External Influences on Hurricane Intensity. Part I: Outflow Layer Eddy Angular Momentum Fluxes

Abstract

Outflow layer winds were objectively analyzed every 12 h for 6 days during the life cycle of Hurricane Elena (1985). A high correlation was found between angular momentum fluxes by azimuthal eddies at large radii and central pressure changes in the storm 27?33 h later. Momentum flux by eddies exceeded that by the azimuthal mean outside the 800 km radius, while vortex spinup by the eddies reached instantaneous magnitudes as large as 25 m s?1/day. Outflow maxima and minima repeatedly appeared more than 1000 km from the hurricane center and tracked inward with time. The results provide evidence of significant environmental control on the behavior of the storm. After reaching hurricane strength, Elena experienced a major secondary intensification associated with a large inward cyclonic eddy momentum flux produced by the passage of a middle latitude trough north of the hurricane. An outflow maximum appeared radially inside of the eddy momentum source, consistent with balanced vortex theory, and tracked inward with the eddy momentum source during the following 24 h. When the outflow maximum reached the storm core, an extended period of rapid pressure fails followed. It is speculated that these pressure falls represented a response to midlevel spinup forced by the outflow layer momentum sourcers. Although environmental forcing dominated the later stages of Elena, the rapid initial intensification of the storm as it moved from land to water appeared to be a precursor to subsequent environmental interactions. The enhanced anticyclonic outflow from this initial deepening reduced the outflow-layer inertial stability, allowing a more radially extended region for external forcing. The secondary intensification of Elena is thus viewed as a cooperative interaction between mesoscale events at the hurricane core and synoptic-scale features in the upper tropospheric environment.