Statistical - Synoptic Analysis of the Extreme Precipitation in Western Iran

Heavy daily rainfall occurs when it exceeds the region's long-term average and becomes an extreme event or climate hazard. The aim of this study is to provide the synoptic and statistical analysis of the most intense daily precipitation in western Iran during a period from 1996 to 2017. To achieve this aim, Precipitation days with a maximum of 70 mm or more were specified and arranged according to the season. Finally, the most intense rainfall events of each season were selected for the analysis. Based on results, the main area of heavy rainfall is located on western slopes of Zagros, including Ilam and Marivan. In winter, the expansion of the Mediterranean system to the western Iran with accompaniment the Red Sea humidity has been recognized as an essential factor for heavy rainfall. In the spring, the formation of a Mediterranean cyclone on the Caspian Sea and the northwest of Iran has caused heavy rainfall. The most intense extreme rainfall of all times have been observed in October and autumn, due to the convergence of the Mediterranean and Sudanese systems and the establishment of a cut-off low over the eastern Mediterranean at the middle level of the atmosphere. The deployment of high-temperature air mass on the Persian Gulf and humidity advection from the southern seas and the Mediterranean have strengthened the system. The Jet stream in the atmosphere's mid-level played an important role in enhancing the convergence of winds and the low-pressure pattern of the Earth's surface. Because, the flow of warm air ascend from the Earth's surface toward the middle of the atmosphere has been out of the area rapidly. Therefore, hot and humid wind currents are absorbed more rapidly towards the low pressure center adjacent to the Earth's surface and with the release of latent heat, the air ascend to higher levels of the atmosphere.
Extended Abstract
1-Introduction
Precipitation is one of the most important climatic parameters in any geographical area. The Precipitation enhances surface and groundwater levels providing soil moisture through Influence into the soil. This climate parameter in mountainous areas such as Zagros in western Iran has wide variations.­ Diversity of topography is one of the factors leading to these changes. Altitude, aspect of slopes, differences in sun exposure, distance and proximity to water sources such as seas and latitude are the most important factors in the complexity of the relationship between topography and precipitation. There is a direct relationship between precipitation and the activity of precipitation systems in western Iran. The Mediterranean systems are transmitting cold air higher latitudes and Mediterranean Sea humidity to the Zagros Mountains. The expansion of the Mediterranean trough in the middle level of the atmosphere toward low geographical latitudes activates the humid low pressure system on Sudan and the Red Sea. The Sudanese system extends over Iran through the movement of the northward or northeastward Mediterranean trough making precipitation. The humidity of the Sudanese low-pressure system is supplied by the Red Sea, the Arabian Sea, and the northern Indian Ocean. Sometimes, the energy and humidity of these systems is so high that it causes very heavy rainfall over the Zagros Mountains in western Iran during one day.
2-Materials and Methods
In this study, daily rainfall data of 22 western synoptic stations of Iran were processed for the period 1996 to 2017. Besides, sea level pressure, atmospheric geopotential height, U-Wind, V-Wind, specific humidity and air temperature data were used for mapping and synoptic analysis. Having checked the rainfall days, maximum daily rainfall of 70 mm or more was considered as a quantitative criterion. According to this criterion, 30 cases of precipitation over 70 mm daily were identified. The most severe of these precipitation were analyzed and studied as the examples of extreme rainfall. These were the February 3, 2006 (162 mm), the April 13, 1996 (131 mm), and the October 29, 2015 (282 mm). The combinatorial maps of atmospheric parameters including sea level pressure, geopotential height, specific humidity, U-Wind and V-Wind and air temperature for levels of 850 and 500 hPa have been analyzed in order to identify the synoptic circulation patterns that cause extreme precipitation events. The composite maps displayed the arrangement of synoptic patterns on a geographical coordinates including latitudes of 10 to 50 degrees north and longitudes 20 to 70 degrees east with a spatial resolution of 2.5 to 2.5 geographical degrees.
3-Results and Discussion
In the cold season of the year whenever the Siberian high-pressure system withdraws from the western parts of Iran, conditions are provided for the establishment of a Mediterranean low-pressure system at sea level. In higher levels of the atmosphere, the Mediterranean trough advances toward Sudan and the Red Sea, resulting in the convergence of western and southern winds.  Extreme specific humidity, severe pressure decreasing, air vorticity increasing, velocity increasing of upward motions, and temperature conflict over the area are logical reasons for the formation of heavy rainfall in western Zagros slopes such as Ilam and Marivan. The air that ascends to higher levels, consumes significant amounts of latent heat to ascend the air, and condenses water vapor in a wide surface. Whatever the humidity content of the system increases, the air column consumes more energy through releasing the latent heat to ascend to higher levels. Increasing the height of the water vapor density contributes to the growth of more cumulus clouds such as cumulonimbus, and the cloud height increases from base to peak. The main cause of the extreme winter precipitation was the expansion of the Mediterranean trough over western Iran, the humidity of the Red Sea and the strong convergence of the southern and western winds. In the case of extreme spring precipitation, the role of the Mediterranean system and the humidity feeding of this system through the Caspian Sea and the Red Sea have been the cause of heavy rainfall. The main reason for heavy rainfall event on the region in autumn season has been the expansion of the Mediterranean cyclone and its integration with the Sudanese low pressure system and the warm and humidity air convection from the lower latitudes.
4-Conclusion
Based on results, the main area of heavy rainfall is located on western slopes of Zagros, including Ilam and Marivan. In winter, the expansion of the Mediterranean system to the western Iran with accompaniment the Red Sea humidity has been recognized as an essential factor for heavy rainfall. In the spring, the formation of a Mediterranean cyclone on the Caspian Sea and northwest of Iran has caused heavy rainfall. The most intense extreme rainfall of all times have been observed in October and autumn, due to the convergence of the Mediterranean and Sudanese systems and the establishment of a cut-off low over the eastern Mediterranean at the middle level of the atmosphere. The deployment of high-temperature air mass on the Persian Gulf and humidity advection from the southern seas and the Mediterranean have strengthened the system. The Jet stream in the atmosphere's mid-level played an important role in enhancing the convergence of winds and the low-pressure pattern of the Earth's surface. Because, the flow of warm air that has ascend from the Earth's surface toward the middle of the atmosphere has been out of the area rapidly. Therefore, hot and humid wind currents are absorbed more rapidly towards the low pressure center adjacent to the Earth's surface and with the release of latent heat, the air ascends to higher levels of the atmosphere.