Synoptic analysis of torrential rains in Gilan province

To receive a master's degree

Department of Geography, Climatology in Environmental Planning

Abstract

In order to identify the synoptic patterns leading to torrential rain, firstly, daily rainfall equal to 20 mm and more during the statistical period of 1991 to 2010 as the basis for analyzing the torrent for the synoptic stations of Astara, Anzali, Rasht and Ramsar with the condition of simultaneity and universality of rainfall. Determining and then identifying the pattern that led to it. During the studied period, 120 cases of precipitation with the above condition were identified. In this research, the environmental approach to circulation has been used. The obtained results showed that seven European high pressure systems, Scandinavian high pressure, Siberian high pressure, Arctic high pressure, Mediterranean low pressure, Arctic low pressure and cyclonic rains are involved in the torrential rains of the southwestern Caspian Sea. Therefore, the synoptic conditions necessary for the occurrence of heavy rains in Gilan province are created by the advection of cold air over the Caspian Sea and the receipt of moisture by the Scandinavian high pressure system, the fall of cold air from the northern Russian region in the upper atmosphere (Northern) to the Caspian Sea, the intrusion of the European high pressure system from the northwest and the north of the Caspian Sea, the intrusion of cold air from the polar side and the intrusion of the low pressure system of the Mediterranean from the south and southwest. Examining the synoptic maps of the earth's surface and 500 hectopascal patterns showed that the majority of widespread rains of more than 20 mm occur when the gyre system established in the north of the Caspian Sea receives moisture from the Caspian Sea in the form of convective convection and carries it to its southwest coast and causes showers. Also, migrant high pressures (including European high pressure and Scandinavian high pressure) and Siberian high pressure create showers with the highest frequency.

