A statistical investigation of the tropical cold point tropopause temperature in Tehran and Shiraz in January and July (2000-2022)

Summary

The tropopause is part of the fundamental structure of the atmosphere, separating the dynamically and chemically distinct troposphere and stratosphere. Based on the concepts; three different tropopauses the name thermal, dynamical, and chemical or ozone tropopause are defined. 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. Chemical tropopause is another type of tropopause that is defined based on the vertical concentrations of the trace gases such as ozone mixing ratio and water vapor.

In tropical latitudes, a useful reference for distinguishing the tropopause from the stratosphere is the cold point, where the temperature minimum occurs. The cold-point tropopause is the temperature minimum in the tropopause region and represents this acts as the thermal boundary between the troposphere and the stratosphere whose height varies from 18 km in the tropics to about 8 km in the poles. This boundary plays a crucial role in the troposphere-stratosphere exchange.

In the present study, radiosonde data used here are obtained from the Integrated Global Radiosonde Archive at the NOAA National Climatic Data Center. Temperature profiles daily over January and July 2000-2022 were taken from two radiosonde stations in the central and south of Iran including Tehran (51.31°E, 35.56°N), and Shiraz stations (52.60°E, 29.53°N), and analyzed.

In Figure 1 using radiosonde temperature profiles (green lines) at Tehran stations in a) January and b) July 2021, cold-point tropopause and monthly mean temperature (black lines) are shown. The blue plus signs are cold point tropopauses from individual profiles and the red plus sign is the average of all blue plus signs. The black thick line is the mean temperature profile and the purple plus sign is the coldest point of the mean temperature profile. ΔT is the difference between the red and purple plus signs.

Although we only showed one month of data in January and July in Figure 1 to visualize the difference between the mean of cold point tropopause and the coldest point of the mean temperature profile, our calculation for ΔT is in the 2000-2022 period.

The results showed that Shiraz station's lowest temperature of the tropical cold point tropopause is colder than the Mehrabad station of Tehran, and the lowest temperature of the tropical cold point tropopause is estimated in July. This difference is related to combining geographical-latitudinal characteristics of the two stations and the development of thermal low pressure in July over Iran, which has caused an increase in height and a decrease in the temperature of the tropical tropopause. Also, the results showed the average tropical cold point tropopause temperature in the stations is 2.5 degrees lower than the average cold point of the vertical temperature profile, and the thermal low pressure and cold high pressure are the cause of this difference.





a) January 2021

b)July 2021



Figure 1: Temperatures profiles for January and July 2021 from Tehran station radiosonde. The green lines are all individual temperature profiles. The blue plus signs are cold point tropopauses from individual profiles and the red plus sign is the average of all blue plus signs. The thick black line is the mean temperature profile and the purple plus sign is the coldest point of the mean temperature profile. ΔT is the difference between the red and purple plus signs.