Extended Abstract (480K)4A.4
The mean structure of vertical velocities and radar reflectivities in the hurricane eyewall as they relate to environmental wind shear
Michael L. Black, NOAA/AOML/HRD, Miami, FL; and F. D. Marks, R. F. Rogers, L. K. Shay, B. A. Albrecht, and H. E. Willoughby
Three widely-recognized factors control tropical cyclone structure and intensity: fluxes of heat and energy to and from the ocean, the storms' internal dynamics, and forcing by the surrounding atmosphere, including upper-level momentum fluxes and vertical wind shear. Of these factors, environmental wind shear is probably the most acknowledged influence on hurricane intensity change. The intensity problem is complex, depending on the interplay between the ocean and atmosphere, the environmental flow, and the internal dynamics of the storm. In recent years, much of the attention in the literature on this problem has been given to the effects of the atmospheric environment, particularly that of vertical wind shear, to the structure and intensity change of the tropical cyclone. The purpose of this study is to analyze the precipitation and vertical velocity structure of the hurricane eyewall from a sample of storms of a variety of intensities and affected by a broad range of environmental wind shears.
The primary data consist of Doppler velocities and reflectivities derived from vertically-pointing rays (vertical incidence or VI) recorded by the 3-cm tail radar system on the NOAA WP-3D aircraft. At intervals of ~750 m along the flight track, Doppler radial velocities and reflectivities are archived in 300 m vertical bins, from just above the sea surface to 15 km altitude. To estimate the vertical winds, the hydrometeor fallspeeds and the vertical motions of the aircraft are removed from the raw Doppler radial velocities. The particle fallspeeds are determined with bulk formulae using radar reflectivity as a function of height. VI Doppler data have been processed for over 300 radial legs, penetrations into or exits from the eye, that were obtained during 28 flights in 11 Atlantic and Eastern-Pacific hurricanes.
In addition to the data from the tail radar, imagery and data from the horizontally-scanning, 5-cm lower-fuselage radar are used to identify asymmetries in the eyewall precipitation structure and to relate these asymmetries to the observed shear and vertical velocity distributions. The calculations of vertical wind shear are from gridded analyses from the European Centre for Medium-Range Weather Forecasts. The data are being provided by John Molinari from SUNY, Albany, NY. The circulation of the symmetric vortex is removed and the resulting shear vector is computed over a radius of 500 km from the storm center and between the 200 mb and 850 mb pressure levels. The shear values are available every 6 or 12 hours so that data for an individual flight mission will be assigned the shear value closest to the time of the flight.
The analyses of this study will be compared to numerically-modeled and individual case studies to see if they are consistent with these observations which cover a broad range of shears and storm intensities. The differences in the analyzed structure may provide insight into the effects of shear on hurricane intensity change. Finally, the results of this study may establish a more refined paradigm of the types of structure most commonly observed in the hurricane eyewall under varying atmospheric wind shear.
Session 4A, Tropical Cyclones and Vertical Shear
Monday, 3 May 2004, 3:45 PM-5:30 PM, Le Jardin Room
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