Descriptive characteristics of the Tropopause on Iran's atmosphere in summer and winter seasons

The tropopause is a thin layer separating the stratosphere from the troposphere and is often characterized by a large change in the thermal, mass and chemical structure of the atmosphere.Compared to global studies on the tropopause and its various features, studies conducted in Iran are very few and the methods used are often less inclusive or the length of the statistical period is limited. For this reason, and considering the importance of the tropopause and its effect on exchanges between the troposphere and the stratosphere, and also due to the lack of information about it in Iran, accurate knowledge of the height of the tropopause in the country using more reliable data sources is a fundamental necessity. To calculate the tropopause, we used daily temperatures of ECMWF reanalysis datasets from January 1979 until December 2018. Gridded data witha spatial resolution of 0.25*0.25 were used. In vertical levels, we used 10 standard isobaric surfaces from 700 to 50 hPa.

The location of the tropopause thermally and dynamically was defined. According to the WMO (World Meteorological Organization), the tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2°C/km.In this study, this index was used to identify the tropopause.In this study, to identify the factors affecting the tropopause, the relationship between the tropopause and spatial variables (latitude and longitude) and altitude was evaluated by general and partial correlations.

The results of this study showed that in the months of cold season, the tropopause pressure level on Iran is followed by latitude, and the tropopause height decreases with increasing latitude, but in the months of the warm season (June, July, and August), the tropopause pressure level is different from the months of the winter season.In these months, the changes in the tropopause pressure levels do not follow the latitude; on the Zagros and Kerman heights, the tropopause height is at its lowest, while the highest tropopause elevation is in these months at higher latitudes than in other months.The temperature of the upper and lower levels of tropopause also showed that the temperature of the lower levels of the tropopause in all seasons was below the temperature of the upper levels of the tropopause and the temperature of the two levels changed with the changes in the levels of tropopause pressure in different months.The study of low and high levels of tropopause showed that during the cold season, the temperature of the two levels around the tropopause, following the tropopause pressure levels, follows the latitude, and with increasing latitude, temperature increases in the two levels around the tropopause.In two studied seasons, the lowest temperature of the two levels of the tropopause is consistent with the highest level of the tropopause, but the highest two-level temperature is only consistent with the lowest tropopause pressure level during the warm season months, and in other months, this observation coordination failed.Investigating the thermal difference between two levels of tropopause showed that the temperature difference between the two levels of the tropopause in the warm season is more significant than that of the cold season, while in the cold season, the temperature difference in most regions of the latitude is obeyed. Slowly, the difference in temperature decreases with increasing latitude.

Examination of the characteristics of the tropopause and its related factors for summer and winter showed that in each season due to local conditions and changes in large-scale factors, the height of the tropopause changes, and therefore the tropopause in each season has completely different characteristics from the other season.Examination of the characteristics of the tropopause and its related factors for summer and winter showed that in each season due to local conditions and changes in large-scale factors, the height of the tropopause changes, and therefore the tropopause in each season has completely different characteristics from the other season.