عنوان مقاله [English]
One of the most important issues that human societies are currently facing is the phenomenon of climate change that attracted the attention of many researchers around the world. One of the effects of climate change is the increase in the frequency and severity of occurrence of extreme weather events, such as frost temperatures and extreme rainfall. The effects of climate change on changes in cold-wave temperatures in recent years, as well as the effects of cold waves on various aspects of human societies, have always been the concern of various scholars in most parts of the world, including cold weather, and atmospheric hazards. Several definitions of cold waves are presented. In general, scientists used cold weather or cold weather for at least 2 days or more at temperatures below zero degrees Celsius. In general, the characteristics of the cold wave can be summarized in three characteristics: 1. the intensity and velocity of the temperature drop 2. the numerical value of the drop or deviation in the daily temperature value 3. the duration of the continuity (time continuity) of the cold wave.
Materials and methods
For the purpose of this research, data from the upper levels of the atmosphere were used to analyze the data of the 20th century. The data listed on the website was https://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.html and was used. NASA's Twentieth Century Data has a good place in comparison with the normal data from time separation, and has a better and more accurate accuracy than it is. The data used are 142 years old from 1871 to 2012. The data are 6 hours and 4 daily observations (0, 6, 12 and 18 Zulu) with spatial resolution of 2 degrees longitude at 2 degrees latitude and 24 pressure levels from 1000 to 10 hPa. For the purpose of this study, at first, the cold waves with a minimum temperature of -15 ° C and continuity of time for at least 2 days in 142 years of statistical period were extracted from a minimum of two-meter surface temperature data. Then the relevant data was categorized in terms of frequency, severity, duration and spatial extent. In order to validate the extracted data, they were controlled to the point where they met the ground stations and ensured the accuracy of the data. For synoptic analysis, the coldest waves occurred in each of the months. For this purpose, combinational maps of ground pressure, geopotential heights and wind current of 500 hPa, bar thickness map, temperature map, and a minimum temperature of two meters of ground level were used.
Results and discussion
The results of the synoptic analysis of the selected days indicate the dominance of the high-pressure system on the surface of the earth and the collapse of the block, followed by the cavern and stack at the upper levels of the atmosphere. In most of the study days, the high pressure (Siberian high-pressure migratory force) system in the Earth has been flowing on the Earth's surface, which has intensified its intensity. The meridian flow formed by the flood-blocking blocks has led to a very cold, northwest northern part of Iran's cold air and, on the other hand, has increased the duration of the deployment of these cold waves.
According to the results, 44 cold currents at -15 ° C and colder in the north east of the country were identified during the studied time period. The waves are mostly due to January, February and December. Which is very good for the influence of Siberian cold pressures, shows such waves. A noticeable decrease has occurred in these waves from 1907 onwards. The frequency of these waves has decreased over the years, but has been added to the severity and duration of its deployment. According to the results of the studies, it can be said that changes in the process of cold weather are not related to global warming and climate change. This cold wave is formed in most cases when the Siberian high-pressure earth is merged with the Western Immigrant High.On the other hand, in the upper levels of the atmosphere, due to the high surface pressure of the western winds, they collide with blocking barriers and convert the orbital flow of the western wind to a meridian stream. The process mentioned above leads to the formation of a heap over Eastern Europe to the middle of the Red Sea and the formation of a ravine over the Mediterranean and Turkey. As explained above, it leads to the very cold weather flowing north to the northern part of Iran. Also, the thinness of barley thickness over the atmosphere of northeastern Iran formed as a result of a very cold, semi-arctic flow; the very cold weather has dominated the northern latitudes for several days. The study of temperature patterns indicates the domination of the northern and northwest winds, and also indicates an increase in the cold intensity of the waves occurring with the increase of the latitude of the source. Very cool temperatures occur when the source of cold air is from latitudes above 60 degrees.