18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001: 8:15 AM
Numerical simulations of the Genesis of Hurricane Diana (1984)
Christopher A. Davis, NCAR, Boulder, CO; and J. G. Powers and L. F. Bosart
We examine the transformation of a sub-tropical weak baroclinic cyclone into Hurricane Diana (1984) through a series of numerical (MM5) simulations designed to elucidate the process by which a warm core disturbance develops and amplifies. We also investigate the sensitivity of the simulation of this process to changes in model physics, horizontal resolution, and initialization time.

As revealed in the simulations, weak baroclinic development begins as a cold-core upper tropospheric trough moves off the Florida coast. Low-level warm advection, induced mainly by the upper-level disturbance, initiates widespread precipitation and latent heating poleward of a stationary surface front. The heating produces numerous low-level positive potential vorticity (PV) anomalies. A dominant PV anomaly develops, amplifies through latent heating, and sweeps surrounding anomalies around itself, growing further by eddy vorticity fluxes as the anomalies are axisymmetrized. This accompanies a transition to warm core that occurs in about 8 hours. In a subsequent 12-h period of quiesence, the boundary layer and lower troposphere moisten. Renewed deepening follows, and the storm reaches hurricane intensity roughly 12 hours later.

Among the more notable sensitivities are variations in initial conditions, cumulus scheme, and horizontal resolution. A simulation with the upper-level trough-ridge disturbance removed produced no storm. Simulations with different cumulus schemes produced different storm intensities and tracks. Schemes which allowed more "resolved" latent heating produced weaker storms and a more westerly track. Simulations with an innermost nest of 3-km resolution, using no cumulus scheme, generally improved the prediction of storm intensity relative to 9-km simulations. Lastly, to explore the impact of resolving deep convective cells, a simulation using a 1.2-km grid was conducted. From only synoptic-scale baroclinic features in the initial condition, a tropical cyclone develops within 48 h. With such resolution, both individual convective cells and organized precipitating bands evolve in the MM5.

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