The vertical structure of cloud radiative heating over the Indian subcontinent during summer monsoon

Abstract

Abstract. Every year the monsoonal circulation over the Indian subcontinent gives rise to a variety of cloud types that differ considerably in their ability to heat or cool the atmosphere. These clouds in turn affect monsoon dynamics via their radiative impacts, both at the surface and in the atmosphere. New generation of satellites carrying active radar and lidar sensors are allowing realistic quantification of cloud radiative heating (CRH) by resolving the vertical structure of the atmosphere in an unprecedented detail. Obtaining this information is a first step in closing the knowledge gap in our understanding of the role that different clouds play as regulators of the monsoon and vice versa. Here, we use collocated CloudSat-CALIPSO data sets to understand following aspects of cloud-radiation interactions associated with Indian monsoon circulation. (1) How does the vertical distribution of CRH evolve over the Indian continent throughout monsoon season? (2) What is the absolute contribution of different clouds types to the total CRH? (3) How do active and break periods of monsoon affect the distribution of CRH? And finally, (4) what are the net radiative effects of different cloud types on surface heating? In general, the vertical structure of CRH follows the northward migration and the retreat of monsoon from May to October. It is found that the alto- and nimbostratus clouds intensely warm the middle troposphere and equally strongly cool the upper troposphere. Their warming/cooling consistently exceeds ±0.2 K day−1 (after weighing by vertical cloud fraction) in monthly mean composites throughout the middle and upper troposphere respectively, with largest impact observed in June, July and August. Deep convective towers cause considerable warming in the middle and upper troposphere, but strongly cool the base and inside of the tropical tropopause layer (TTL). Such cooling is stronger during active (−1.23 K day−1) monsoon conditions compared to break periods (−0.36 K day−1). The contrasting warming effect of high clouds inside the TTL is found to be double in magnitude during active conditions compared to break periods. It is further shown that stratiform clouds (combining alto- and nimbostratus clouds) and deep convection significantly cool the surface with net radiative effect in the order of −100 and −400 W m−2, respectively, while warming the atmosphere in the order of 40 and 150 W m−2. While deep convection produces strong cooling at the surface during active periods of monsoon, the importance of stratiform clouds, on the other hand, increases during break periods. The contrasting CREs in the atmosphere and at surface, and during active and break conditions, have direct implications for monsoonal circulation.