Formation of Atlantic Hurricanes from Cloud Clusters and Depressions

Abstract

The role of large scale eddy processes in the transformation of cloud clusters and depressions into hurricanes is investigated by using different initial conditions in numerical integrations of the Naval Research Laboratory limited-area hurricane model. With initial conditions specified from the Colorado State University composite datasets of Professor William Gray for Atlantic nondeveloping cloud clusters, wave trough clusters and depressions, no hurricane formation takes place in any of the model integrations. With initial conditions specified from the datasets for Atlantic developing cloud clusters and depressions, a hurricane develops in the course of each model integration. One characteristic difference between the developing and nondeveloping disturbances is that the former exhibit large, well-organized eddy flux convergences of angular momentum associated with wavelike disturbances in the upper troposphere and lower stratosphere, whereas the latter exhibit weak, poorly organized eddy momentum fluxes. In order to assess the role played in hurricane formation by the large-scale eddy processes in the developing cases, we performed additional integrations with initial conditions in which the eddies were removed from the datasets for the developing cloud cluster and depression. This was accomplished by using only the symmetric components of the wind and moisture fields in these datasets. In these integrations, the initial disturbances failed to develop. We take this as evidence to support the view that wavelike asymmetries in the upper troposphere and lower stratosphere may be necessary for hurricane development from Atlantic cloud clusters and depressions. Such asymmetries may act through the agency of eddy fluxes of heat and/or eddy fluxes of momentum. In this paper, we concentrate mainly on the role of eddy fluxes of momentum. Mechanistically, these fluxes exert an upper-level cyclonic torque on the initially weak vortex. Such a process induces upper level divergence and lower level convergence. The air converging in the lower boundary layer over a broad stretch of warm ocean brings moisture inward, organizing and concentrating the convection which fuels the development of the hurricane.