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

E Johansson,A M L Ekman,M Tjernström,M Tjernström,T L'Ecuyer,A Devasthale

doi:10.5194/acp-15-11557-2015

Abstract

Abstract. Clouds forming during the summer monsoon over the Indian subcontinent affect its evolution through their radiative impact as well as the release of latent heat. While the latter is previously studied to some extent, comparatively little is known about the radiative impact of different cloud types and the vertical structure of their radiative heating/cooling effects. Therefore, the main aim of this study is to partly fill this knowledge gap by investigating and documenting the vertical distributions of the different cloud types associated with the Indian monsoon and their radiative heating/cooling using the active radar and lidar sensors onboard CloudSat and CALIPSO. The intraseasonal evolution of clouds from May to October is also investigated to understand pre-to-post monsoon transitioning of their radiative heating/cooling effects. The vertical structure of cloud radiative heating (CRH) follows the northward migration and retreat of the monsoon from May to October. Throughout this time period, stratiform clouds radiatively warm the middle troposphere and cool the upper troposphere by more than ±0.2 K day−1 (after weighing by cloud fraction), with the largest impacts observed in June, July and August. During these months, the fraction of high thin cloud remains high in the tropical tropopause layer (TTL). Deep convective towers cause considerable radiative warming in the middle and upper troposphere, but strongly cool the base and inside of the TTL. This cooling is stronger during active (−1.23 K day−1) monsoon periods compared to break periods (−0.36 K day−1). The contrasting radiative warming effect of high clouds in the TTL is twice as large during active periods than in break periods. These results highlight the increasing importance of CRH with altitude, especially in the TTL. Stratiform (made up of alto- and nimbostratus clouds) and deep convection clouds radiatively cool the surface by approximately −100 and −400 W m−2 respectively while warming the atmosphere radiatively by about 40 to 150 W m−2. While the cooling at the surface induced by deep convection and stratiform clouds is largest during active periods of monsoon, the importance of stratiform clouds further increases during break periods. The contrasting CREs (cloud radiative effects) in the atmosphere and at surface, and during active and break periods, should have direct implications for the monsoonal circulation.

Highlights

Clouds cover about 70 % of the Earth’s surface area (Stubenrauch et al, 2012) and are a key component of the Earth’s energy and water cycle
The role of latent heating has been previously investigated in a few studies to some extent (Zuluaga et al, 2010, and references therein), but the radiative component of heating has received very little attention so far due to a lack of suitable observations. This is the knowledge gap that the present study aims to address over the Indian subcontinent by documenting radiative heating/cooling contributions from different cloud types and their intraseasonal evolution from May to October
The onset of the northeast monsoon causes increased cloudiness in October compared to September

Summary

Introduction

Clouds cover about 70 % of the Earth’s surface area (Stubenrauch et al, 2012) and are a key component of the Earth’s energy and water cycle. They have multiple influences on the state of the atmosphere, occurring over a wide range of spatio-temporal scales. Apart from modulating the net surface radiation, clouds substantially influence the diabatic heat budget of the atmosphere through radiative heating/cooling and latent heat release (Fueglistaler et al, 2009a; L’Ecuyer and McGarragh, 2010). Clouds are an important part of the landocean-atmosphere coupling mechanisms that regulate atmo-. E. Johansson et al.: The vertical structure of cloud radiative heating spheric circulation.

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