Tuesday, 15 January 2002: 10:45 AM
Imaging the sea surface imprints of atmospheric vortex streets by space-borne synthetic aperture radar
The sea surface imprint of atmospheric vortex streets (AVS) observed on four RADARSAT Synthetic Aperture Radar (SAR) images of the Aleutian Islands in the western Gulf of Alaska during NOAA's Alaska SAR demonstration period (1999-2001) are analyzed. The AVS's are interpreted as the atmosphere analog of classic Von Karman vortex streets. The AVS length is typical over 200 km, and there are several pairs of vortices within the AVS. Although atmospheric vortex shedding from large islands was suggested as early as the 1930's, it was not until the early 1960's that researchers observed the AVS pattern in the atmosphere in cloud images taken by the first generation of earth orbiting satellites. AVS's were not observed prior to the advent of satellites due to the AVS scale of 100 to 400 km; too small to be delineated by a synoptic observation network and too large to be observed by a single station
The surface wind field associated with an AVS can also modify the sea surface roughness. A synthetic aperture radar (SAR) senses the ocean surface roughness through a Bragg scattering mechanism. Therefore, SAR can observe any atmospheric process that modulates near surface wind field.
In this study, we analyze the first few cases of AVS observed by SAR. Using SAR image, we can measure the basic properties of AVS, i.e., the ratio of vortex spacing (a) and the width of the Karman vortex street (h). We found that the a/h ranges between 0.35and 0.45. On the basis of these SAR images, surface pressure maps and nearby radiosonde station observations, the vortex shedding period, the tangential velocity at the outer edge of the vortex, the vortex lifetime at the SAR acquisition time, and the total energy dissipation are estimated using the Von Karman vortex streets theory. We also calculate the range of Reynolds number, the Strouhal number (S), the Lin number and kinematic viscosity number, which can support the AVS shedding.