Comparison of RADARSAT Observations of Secondary Circulations in Hurricane Floyd's Boundary Layer With Concurrent WP-3D Aircraft Observations

RADARSAT images of several hurricanes, notably Mitch, Georges and Floyd, have shown distinctive streaks parallel to the surface wind which appear to confirm the existence of secondary 'roll-vortex' circulations in the hurricane boundary layer. These features had been noted earlier by NEXRAD and DOW Doppler radar observations in the hurricane boundary layer over land. However, their existence over the ocean in widespread regions of the hurricane circulation suggest a new scale of motion that will be very important for modulating surface air-sea fluxes.

Hurricane Floyd on 14 Sept, 1999 presented an especially well-documented case of these along-wind features. FFT analysis of the RADARSAT image showed that the dominant spacing of the streaks ranged from 1.8 - 3.5 km, approximately 2 - 4 times the average boundary layer depth. This is larger than the spacing found from the Doppler radar observations, which were on the order of the estimated boundary layer depth.

One of the NOAA WP-3D aircraft flew an east-west pattern cross the south and north sections of the image, dropping GPS dropsondes and obtaining surface emissivity and ocean backscatter measurements from an on-board Stepped Frequency Microwave Radiometer (SFMR) and a Scanning Radar Altimeter (SRA). These observations establish the boundary layer depth, independently confirm the RADARSAT surface modulations with airborne remote sensing data and establish the orientation of the streaks being parallel to the surface wind direction. Future airborne gust probe measurements are planned to directly measure the effects of these streaks on surface fluxes via eddy correlation analysis.