عنوان مقاله [English]
نویسنده [English]چکیده [English]
Introduction: The position of Iran between extratropical and subtropical latitudes, the location of a major part of Iran on the desert belt of the world from North Africa to Central Asia, and the role of various geographical phenomena in Iran have caused all kinds of atmospheric hazards in this region. To have an accident (Alijani, 2018). Such risks are inherent in Iran's climate and have occurred regularly since the distant past, as the natural environment and human activities in different regions of Iran have adapted to these phenomena. As the behavior of these phenomena has been repeated somewhat regularly and the climatic changes and short-term atmospheric processes have been less. These phenomena include droughts and droughts, heavy rains and floods, floods and dust storms, frosts, heat and cold waves (Omidvar-Kamal 2013). But in recent decades, with the increase in the temperature of the earth after the industrial revolution (1850), and with the intensification and rapid trend of temperature in the recent decades from 1970 onwards, the atmospheric hazards in the world, including Iran, have become abnormal. Such heavy and short-term rains cause a lot of damage related to agriculture, severe soil erosion, and destruction of transportation infrastructure and flooding of cities and villages. In recent years, in connection with global warming and changes in the behavior and anomaly of rainfall in the world, the amount of rainfall has decreased in this region, but the rainfall has mainly changed its behavior to heavy and short-term rainfall. Therefore, knowing the patterns that lead to the occurrence of these precipitations will be useful in the first place for predicting and managing precipitations. The aim of this research is to identify and analyze the atmospheric circulation patterns leading to the occurrence of heavy rain and short-term flooding based on the data of the stability rain gauge in Khorasan.
Materials and methods: in this study, the daily rainfall data and the rain logger of the stability station were used. Among the stations in the region, those that have a proper distribution in terms of spatial distribution and also have long-term statistics (1993-2017) were selected. The synoptic stations were chosen to cover all the regions of the province with different topography and climate diversity. In order to determine heavy rains, the 95th percentile index of daily rains in each station was used. In the following, days with heavy rainfall and more than 95th percentile were extracted in each of the stations using these indicators. The spatial distribution of these rainfalls in the whole province was determined by using the number of these rainfalls in the entire region. , heavy rain. In connection with the purpose of this study, determining the atmospheric patterns of heavy rainfall in Khorasan, after determining the days with heavy rainfall in the previous stage, the atmospheric patterns of rainfall were determined and synoptically analyzed. For this purpose, reanalyzed daily grid data with a resolution of 2.5 degrees were extracted from the National Center for Environmental Prediction and Atmospheric Research (NCEP/NCAR) for the above days. The selected window for receiving network data in this study is 15 to 70 degrees north and 15 to 80 degrees east, so that the patterns affecting Khorasan precipitation in this range can be identified.
Results and discussion:
On February 17, 2017, in the geopotential height map of the middle surface of the atmosphere, a trough is drawn over the entire western half of Iran and the western coast of the Caspian Sea, and the center of this trough is a closed cell with a height of 5280 geopotential meters over the northeast of the Caspian Sea and Kazakhstan. This deep trough extends to Saudi Arabia and passes through the center of Saudi Arabia at a height of 5750 meters and shows the penetration of cold air in the upper atmosphere to the warm southern sea of Iran. At the same time, in the east of the Mediterranean Sea, there is a Rex-type blocking. These atmospheric conditions caused persistence and slow movement of the circulation system in the synoptic scale in Southwest Asia. At the same time, there is a low pressure center with a pressure of 1010 HP in front of the curved ship in the east of Iran. The east of this trough extends from the southwest and the western half along the southwest to the northeast direction of Iran, which caused instability in the northeast of Iran. On April 1, 2016, the arrangement of westerly winds in the entire Eurasia area shows a large anomaly in the westerly wind waves, naturally in late winter and early spring, due to the reduction of temperature and pressure differences in the northern hemisphere, westerly winds move meridian. And the number of long waves also increases hemispherical. In this map, there are two meridional patterns of westerly winds in the north and south, the first pattern in the north with a deep and curved channel with a west-east direction and closed with two equal heights, cut off low respectively over Western Russia and Kazakhstan respectively with There are 5280 and 5440 meters, That on the surface of the earth in these two areas, two low pressures with a pressure of 1003 and 1011 HP, respectively, indicate a strong rotation and instability in these areas.
Conclusion: Heavy rains and floods are one of the most important weather hazards that cause great damage to nature and humans every year. This type of precipitation, which is an inherent feature of arid and semi-arid climates, has increased significantly in recent years due to global warming and the increase in climate extremes.