On the Sub-tropical Jet Stream and its Role in the Development of Large-scale Convection

This paper contains mainly the results of a synoptic and climatological study of the large-scale convection in northern India and Pakistan during the three months preceding the onset of the southwest monsoon. It has been shown that the sea-level and lower tropospheric charts give little clue to the development of the large-scale convection and that the latter is overwhelmingly determined by the divergence in the waves in the sub-tropical jet-stream. It has further been shown from detailed synoptic evidence that nor’westers, andhis and the majority of the thunderstorms without squalls in northern India and Pakistan in the pre-monsoon period are fundamentally the same phenomenon. The role of cold-air advection in the middle and upper troposphere in the development of large-scale convection has also been discussed. This study has further revealed that the regions of upper-divergence and convergence can be qualitatively located by identifying certain typical patterns on the high-level maps more than 12 hours before the usual time of commencement of convection and that, consequently, these maps can be used as effective tools in the issue of area-warnings against thunder in general and nor’westers and andhis in particular.

A general study has also been made of the large-scale convection in southeast Australia, Union of South Africa, Bechuanaland, Southern Rhodesia, northeast Argentina, Uruguay, southeast Brazil and southeast United States and the similarities between the large-scale convection in these countries and in Indo-Pakistan have been brought out. On the basis of these studies, it has been suggested that the jet stream plays the very important role of producing large-scale convection in the subtropics all over the world wherever it over-runs on its equatorward side, moist air possessing a high degree of latent instability.