P2E.3 Images of Katrina (2005) below the cloud

Thursday, 1 May 2008
Palms ABCD (Wyndham Orlando Resort)
Timothy J. Dunkerton, NorthWest Research Associates, Bellevue, WA; and B. A. Walter, W. A. Perrie, D. G. Long, C. Nie, J. A. Zhang, P. G. Black, and R. F. Rogers

A remarkable coincidence of two independent satellite images from Radarsat-1 synthetic aperture radar (SAR) and SeaWinds/QuikSCAT scatterometer, depicting the state of the sea surface, and HRD/NOAA aircraft reconnaissance including a Stepped Frequency Microwave Radiometer (SFMR), occurred in Hurricane Katrina near the time of its maximum intensity on August 28th, 2005. The satellite images were acquired within 6 seconds of each other near 2345 UTC. The eye, primary and secondary eyewalls, and outer rainbands were traversed by the aircraft during the time of image acquisition, and all of these features are visible on the images, both of which captured most of the storm area. Comparison of SAR and scatterometer images indicates good agreement in the level of backscatter pixel-by-pixel when SAR pixels (~50 m) are averaged to match scatterometer pixels (~2.5 km). Comparison with airborne SFMR also indicates good agreement when the aircraft data are rotated slightly in azimuth to account for advection by the tangential surface winds over a period of 0-6 minutes. Comparison of surface and flight level data in the primary eyewall indicates an outward tilt of the axis of maximum winds with height similar to that seen in the Doppler composite structure obtained around this time. Surface winds appear stronger than flight-level winds in the primary eyewall but not in a secondary eyewall farther out. Retrieval of surface wind from the microwave imagery is complicated by regions of intense precipitation in the primary and secondary eyewalls and in the outer bands. The coincidence of measurements in two different microwave bands (C, Ku) together with a third view from the airborne radar provides an unprecedented opportunity to assess the impact of hydrometeors on microwave wind retrieval. Because the eye is devoid of rain droplets at this time, satellite-derived estimates of surface wind in this region of the storm are more reliable than elsewhere. Enhanced surface roughness and derived windspeed are found to coincide with a horizontal plume of air and low stratocumulus across the center of the eye, attributable to a pair of eye mesovortices. SFMR measurements in the eye cross-validate these enhanced values of retrieved windspeed. Surface winds in the plume significantly exceed those of typical dropsonde measurements and support a finding from a new climatology of scatterometer winds in hurricanes suggesting rms values of 10-20 m/s at the origin. The impact of such enhanced winds on air-sea exchange and their role in maintaining elevated values of moist entropy within the eye is assessed quantitatively.
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