Model hurricane formation in the presence of a basic state current



Numerical simulations with the Naval Research Laboratory limited area model were performed using initial conditions derived from gridded analyses of the Colorado State University composite Atlantic prehurricane cloud cluster and depression. For each type of disturbance, simulations were made with initial conditions consisting of the zonal mean wind superimposed on the axially symmetric component of the flow about the vortex center. These simulations were compared with ones in which the initial conditions were specified from the gridded analyses in their full 3-dimensional complexity. When the full asymmetries were retained in the initial conditions, both the prehurricane cloud cluster and the prehurricane depression developed into a mature hurricane in the numerical integrations. When the asymmetries, with the exception of those associated with the zonal mean wind, were not present in the initial conditions, the prehurricane depression showed no significant development during the first 60 hours of integration, after which it intensified slightly, and the prehurricane cloud cluster developed into a weak depression. Associated with the wave-like asymmetries in the initial wind and temperature fields are large-scale eddy fluxes of angular momentum and heat. The data reveal that these are organized in such a way as to induce a secondary radial circulation which picks up moisture as it spirals inward over a large stretch of ocean and pumps dry air outward in the upper troposphere and lower stratosphere. We argue that it is this circulation which initially organizes the convection over warm ocean water and triggers a chain of events that ultimately leads to the intensification of the disturbance into a hurricane. The absence of such fluxes when the initial conditions consist of the zonal mean wind superimposed on the axially symmetric component of the flow is sufficient to keep the disturbance from intensifying into a hurricane.


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