A Statistical Analysis of the Tropopause Characteristic over Tehran and Shiraz in January and July (2000-2022)

ABSTRACT
This paper analyses the characteristics of the tropical tropopause, mid-latitude, and polar tropopause based on the sounding temperature data at Mehrabad and Shiraz airport stations in January and July in the statistical period of 2000-2022. The results showed that the observed frequency of the tropical tropopause in Iran in January (41 and 59 present in Mehrabad and Shiraz, respectively) is less than in July January (95 and 94 present in Mehrabad and Shiraz, respectively), and the frequency of the observed tropical tropopause in July is more than that of the mid-latitudes. The reason for this difference can be found in the increased thermal energy of the atmosphere in the warm seasons. In July, due to the development of thermal low pressure over Iran, the thermal energy, the air temperature, and the thickness of the atmosphere increased. As a result, the tropopause elevates and approaches the level of the tropical tropopause. It was also found that the tropical and mid-latitude tropopauses have a higher height in the warm month and are placed in lower pressure levels. For this reason, the temperature of these two tropopauses in the warm month is lower than the corresponding value in the cold month. Based on the results, the average height and temperature in tropical tropopause levels were estimated between 16.5 to 17.4 kilometers and -65 to -78 degree Celsius, respectively, in different regions of Iran. Also, these parameters for mid-latitude tropopause level were estimated from 11.5 to 12.8 kilometers and -52 to -59 degree Celsius, respectively
Extended Abstract
Introduction
The tropopause is usually defined as the transition region separating the stably stratified and turbulent troposphere. These two atmospheric regions differ in numerous dynamic and chemical constituents. Depending on season and latitude, the tropopause is typically found at around 18 km in the tropics and around 8 km at high latitudes. Tropopause is defined based on up to three different definitions. The conventional tropopause is the thermal one which is usually characterized by an abrupt change in temperature lapse rate. Its definition is based on the fact that the stratosphere is much more stably stratified than the troposphere. The thermal tropopause is defined as the lowest level above 500hPa at which the lapse rate decreases to 2 K/km or less, provided that also the average lapse rate between this level and all higher levels within 2 km does not exceed 2 K/km. The dynamical tropopause is defined in terms of sharp changes in the potential vorticity, which measures the stratification and wind shears in air masses. The original concept of the dynamical tropopause is based on the isentropic gradient of potential vorticity. It is typically determined in a thin layer with absolute potential vorticity values within 1 and 4 potential vorticity units. The chemical tropopause is another type defined based on the vertical concentrations of trace gases such as ozone and water vapor. In this paper, the tropical, mid-latitudes, and the polar tropopause are defined based on latitude and geographical characteristics. The characteristics of the different tropopauses are analyzed regarding the temperature profiles from radiosonde data of the Mehrabad and Shiraz airport stations in January and July on the statistical period of 2000-2022.
 
Materials and methods
The radiosonde data are obtained from the Integrated Global Radiosonde Archive of the NOAA National Climatic Data Center. The temperature data in January and July 2000-2022 are taken from the two radiosonde stations in the central and southern regions of Iran, including Mehrabad (51.31°E, 35.56°N) and the Shiraz airport stations (52.60°E, 29.53°N). The individual sounding profiles are exerted to determine the location and analyze the lowest tropopause and, if present, the second or the third tropopause based on the definition of the Commission for Aerology of World Meteorological Organization.  In former studies, the data of the 200-hPa pressure surface were often used to measure the mid-latitude tropopause and that of the 100-hPa for the tropical tropopause. However, sounding measurements confirm that a constant pressure surface is a flawed assumption for detecting tropopauses. In this study, the lower tropical tropopause, the mid-latitude tropopause, and the polar tropopause levels data are used (based on the mean pressure of thermal tropopause) in the 0-30°N, 30-60°N, 60-90°N regions to analysis their characteristics.  
 
Results and discussion
Comparing the frequency of tropopauses detected in Mehrabad and Shiraz airport stations shows that the frequency of days with two tropopauses detected over Mehrabad airport is lower than in Shiraz station in January. However, the number of days with three and four tropopauses at Mehrabad airport is more meanwhile the not detected tropopause, i.e. the break-down ones is more frequent in Mehrabad station. The days with one tropopause are more frequent in Mehrabad airport in January, but the number of days with two tropopauses is the same. The significantly elevated tropopause of the subtropical region in the warm season is the reason for the detected differences in which the radiosonde may not pass over the tropopause levels. Comparing the frequency of tropical and mid-latitude tropopause shows that at Mehrabad airport (Shiraz station) in January, the number of detected mid-latitude tropopauses is more (less) than that of tropical ones. This difference is related to the combined geographical-latitudinal characteristics of the two stations. The tropical tropopause in July is the most frequent in both stations. 5 up to 6 percent of them are due to subsidence inversion.   Investigations also showed that the average temperature of the tropical tropopause in Shiraz station is lower than Mehrabad airport in January. Mid-latitudinal tropopause temperature is almost the same in both stations, but the mean polar tropopause temperature in January over Mehrabad airport station is lower than in Shiraz station. The analysis of the January precipitation variability of these stations (in the 2000-2022 statistical period) shows that Shiraz is much greater than that of Mehrabad airport, so the average precipitation in this month in Mehrabad airport is 34 mm and in Shiraz station is 70 mm. It seems that in January, the release of latent heat caused by the condensation process in the upper parts of the troposphere and the frequency of the turbulent pressure systems over the Shiraz station was more than that of the Mehrabad airport, which caused the higher polar tropopause temperature in Shiraz station than the Mehrabad airport.
 
Conclusion
This paper analyses the characteristics of the tropical tropopause, mid-latitude, and polar tropopause based on the sounding temperature data at Mehrabad and Shiraz airport stations in January and July in the statistical period of 2000-2022. The results showed that the observed frequency of the tropical tropopause in Iran in January is less than in July, and the frequency of the observed tropical tropopause in July is more than that of the mid-latitudes. The reason for this difference can be found in the increased thermal energy of the atmosphere in the warm seasons. In July, due to the development of thermal low pressure over Iran, the thermal energy, the air temperature, and the thickness of the atmosphere increased. As a result, the tropopause elevates and approaches the level of the tropical tropopause. It was also found that the tropical and mid-latitude tropopauses have a higher height in the warm month and are placed in lower pressure levels. For this reason, the temperature of these two tropopauses in the warm month is lower than the corresponding value in the cold month. Based on the results, the average height, pressure, temperature, and potential temperature in tropical tropopause levels were estimated between 16.5 to 17.4 kilometers, 92 to 96hPa, -65 to -78 degree Celsius, and 386 to 411 Kelvin, respectively, in different regions of Iran. Also, these parameters for mid-latitude tropopause level were estimated from 11.5 to 12.8 kilometers, 200 to 213hPa, -52 to -59 degree Celsius, and 335 to 386 Kelvin, respectively.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved thecontent of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.