Mid-Season Typhoon Development and Intensity Changes and the Tropical Upper Tropospheric Trough

Abstract

A mid-season test of a synoptic model for typhoon development (Sadler, 1976a) was conducted for July 1972 during an outbreak of four simultaneous typhoons. In agreement with the model, the rapid deepening phase of typhoons Rita, Phyllis and Tess was associated with multi-directional outflow channels to the large-scale flows of the upper troposphere. The depressions were positioned beneath the subequatorial ridge with normal outflow to the south into the equatorial easterlies. Vigorous outflow to the north was initiated by the proper positioning, through relative motion, of intense cells in the tropical upper tropospheric trough. Each storm, filled to minimal typhoon strength when either of these efficient outflow channels was denied to it. Susan was a study in contrasts. She spent most of her lifetime under large-scale unidirectional northeast flow aloft and attained doubtful minimal typhoon intensity for a few hours. The upper flow became favorable for intensification just prior to landfall.