عنوان مقاله [English]
Natural disasters have always occupied the human mind, and in spite of the increasing progress of human knowledge, human beings are always weak against these disasters. Floods are one of these disasters. Identifying floods is one of the most important steps in controlling it, and in this direction, data and their analysis can play a very decisive role. The first factor in causing floods is the intensity of rainfall, and the largest floods are related to heavy rainfall with a maximum instantaneous peak that occurs at a certain point in time. Heavy rainfall have the potential to be considered a threat in most parts of the world, as we have seen floods and the death of many people around the world by floods. Precipitation is the main atmospheric phenomenon resulting from the complex interactions of the climate system and has a complex behavior due to the connection with different components of the climate system. The occurrence of precipitation requires several conditions. Moisture availability, deep instability availability and cooling are necessary conditions for precipitation to occur. For heavy cloud precipitation to occur, these conditions must emerge at their strongest. Various definitions have been proposed for heavy rainfall in the world: criteria such as events greater than 50 or 100 mm or a probability of occurrence of less than 10, 20 and 25% have been introduced Existing studies based on hydrological statistics and flood events show that the southern and southwestern regions of Iran are among the most flood-prone parts of the country. In this study, heavy and pervasive rainfall in the southern and southwestern regions of Iran in the period 1986 to 2015 is statistically and synoptically examin. so, daily rainfall data from 17 selected synoptic stations in south and southwestern Iran during 1986–2015 were used. The dataset is also used by the National Center for Environmental Prediction / National Center for Atmospheric Research (NCEP / NCAR) with a spatial resolution of 2.5 x 2.5 in latitude and longitude. The variables used are geopotential data, sea level pressure maps (SLPs) specific humidity (q), zonal and meridians components of wind (u, v). We use the basic percentile index to determine the days associated with heavy rainfall and to separate them. The base percentile index is the most common method for calculating and determining severe and limit precipitation. It is mainly used to determine the number of days with high, very severe and extreme rainfall, specific percentile thresholds. According to international agreement, the total number of rainy days, days above or equal to the 99th or 95th percentile are considered as severe and limiting precipitation, respectively. In this study, the 95th percentile index is used. Considering the occurrence of events in 50% of the stations studied, heavy and pervasive precipitation events are separated. The average of 30-year rainfall (1986-2015) in the study area (southwest and south of Iran) shows that the northern regions of the study area have more average rainfall than other areas such as Koohrang, Shahrekord, Abadeh, Dogonbadan stations are located in this part and the lowest rainfall is allocated to the southeast and northwest corners of the study area, which includes very low altitude areas (including Abadan, Ahvaz, Bandar Lengeh, Bandar Abbas, Kish and Minab stations. Central regions such as Fasa and Shiraz have moderate rainfall. After separating the heavy rainfall events, we studied one day as a synoptically sampled event. Statistical analysis of the data shows that during the period under study, in 1991 and among the stations, Koohrang station had the most days with heavy rainfall. In synoptic studies, we have two types of approaches: circular to environmental and peripheral to circular. Classification and environmental approach to rotation are used in the study of precipitation because the temporal and spatial variations of precipitation are severe and the environmental method of rotation allows the researcher to focus on precipitation and consequently the synoptic patterns that are intended to be studied. Has. After identifying heavy rainfall events in the study area, a typical event was identified in the south and southwest of Iran. The heavy rainfall event on December 25, 2004 is significant in terms of intensity and prevalence. On December 25 and 26, 2004, 455 mm of rain fall reported from selected stations in southern and southwestern Iran in one day. The highest amount of rainfall is related to Yasuj station with 91 mm. analysis of synoptic maps showed that The strength of Sudan's low-pressure system provides the basis for instability and precipitation on heavy rainy days that affect the study area. At 850 hPa, southwestern currents combine the humidity of the Arabian Sea, Oman, the Gulf of Aden, and part of the humidity of East Africa and send it from the Red Sea along the southwest-northeast axis to the study area. also pressure gradient between The high pressure of Europe and the low pressure of Sudan creates instability and cyclonic conditions in the region. These conditions absorb moisture from moisture sources such as the Red Sea and the Arabian Sea and send it to the study area. At 500 hPa, on the eastern part of the Arctic shore, there is a turbulence from the eastern Red Sea to the Caspian Sea, which causes low pressures of 1000 hPa to rise to high altitudes. The high humidity seen on the map this day has caused heavy rainfall. On this day, subtropical jet stream is located next to the tropics at a speed of 65 meters per second in the west-east direction on Iran. With the relocation of the polar front to low latitudes and the displacement of the tropical side to the east, the amount of precipitation increases significantly.