Rainfall over the Arabian Sea during the onset of the 1979 monsoon

Abstract

One path to understanding the summer monsoon of south-east Asia lies through its regimes of rain. There the main scales of interest range, on the small side, down at least to component rainstorms: that is, extending over hundreds of kilometres and lasting about a day. On the large side, interest might be extended to include the whole quasi-planetary monsoon system, although here, for practical reasons, the limits are taken to be 1 week and several thousands of kilometres. The main branch of the summer monsoon forms over the Arabian Sea. The rainfall of this sea, roughly 3,000 km in its dimensions, is known to us from about 40 coastal stations, mostly in India, plus six instrumented islands and the reports of seagoing ships. The ships stay mainly in a few lanes and in any case only record the frequency of rain, not the amount. Thus, even taken all together, these reports are hardly adequate for building a picture of the monsoon rains. We report here a new technique for inferring daily rainfall from visible or IR images obtained by a geostationary satellite, applied in a simplified form to the Arabian Sea during the onset of the 1979 summer monsoon. The rain area estimated increased spectacularly (from 1 to 14%) over a 1-week period coincident with the appearance of the Somali jet and development of the onset vortex.