Tuesday, 23 May 2000: 4:15 PM
The operational forecast fields from the Geophysical Fluid Dynamics Laboratory
Hurricane Prediction System (GFDL model) for Hurricane Opal (1995) are used to
diagnose the rapid intensification of a tropical cyclone. The environmental
wind is computed in an averaging annulus. An episode when the
component of the
environmental wind perpendicular to the direction of storm
motion vanishes is coincident with the most rapid
intensification of the storm. Using a mean tangential wind budget, positive
eddy vorticity fluxes aloft are identified in the vicinity of Opal
(~600 km), but this signature is as equally present during the intensifying
stage as in the subsequent weakening stage of the storm. A detailed examination
of each of the terms of the budget (mean and eddy vorticity flux, mean and
eddy vertical advection, and friction) shows a greater vertical extension of
total forcing for mean tangential winds near the center of the
storm, particularly from the mean and
eddy vertical advection terms. Upper-level divergence exhibits
significant vertical
structure, such that single level or layer average analysis techniques will not
capture the divergence signature aloft. Divergence aloft is also
significantly influenced by far-field convective events that are, perhaps, only
indirectly influenced by the hurricane. The environmental
vorticity distribution is examined for asymmetries (particularly in relation
with an upper level trough to the northwest) and a mechanism for their
removal is presented as a prelude to an attribution study of the specific
dynamical effects of environmental features on a hurricane.
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