Analyzing the Circulation Patterns Affecting Rain Showers in the Mountainous Regions of Iran

Rainfall is the most significant phenomenon or feature of the environment. The factors causing rainfall have been the subject of numerous studies. Both the rainy or very dry years have considerable impact on the quality of humans’ living environments. Rainfall occurs when there is enough moisture as an effective factor. Both of these conditions are provided through circulation patterns of the atmosphere. The purpose of the present study is to identify the circulation patterns and factors causing rain showers in the mountainous regions of Iran (i.e. Southern Alborz and Eastern Zagros mountain range); moreover, it is attempted to explain how they are formed, during which period of the year they could evolve more significantly, and at what times they occur in the region, along with their similarities and differences. Given the breadth and considerable socioeconomic significance of the region being studied (i.e., Southern Alborz and Eastern Zagros mountain range) it is necessary to identify the circulation patterns causing such damaging rain showers. To this end, the occurrence of this phenomenon could be predicted by observing the commencement of the sequences of patterns leading to torrential rain showers at least one or two days beforehand. In this case, there would be enough time to make necessary preparations. In this study, the circulatory synoptic approach to environment was used; in this regard, first the statistics and data on the climate codes of past and present in stations including codes 80 to 99 related to showers or heavy rainfalls accompanied by lightning were extracted from 50 synoptic stations in the region of the study during a 30-year period (1985-2014). Next, the days of occurrence (code 80-99) were identified followed by the days with rain showers among the entire statistical data which had been observed in at least 50% of the stations of the region; ultimately, 80 days of rain showers were identified. Then, clustering approach was used on data in order to identify circulation patterns causing such showers based on Euclidean distances and integrate it with Ward’s method in SPSS. It was carried out according to the elevation data at 700 hectopascal level of these days, which is the best level for demonstrating how rain showers are formed. Furthermore, data on the vertical speed (omega) and components of orbital, meridional and vorticity winds obtained from clustering patterns at 10-80 E and 10-70 N with a spatial resolution of 2.5 2.5 degrees were provided. In the end, the related maps were drawn and analyzed in Surfer and GrADS softwares. According to the findings obtained from principal components analysis, seven primary components constituting 80% of total data diffraction were selected so as to indicate climate circulation patterns. Given 80 days representing the entire days being studied (19333 days) in 50 synoptic stations during a 30-year period (1985-2014), 7 clusters (circulation patterns) causing rain showers were identified through the aforementioned method as well as conducting cluster analysis with Euclidean distances based on data at the level of 700 hectopascal using Ward’s integration method. Drawing conclusions on the analysis of seven clusters at the region of the study showed the presence of drought in the western part of the region as well as blocking and cut-off low at the level of 700 hectopascals, low-pressure systems in Saudi Arabia and Eastern Iran and a high-pressure system at the northern part of the region, and dominant synoptic phenomena during days of rain showers at the region. Examining the patterns of climate’s upper levels in the region studied shows that the highest volume of rainfall occurs when systems are formed as blocking, while the high-altitude subtropical center moves toward more southern latitudes. Given the analysis of drawn synoptic patterns, according to the data of level 700 hectopascals, in general, it must be pointed out that among the obtained patterns from clustering the aforementioned level data, the highest number of days with rain shower occurrences in the region follows the cut-off low pattern of the Black Sea as well as the deep drought of North-Western Iran; these patterns have been formed mainly during Spring (March, April, and May). The highest amount of rainfall in this pattern has occurred in the Western and North Western of the region. The examination of this pattern demonstrated the presence of a deep and strong closed low-altitude center on the Black Sea. While this low altitude contour gradient center in the region moves, it paves the way for the formation and expansion of baroclinic waves in the eastern regions of the trough. As the low altitude center (cut-off low) is closed, the movement of Western systems over the Iranian mountainous region is slowed down and rainfall continues. On the other hand, the western and north-eastern parts of the region under investigation are influenced by the 1025 hectopascal high pressure of Western Europe in this pattern while the eastern parts of the region are affected by the low-pressure center. The presence of a strong ridge in the Northern Mediterranean Sea as well as a deep scot-off low in the Eastern Black Sea have strengthened baroclinic waves at the lower level of the atmosphere; consequently, considering the proper convergence in low levels, suitable conditions for ascension and positive vorticity, divergence at upper levels of the atmosphere, and desirable rain showers (more than 40 mm) have been observed, particularly, in the north-eastern parts of the region. In this pattern, humidity transfer has occurred in the north-eastern and eastern parts of the region over the Caspian Sea, through high-pressure tabs. In addition, the role of the cyclonic movement of south-eastern systems in transferring and enhancing the humidity of the Arabian Sea and the Persian Gulf cannot be overlooked. According to the 7 synoptic patterns formed in this clustering, the southern and south-western low-pressure systems are of substantial importance in offering convergence among low levels and rain showers. Rainfalls occur in high volumes when systems appear as cut-off low or deep and strong low-altitude centers in the upper layers of the atmosphere. Furthermore, the presence of a strong ridge in Western Europe and the subsidence of cold weather from high latitudes have enforced pass pression center at low levels while its tabs have led to the northern streams and the injection of Caspian humidity to the western parts of the region under investigation.