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ABSTRACT
Analysis of radar data taken from the three intense hurricanes that passed close to WSR-57 and WSR-88D radar sites at their point of landfall illustrate small-scale spiral bands that are frequently observed within 100 km or so of the hurricane center. Faintly visible in the radar reflectivity images, these bands have scales of 10 km across the band and can extend for 100 km as they spiral outward in a clockwise fashion. They appear to move around the hurricane with speeds close to the tangential wind at the level of the bands and are characterized by enhanced updrafts with higher equivalent potential temperature in the regions of elevated reflectivity. They induce wind speed variations of at least 8 m s ' across the bands. The authors suggest that these small-scale hurricane spiral bands are similar to boundary layer rolls although they extend through depths of 5-6 km, which is more than would be expected for rolls in the boundary layer near the sea. The data presented here are not sufficient to completely describe the structure of the spiral bands, so their role in hurricane dynamics is not known.
1. Introduction
Recent observations of three landfalling hurricanes, Hugo, Andrew, and Erin, have provided measurements of ubiquitous, small-scale spiral bands. These observations include the passage of Hurricane Hugo over the Charleston, South Carolina, WSR-57 radar on 22 September 1989; Hurricane Andrew just south of the Miami, Florida, WSR-57 radar on 24 August 1992; and Hurricane Erin close to the Mobile, Alabama, WSR88D on 3 August 1995. In each case, continuous radar coverage of the hurricane was provided for over an hour prior to landfall, and in each case, the hurricanes contained small-scale spiral bands that have not been described in detail in the literature previously. These spirals appear to be distinct from the rainbands that have been described from radar studies for decades (Senn and Hiser 1959). The observations of the landfall of Hurricane Erin also provided measurements of radar radial velocity perturbations in the small-scale hurricane spiral bands.
A growing body of evidence suggests that important small-scale circulations and features can be found in the inner hurricane rainbands and near the eyewall of a hurricane, and that many of these features are related...