Influence of upper level troughs and ridges on the formation of post-monsoon cyclones in the Bay of Bengal

Abstract

The upper air conditions at the formative stages of several depressions and cyclones in the Indian seas were studied by Desai and Rao in the light of Riehl model for the formation of tropical cyclones. The recent introduction of twice daily radiosonde-rawin ascents at some stations in India provided additional facilities for similar studies in this area. The available upper air conditions associated with the formation of two post-monsoon cyclones of 1955 in the Bay of Bengal  were accordingly studied with the idea of finding how far Riehl's theory of tropical cyclones could be applied in these cases.

The present study of the post-monsoon cyclones of 1955 reveals that two radiosonde ascents a day from a reasonable network of stations should be minimum requirement for studies of this kind, Time cross-sections showing 24-hour changes of heights at intervals of 12 hours are presented to illustrate the sequence of movement of upper troughs and ridges. As postulated by Riehl, the formation of both the cyclones was apparently associated with the passage of troughs in the westerlies across northeast India, i.e., to the north of the area of cyclogenesis. But the upper air data from the available network of stations were too inadequate for drawing any definite conclusion as to whether the initial mass divergence at upper level was caused by the southward extension of the westerly trough and the superposition of its forward portion over the surface incipient low, or by the in-phase superposition of the upper trough in the westerlies with the upper tropospheric vortices moving across the Bay of pour Bengal, Dynamical development to the south of a confluent trough in the westerlies is suggested as a possible , contributory factor for the cyclogenesis in the south Bay of Bengal.

The influence of the middle-latitude trough on the recurvature of one of the cyclones while out at sea and of 453474 another similar cyclone of October 1949 which had recurved only after crossing the coast, and the unexpected weakening of the former while out at sea, have been discussed.