Abstract
The complete transformation of a weak baroclinic disturbance into Hurricane Diana is reproduced by numerical simulations using the fifth generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model. Three distinct phases of the evolution are evident. First, baroclinic and barotropic development, strongly modified by the effects of latent heating, occurs. During the latter part of this phase, the low-level circulation is strengthened through the axisymmetrization of remote potential vorticity anomalies that are generated by condensational heating and then advected toward the incipient storm. The axisymmetrization process evinces properties of both nonlinear, discrete vortex merger and vortex Rossby wave dynamics. The transformation from cold-core to warm-core vortex occurs in this development stage.
In the second phase, lasting 10–12 h, little deepening occurs. Spiral bands of convection begin to form and the core of the storm moistens, eventually reaching 95% humidity averaged between the top of the boundary layer and 600 hPa at the radius of maximum wind. The third stage ensues, driven mainly by the positive feedback between fluxes of latent heat and the increase of the tangential wind. In this stage, the storm readily develops a clear eye. The transition to the hurricane stage occurs 12–24 h sooner in the model than in nature. The maximum intensity was also underestimated, with peak winds in the model being about 42 m s−1 (at 40 m above ground level) whereas sustained winds of nearly 60 m s−1 were observed.
Corresponding author address: Christopher A. Davis, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. Email: cdavis@ucar.edu
