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Changes in the Low-Level Kinematic and Thermodynamic Structure of Hurricane Alicia (1983) at Landfall

Mark D. PowellNOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida 33149

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Abstract

Aircraft, land station, and buoy data were composited with respect to the center of Hurricane Alicia (1983) for three 8-h periods corresponding to prelandfall in the open Gulf of Mexico, landfall in the Galveston area, and postlandfall in the vicinity of Houston.

Comparison of the wind analyses before, during, and after landfall emphasizes the land-sea frictional asymmetry at landfall. In addition, other asymmetries in the surface wind field and differences between the flight-level and the surface wind fields are revealed. The asymmetric structure of the surface wind field may be interpreted as having resulted from the combined effects of land-sea roughness differences, background environmental flow, and storm translation. The land-sea frictional difference acted to oppose the mean vortex flow over land and reinforce it over water. The southwest background environmental flow acted nearly parallel to the coastline, producing surface inflow on the left side and outflow on the right side, while the effect of the storm translation increased winds on the right and decreased winds on the left. At landfall, the analysis revealed a broad region of high wind speeds and a mesoscale divergence-convergence couplet along the outer rainband axis just offshore on the northeast (right) side of the storm. The outer rainband axis acted as an obstruction to the surface flow, separating the warmer central core of the storm from the environment through which the storm moved. In contrast to recent numerical model studies, surface convergence was also noted on the left side of the storm just offshore, despite outflow at flight level.

Analyses of temperature, dewpoint, and equivalent potential temperature indicate that loss of the oceanic heat and moisture source, combined with advection of drier air on the landward side of the storm, was responsible for cooling and drying of the inflowing boundary layer air. Upon introduction of this air into the core convection and vertical ascent, a decrease in the release of latent heat could then lead to cooling in the middle levels of the storm and a subsequent increase in the central sea-level pressure.

Abstract

Aircraft, land station, and buoy data were composited with respect to the center of Hurricane Alicia (1983) for three 8-h periods corresponding to prelandfall in the open Gulf of Mexico, landfall in the Galveston area, and postlandfall in the vicinity of Houston.

Comparison of the wind analyses before, during, and after landfall emphasizes the land-sea frictional asymmetry at landfall. In addition, other asymmetries in the surface wind field and differences between the flight-level and the surface wind fields are revealed. The asymmetric structure of the surface wind field may be interpreted as having resulted from the combined effects of land-sea roughness differences, background environmental flow, and storm translation. The land-sea frictional difference acted to oppose the mean vortex flow over land and reinforce it over water. The southwest background environmental flow acted nearly parallel to the coastline, producing surface inflow on the left side and outflow on the right side, while the effect of the storm translation increased winds on the right and decreased winds on the left. At landfall, the analysis revealed a broad region of high wind speeds and a mesoscale divergence-convergence couplet along the outer rainband axis just offshore on the northeast (right) side of the storm. The outer rainband axis acted as an obstruction to the surface flow, separating the warmer central core of the storm from the environment through which the storm moved. In contrast to recent numerical model studies, surface convergence was also noted on the left side of the storm just offshore, despite outflow at flight level.

Analyses of temperature, dewpoint, and equivalent potential temperature indicate that loss of the oceanic heat and moisture source, combined with advection of drier air on the landward side of the storm, was responsible for cooling and drying of the inflowing boundary layer air. Upon introduction of this air into the core convection and vertical ascent, a decrease in the release of latent heat could then lead to cooling in the middle levels of the storm and a subsequent increase in the central sea-level pressure.

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