Numerical simulations of the genesis of Hurricane Diana (1984). Part I: Control simulation

The complete transformation of a weak baroclinic disturbance into Hurricane Diana is reproduced by numerical simulations using the fifth generation Pe nnsylvania State University-National Center for Atmospheric Research Mesosc ale Model. Three distinct phases of the evolution are evident. First, baroc linic and barotropic development, strongly modified by the effects of laten t heating, occurs. During the latter part of this phase, the low-level circ ulation is strengthened through the axisymmetrization of remote potential v orticity anomalies that are generated by condensational heating and then ad vected toward the incipient storm. The axisymmetrization process evinces pr operties of both nonlinear, discrete vortex merger and vortex Rossby wave d ynamics. The transformation from cold-core to warm-core vortex occurs in th is 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 6 00 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 wi nds in the model being about 42 m s(-1) (at 40 m above ground level) wherea s sustained winds of nearly 60 m s(-1) were observed.