of low cloud occurs at Bandar- Anzali station in Gilan Province with max of 93.03% and Ramsar station in Mazandaran province with max of 94.34% which both occur in October. In the warm season (roughly spring and summer) is related to Ramsar station with Max of 99.10% and Noshahr station with 98.72 % which both occur in September. The minimum of low cloud occurrence is related to Zabol station in Sistan- Baluchestan province with 32.88% and Iranshahr station in Sistan- Baluchestan province with 37.93% in clod season which both occur in October. In warm season Bushehr station in Bushehr province and Abadan station in Khuzestan province have 15.79% and 16.34% respectively which both occur in September. Due to location of Caspian Sea in North of Iran and Alborz Mountain in southern part of it, therefore the most low level clouds occur in moist and rainy climate.
Long term surface pressure pattern show formation of suitable condition for amplification of cloudiness amount in cold season which with falling of cold weather in North and Northwest associated with Siberian high pressure and amplification of warm and humid weather provide suitable condition for increasing of humidity specially in Midwest of country. Formation of thermal low pressure at the surface causes decreasing of coverage of low cloud and precipitable water over the region in summer.
Although application of satellite data for investigating of clouds is prevalent due to their expand coverage, but the accuracy and importance of surface observation is not deniable. However, in order to achieve more accurate mapping of low level cloud over Iran, due to the shortage of number of synoptic stations with long term observation using satellite data is necessary.