Small-Scale Spiral Bands Observed in Hurricanes Andrew, Hugo, and Erin

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?1 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.