External Influences on Hurricane Intensity. Part II: Vertical Structure and Response of the Hurricane Vortex

Abstract

The vertical structure of the interaction of Hurricane Elena (1985) with a baroclinic wave was evaluated using analyses from the European Centre for Medium Range Weather Forecasting. During the period of interaction, azimuthal eddies produced a localized flux convergence of cyclonic angular momentum in the upper troposphere which shifted to progressively smaller radii prior to major secondary deepening of the storm. These momentum fluxes decayed above and below the outflow layer. Eddy heat fluxes showed maximum cooling in the middle and upper troposphere and warming in the lower stratosphere, reflecting the temperature structure of the baroclinic wave as it moved into the hurricane volume. The response of the hurricane vortex to the fluxes of heat and angular momentum was determined by solution of Eliassen's balanced vortex equation. The balanced solutions showed a band of upward motion, with deep inflow and narrow outflow, which shifted inward from the 500 km radius to the hurricane core in the 24 hours prior to the secondary deepening. The position and timing of this feature corresponded to the contracting outflow maximum found in Part I. Eddy heat fluxes contributed to the induced circulation in the same manner as momentum fluxes near the core, but with smaller magnitude and areal coverage. The contracting outflow maximum thus appeared to represent the upper branch of a secondary circulation excited primarily by the eddy momentum fluxes. The reintensification of hurricanes is often directly associated with formation of a wind maximum at inner radii which replaces or reinforces the original eye wall as it contracts. Such a feature was seen in reconnaissance data in Elena at the time the secondary circulation reached inner radii. It is speculated that the relatively weak secondary circulation evolved into a local wind maximum through the actions of diabatic heat sources. The approaching trough is thus viewed not as a direct cause of deepening, but as a catalyst which organized the diabatic sources in such a way as to excite internal instabilities of the system.