Tropical Storm Formation from Easterly Waves: A Criterion for Development

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  • 1 National Hurricane Center, National Weather Service, NOAA, Coral Gables, Florida 33124
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Abstract

A triggering mechanism is presented for the transformation of a wave in the easterlies to an intensifying tropical depression. Thermodynamic processes appear to be of secondary importance at this early stage of tropical storm formation. A development criterion is presented that measures the importance of nonlinear vorticity advection for the dynamics of the wave disturbance. If the contributions of the nonlinearities become significant then formation of an intensifying depression is hypothesized. The hypothesis allows one to predict the tune and place of tropical.storm development. Both climatology and the 1975 hurricane season are analyzed in order to test the theory for Atlantic easterly waves. The development criterion is found to have predictive ability in anticipating tropical storms during August and September 1975, several days prior to development.

Abstract

A triggering mechanism is presented for the transformation of a wave in the easterlies to an intensifying tropical depression. Thermodynamic processes appear to be of secondary importance at this early stage of tropical storm formation. A development criterion is presented that measures the importance of nonlinear vorticity advection for the dynamics of the wave disturbance. If the contributions of the nonlinearities become significant then formation of an intensifying depression is hypothesized. The hypothesis allows one to predict the tune and place of tropical.storm development. Both climatology and the 1975 hurricane season are analyzed in order to test the theory for Atlantic easterly waves. The development criterion is found to have predictive ability in anticipating tropical storms during August and September 1975, several days prior to development.

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