What is the trigger for tropical cyclogenesis?

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Abstract

The development of a tropical cyclone from a pre-existing, weak, warm-core vortex is investigated with high-resolution cloud-resolving simulations using the Weather Research and Forecast Model (WRF). The simulation design and initial conditions are quite favourable for tropical cyclogenesis: the environment has a tropical sounding with no mean wind or wind shear, and the sea-surface temperature is held constant at 29°C. Nonetheless, it is found that sporadic convection must occur for 48 to 72 hours before genesis and rapid intensification begins. During this time, before intensification, the vortex is found to go through important structural changes in both its wind field and its thermodynamics. While the low-level wind field decays due to friction, the inner core slowly becomes humidified due to moist detrainment and precipitation from deep convective towers. As the relative humidity in the core exceeds values of 80 per cent over most of the depth of the troposphere, a mid-level vortex forms, contracts and intensifies. Once the mid-level vortex has reached a sufficient strength, and the inner core is nearly saturated, a smaller scale vortex forms very rapidly at the surface. This smaller vortex becomes the core of an intensifying tropical cyclone. This process is explored through careful study of the inner-core dynamics and thermodynamics, with close attention paid to the changes in the moist convection as the ijrmer core approaches saturation. While the frequency of deeper and stronger updraughts increases with time, the frequency of cool downdraughts remains essentially unchanged. In the hours before genesis, the intensification of the mid-level vortex leads to a large increase in the efficiency of the conversion of latent heat energy to the kinetic energy of the cyclonic wind field. The relative importances of the mid-level vortex and inner-core saturation are illustrated with additional simulations with different initial conditions and environmental soundings. Implications of these results for identifying and forecasting tropical cyclogenesis are discussed.

Original languageEnglish (US)
Pages (from-to)241-266
Number of pages26
JournalAustralian Meteorological Magazine
Volume56
Issue number4
StatePublished - Dec 1 2007

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ASJC Scopus subject areas

  • Atmospheric Science

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