Observations of extremely low tropopause temperature over the Indian tropical region during monsoon and postmonsoon months: possible implications

Abstract

The present study examines the possible role of the "freeze and dry effect", which is generally believed to occur when the tropopause temperature is lower than 191 K. So far, there is observational evidence to support the role of the "freeze-dry mechanism" over the western Pacific region, in particular, over Indonesia. The paper presents observations from four intensive radiosonde/GPS sonde campaigns during summer monsoon and postmonsoon seasons from a number of stations in the tropical Indian region. Observations provide clear evidence of the existence of tropopause temperature lower than 191 K. Analysis suggests that cold tropopause occurs over a wide area of 5°×5° in latitude and longitude and more often around local midnight hours. Cold tropopause temperature appears to occur more often over the ocean as compared to the coastal stations. The observations at the coastal stations show that on the days of low temperature, the tropopause is cooler by ~8 K than the climatological temperature for the Indian equatorial region and ~3.5 K from the mean of all observations. It is noted that on the days of observed cold tropopause, the lapse rate just below the tropopause is closer to the dry adiabatic lapse rate. This indicates a rapid cooling in a narrow height range. The present observations thus provide evidence that the freeze-dry mechanism could be operating over a wide area, which includes the Indian tropical region. The possible role of atmospheric convection and also of atmospheric waves in producing low tropopause temperatures is examined and discussed.