Variability of the thermal structure of the atmosphere during wet and dry spells over southeast India



Surface and upper-air observations of meteorological parameters at Gadanki (13.5°N, 79.2°E) are utilized to understand the thermal structure of the atmosphere in dry and wet spells and its effect on draught core statistics. The temperature and humidity variations from wet to dry spell are quite pronounced near the surface and in the atmospheric boundary layer (ABL), but not above the ABL. Interestingly, convective available potential energy (CAPE) is found to be higher during wet spells than during dry spells, in contrast to the earlier studies made elsewhere over warm oceans and tropical land masses. The stratified CAPE values based on rainfall occurrence are also larger during wet spells than during dry spells, irrespective of whether the sounding is made before, during or after the rain occurrence. Surprisingly, large CAPE values are observed after the rain occurrence in both spells. The high CAPE during wet spells (and also after rain occurrence) corresponds well with larger surface equivalent potential temperature (θe). During wet spells, the positive buoyancy is not only larger but also vertically extended to higher altitudes. On the other hand, buoyancy profiles during dry spells are punctuated by several negative buoyancies (associated with stable layers), reducing CAPE values in those spells.

A synthesis of all measurements reveals that favourable environmental conditions (high CAPE, humid atmosphere and weak stable layers) allow the convection to grow into a deep system in wet spells. On the other hand, strong stable layers, weak CAPE coupled with relatively less humidity above the ABL inhibits the growth of convective cloud during dry spells. These observations explain the differences in draught core statistics in wet and dry spells. Copyright © 2012 Royal Meteorological Society