Microwave imagery and in situ validation of eye mesovortex structure in Hurricane Katrina (2005) at peak intensity

Eye mesovortices are thought to be responsible for the swirling and folded structure of eye stratocumulus in a major hurricane. These complex patterns represent the accumulated effect of horizontal and vertical motions on the time scale of one hour or less. The instantaneous horizontal structure of such motions has (with one exception) never been observed. Here, the simultaneous acquisition of C- and Ku-band microwave imagery from RadarSAT-1 and QuikSCAT, together with in situ flight reconnaissance including passive microwave measurements from the SFMR, are intercompared to test the hypothesis that horizontal motions are responsible for surface roughness variations seen in the microwave. The retrieved magnitude of surface wind variation is on the order of 20 m/s, a value of wind speed several times larger than commonly observed by dropsondes at the calmest part of the eye. The subject of interest is Hurricane Katrina (2005) on August 28th, during the time of her peak intensity. It is shown that the solid body rotation rate of the eye is significant in this storm (comparable to that of the eyewall) but highly variable, and that azimuthally asymmetric motions from the inner eyewall penetrating further inward exist on top of the solid body rotation which are of the right magnitude to explain the observed surface roughness variations. Implications for the moist entropy budget of the lower eye are briefly discussed.