Evolution of the Net Surface Shortwave Radiation over the Indian Ocean during Summer MONEX (1979): A Satellite Description

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Abstract
In this paper we briefly describe the method applied to derive the net shortwave radiation (NSW) at the surface from combinations of calibrated visible geostationary satellite data. We then discuss the anticipated accuracy of the satellite estimations of NSW as a function of averaging time and space scales, as well as some intermediary results (surface albedo field). The main purpose of the paper is, however, to describe and analyze the evolution of the NSW fields during the monsoon of 1979. We show that during the pre-onset phase (1 May–11 June), the characteristics of the NSW field are dominated by a strong maximum in the entire Arabian Sea (about 300 W m−2) and by a strong minimum in the central and eastern equatorial Indian Ocean (East of 80°E, in a band from 10°N to 10°S). This minimum is associated with the intense convective activity occurring in that region; very little activity exists in the western part of the Indian ocean. As the season evolves, the minima of NSW associated with the large-scale convective activity occur more and more westward in the equatorial ocean, as is illustrated by a time sequence of NSW fields. Just a few days prior to the monsoon onset, several consecutive large-scale convective systems are present in the equatorial regions. These systems are then found more and more northwards. During the few days surrounding the onset (11 June–20 June), the entire field of NSW is drastically modified. The Arabian Sea maximum has retreated towards the Somalia coast, and most of the sea then experiences a strong minimum of NSW associated with the intense precipitation occurring along the southwestern coast of the Indian subcontinent.

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