Abstract
Numerical simulations and diagnostics are performed for Typhoon Tip and Tropical Storm Faye, both of which occurred during 1979, the year of the First Global GARP (Global Atmosphere Research Program) Experiment (FGGE). The simulations are started from early in the life cycles of both disturbances, the former of which developed into a super typhoon, and the latter of which did not develop beyond the tropical storm stage. The numerical model employed was that of Madala et al. and is a modification of the one used in previous simulations by the authors. The primary modifications are the inclusion of a more sophisticated boundary layer parameterization, based on similarity theory, and the inclusion in the Kuo cumulus parameterization scheme of the nonmeasurable mesoscale latent heat release, as described by Krishnamurti et al. The initial conditions for both simulations were derived from the FGGE dataset of the European Centre for Medium-Range Weather Forecasts and from monthly mean sea surface temperatures provided by the National Meteorological Center (now the National Centers for Environmental Prediction). The initial intensities and the underlying sea surface temperatures were approximately the same for the two disturbances. In the simulations, Tip developed into an intense typhoon and Faye did not develop, as observed in the atmosphere, although the minimum surface pressures and maximum wind speeds attained do not agree quantitatively with the reported values.
The primary question the authors set out to answer is what special conditions exist at the early stages of the life cycles of tropical disturbances that allow one system to develop and another to fail to develop into a typhoon. The most significant difference found in the initial states of Tip and Faye was a large-scale eddy flux of angular momentum from the surroundings into the former and out of the latter, with maximum amplitudes located around 200 mb at radial distances from the vortex centers greater than 1000 km. These fluxes persisted for at least 24 h prior to the time the numerical simulations were started. While there were differences in the eddy heat fluxes as well, these were less significant. Diagnostic calculations reveal that the secondary radial circulation induced by the eddy fluxes of momentum and heat transported water vapor inward for Tip and outward for Faye, with the result that convection broke out at an early stage in the vortex center of Tip, but not in Faye. The convection intensified with time in Tip and subsequently became the dominant factor contributing to the moisture inflow and rapid vortex intensification.
The authors’ interpretation of the results of their numerical simulations and diagnostic calculations is that the secondary radial circulation induced by large-scale eddy fluxes of heat and momentum can serve either as a catalyst for typhoon formation or as a mechanism for inhibiting the further development of an incipient tropical disturbance, depending on the direction of the water vapor transport (into or out of the vortex core).
Corresponding author address: Prof. Richard L. Pfeffer, Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, FL 32306-4360.
Email: pfeffer@gfdi.fsu.edu
