Climatic-Synoptic Classification of Circulation Patterns conducive to pervasive frost in Iran

Frost, which results from temperature variations at the time, is among the significant phenomena under study in climatology. The current research is intended to identify the mechanisms responsible for Iran’s pervasive frosts through classification of synoptic patterns associated with this phenomenon. Thus, based on a spatial principle, the days when 65% or larger percent of Iran’s surface area underwent zero or below zero (̊ C) temperatures were selected as pervasive frost days in Iran. Total number of such days reached 835 during the total duration of the present study. Accordingly, the average daily data of geo-potential height (sea surface pressure) were extracted for the respective domain and Principle Component Analysis was performed on their matrix in order to identify and investigate the circulation patterns of 500 hPa level (sea surface pressure). Results of this analysis indicated that, using 15 components, it would be possible to explain 89.6% of geo-potential height changes as well as 84.9% of sea surface pressure changes at the times of Iran’s pervasive frosts. Then, to detect the circulation patterns related to Iran’s pervasive frosts, outputs of Principle Component Analysis were used as the inputs of cluster analysis, and finally, five patterns were introduced for geo-potential height of 500 hPa level and 3 patterns were identified for sea surface pressure. The research findings revealed the fact that a deep ridge needs to be formed at 500 hPa level around the Black Sea, and also, there should be an eastward spread of high-pressure European tongue or a westward spread of high-pressure Siberian tongue for occurrence of pervasive frosts in Iran