A study of convective systems, water vapor and top of the atmosphere cloud radiative forcing over the Indian Ocean using INSAT-1B and ERBE data

Abstract

The distribution of cloud radiative forcing (CRF) at the top of the atmosphere over the Indian Ocean is investigated using satellite observations. Two key regions are considered: The eastern Indian Ocean and the Bay of Bengal which experience maximum upper-level cloudiness in winter and summer respectively. It is found that longwave CRF in the Bay of Bengal during summer is similar to that over the eastern Indian Ocean during winter. On the other hand shortwave CRF magnitude is larger in the Bay of Bengal. These differences explain the net CRF difference between the two regions. The stronger shortwave forcing seems to be related to the Upper-Level Cloudiness being larger over the Bay than over the eastern Indian Ocean. The reasons for the longwave CRF similarities are analysed in more details. Using the results from a convective system classification method, it is first shown that the longwave radiative properties of the individual systems do not vary much from one region to another. The distribution of the different kind of systems, a proxy for the vertical cloudiness structure, does not either indicate strong difference between the regions. It is then proposed that the substantial precipitable water vapour amount observed over the Bay of Bengal damps the effects of the upper-level cloudiness on radiation compared to the relatively dryer eastern Indian Ocean area; yielding to similar LW CRF in both region despite more Upper-Level Cloudiness over the Bay of Bengal. These observations are supported by idealised radiative transfer computations. The distribution of cloudiness and radiative forcing is then analysed over the whole tropical Indian Ocean for each season. July is characterized by a low longwave CRF regime (relative to January) over the most convectively active part of the Ocean. The non linear damping effect of water vapor on longwave CRF is also shown to contribute to this regime. Overall, this study reaffirms the need for simultaneous documentation of the cloud systems properties together with their moist environment in order to understand the overall net radiative signature of tropical convection at the top of the atmosphere (TOA).