Key words: overview, precipitation, shower, analysis, Gilan province

Chapter 1

Research overview

Introduction

Investigation of weather conditions in Iran due to its diversity Its climate is very important, the shores of the Caspian Sea and the north of Alborz have a more sensitive balance. Due to the fact that Caspian has a humid climate and its rainfall is higher than other regions of Iran, it is necessary to investigate and research how the rainfall trends and systems affect it. To be fully aware of the results, and this awareness will also help in more goals and precise planning, based on the divisions of the Ministry of Energy (Figure No. 1) related to Rasht, Anzali, Astara, Ramsar stations in the study area. One of the distinctive features of this region is abundant rainfall throughout the year, so that it decreases with the distance from this region in any geographical direction in the country. The purpose of this research is to know the synoptic patterns leading to rainfall in the region, widespread rainfalls of more than 20 mm and its movement and source of moisture, and what is the performance of the above systems. which causes a difference in the amount of precipitation in terms of time and place in the region. The statistical population includes: synoptic stations, Rasht, Anzali, Astara, Ramsar. Air ascent is one of the precipitation factors in a region. In our country, the mechanisms of ascent cause widespread rains. The establishment of westerly wind wave systems is in the middle latitudes, which happens in the cold period of the year, investigating the warm seasons of the year when the abundant rains of the cold period are deprived, in order to store water from rainfall and strengthen underground water resources, is of particular importance, on the one hand, to know and understand the laws governing the temporal and spatial distribution of precipitation in Iran, it is necessary to identify the pressure patterns governing the precipitation period, and according to the pressure distribution and changes in the society, it is necessary to determine the precipitation patterns, this is the necessity of the program The economy of the country should be considered as a vital factor, also because in the area that needs water in early spring, it is important to not need water in late summer for agricultural products. The establishment of high pressure near the tropics in the warm period of the year and the wave system of the western winds of the middle latitudes in the cold period of the year has caused a large area of ??the country to be deprived of precipitation and the majority of the country's rains are limited to the cold period of the year. The severe lack of water resources on the one hand and the occurrence of heavy rains and showers on the other hand require that this issue be carefully evaluated. Due to its proximity to the moisture source of the Caspian Sea, Gilan province experiences different conditions in terms of rainfall compared to other regions of the country. In connection with torrential rains, extensive studies have been conducted in Iran and abroad.Among the researches of Ron Kahana et al. (2002) regarding the synoptic climatology of the main floods in the Negev (NEGEV) is the use of hydrology to determine the sample occurrence by Yarnall and Frakes (1997). Also, the use of land surface and atmospheric information in determining synoptic weather and land surface changes (Kidson 1997) about heavy rains, they investigated several variables about the characteristics of the land surface and the 250 and 800 hectopascal level. In the synoptic survey of waves caused by storms in the Caspian Sea, Jenin infers that the storms in the Caspian Sea are caused by the establishment of high-pressure fronts that cause strong winds from the north and northwest. or northeast, they arise, a synoptic study and statistical analysis of the rainfall of the southern shores of the Caspian Sea has been done by Vahidi (1376). Mohammadi and Masoudian (2009) in the simultaneous analysis of heavy rains in Iran with an environmental approach to circulation showed that the high pressure pattern of Europe - low pressure of Iraq was effective in the rainfall event from 12th to 17th of Aban 1373. At this time, the high pressure of Europe and the Black Sea penetrated from the north-west of the country and caused a severe shift with the low pressure of Iraq. They believe that under this trend of pressure and integration of subtropical winds and the polar front over Iraq, deep descent over Cyprus, the moisture supply of the Persian Gulf at the levels of 925 and 850 hectopascals, the moisture supply of the Red Sea, the Mediterranean and the Black Sea at higher levels, create these heavy rains. Khushal et al. (2008) in order to identify the origin and moisture path of super heavy rains in Bushehr Province showed that the moisture entry paths in the region are mainly south, southeast and southwest and the high pressure systems of Siberia and Arabia, the dynamic low pressure center of Eastern Europe, the Sudanese low pressure system and the European high pressure center at the sea level and the Wacharkhand systems of Eastern Arabia, the North African Nave and the Spanish Wacharkhand in the lower and middle atmosphere are effective systems for transferring moisture over the studied area. are Azizi et al. (2005) analyzed the synoptical analysis of heavy rains in the west of the country during March 16 to 24, 2015 and concluded that the low pressure system of the Mediterranean Sea and the Sudanese low pressure zone caused the rainfall during this period and that the Mediterranean Sea, the Black Sea and the Red Sea played a role in strengthening these systems on the surface of the earth. Amidar et al. (2009) consider the main cause of heavy rains in the central and southwestern regions of Iran to be the penetration and establishment of the Siberian high-pressure system and the formation of a large cold front over the southern Caspian Sea and central Iran. At the same time, the unstable western system changes its direction and moves to lower latitudes, and the western systems, by moving over the southern waters of the country, gain a lot of moisture and penetrate into the country from the southwest. Rahimi et al. (2009) believe that the most important factor in the formation of heavy rains in Kerman province is the movement towards the east and the strengthening of the descent of the eastern Mediterranean in the middle troposphere, and when it is accompanied by the downward movement of the polar vortex system, heavy rains occur over the stations of the province. Hamidianpour et al. (2009) by identifying the synoptic patterns of heavy rains in northeastern Iran, concluded that three Sudanese low pressure patterns, combined Sudanese-Mediterranean low pressures, and Mediterranean low pressures were effective in heavy rainfall in the region, and among these three patterns, the heaviest rains are related to the Sudanese pattern. Also, Alijani et al. (2008) in the synoptic analysis of the heavy rain on the 6th of January 2008 in the southeast of Iran, believe that the occurrence of rain in this area is related to the presence of conditions such as high humidity in the lower level of the atmosphere and a strong discharge of moisture from the Persian Gulf, severe negative anomalies in the middle levels of the atmosphere and the establishment of the tropical monsoon in the west of the region. Gandhamkar (2009) by investigating the occurrence of heavy rains in the southern areas of Bushehr province, showed that the main cause of heavy rains and mudslides in the southern areas of Bushehr province is the entry of the Sudanese low pressure system from the south and southwest of the country into this area and the favorable conditions of local ascent such as the heat of the earth. Khosh (1376) in order to analyze and present climatology synoptic models for precipitations of more than 100 mm on the southern shores of the Caspian Sea, showed that these precipitations fall on this sea both due to the arrival and establishment of the gyre systems and due to the arrival and establishment of gyre systems.