Characteristics of the Upper-Tropospheric Environmental Flow around Hurricanes

Robert T. Merrill Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

The upper-tropospheric flow out to a radius of 2000 km around Atlantic hurricanes is described using rotated coordinate composite analysis of the NOAA National Hurricane Center operational wind set. The rotated coordinate methodology, designed to preserve some of the asymmetry of hurricane outflow during compositing, is described in detail.

A rotated coordinate composite of all hurricanes from a five-year period is used to study the general properties of the hurricane outflow layer. Coordinate rotation improves the representation of the outflow jet and the associated extrema of radial and tangential wind, but tends to obscure the geographically persistent features of the upper-tropospheric environment such as the midlatitude westerlies. The amplitude of the asymmetric radial wind is twice that of the symmetric, while the amplitudes of tangential winds are equivalent. A comparison of geographic and rotated coordinate composites indicates that both the outflow jet and the midlatitude westerlies are important structures for the import of angular momentum into the hurricane by horizontal eddy fluxes.

Separate composites of eight characteristic outflow patterns are also presented. Pattern variability arises from the juxtaposition of the hurricane circulation with surrounding synoptic features.

Abstract

The upper-tropospheric flow out to a radius of 2000 km around Atlantic hurricanes is described using rotated coordinate composite analysis of the NOAA National Hurricane Center operational wind set. The rotated coordinate methodology, designed to preserve some of the asymmetry of hurricane outflow during compositing, is described in detail.

A rotated coordinate composite of all hurricanes from a five-year period is used to study the general properties of the hurricane outflow layer. Coordinate rotation improves the representation of the outflow jet and the associated extrema of radial and tangential wind, but tends to obscure the geographically persistent features of the upper-tropospheric environment such as the midlatitude westerlies. The amplitude of the asymmetric radial wind is twice that of the symmetric, while the amplitudes of tangential winds are equivalent. A comparison of geographic and rotated coordinate composites indicates that both the outflow jet and the midlatitude westerlies are important structures for the import of angular momentum into the hurricane by horizontal eddy fluxes.

Separate composites of eight characteristic outflow patterns are also presented. Pattern variability arises from the juxtaposition of the hurricane circulation with surrounding synoptic features.

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