P5.7
Transition from unstable to stable PBL in the coastal region observed by SAR
Ralph C. Foster, APL/ University of Washington, Seattle, WA; and D. C. Vandemark, H. L. Stern, L. Mahrt, and P. D. Mourad
We present analysis of SAR imagery collected on 25 November, 1999 as part of the Shoaling Waves experiment off the North Carolina coast near Duck. This is an interesting example of a very complex boundary layer situation. In situ surface fluxes and high resolution radar and laser wave mean square slope data were measured by the low-flying LongEZ aircraft along flight legs timed to coincide with the SAR overpass. The SeaWinds on QuickSCAT scatterometer overpassed this region 10 minutes prior to the SAR image and two NDBC buoys are within the scene. In this case we have easterly flow off the Gulf Stream. As the flow crosses the western edge of the Gulf Stream north wall, the stratification switches from unstable to stable thus reducing the downward transfer of horizontal momentum and hence the surface wind. Over the Gulf Stream wind speeds are around 5 to 6 $ms^{-1}$. Immediately west of the Gulf Stream the surface winds measured on the LongEZ drop to $~2ms^{-1}$ or lower which is less than the threshold for wind-induced gravity-capillary wave formation resulting in very low SAR backscatter. We postulate that an internal boundary layer has formed due to the SST front and resulting change in atmospheric stratification. The implied absence of capillary waves is confirmed by laser glint data measured on the LongEZ. Flight observers reported the sea surface to be free of surface slicks in the reduced backscatter region. This is the second field observation of the threshold wind speed necessary for gravity-capillary wave formation that we know of. We also consider the effects of sub-scatterometer footprint variability, which can be imaged via SAR, on the scatterometer wind retrievals.
Poster Session 5, ABL Parameterizations; Marine BLs
Thursday, 18 July 2002, 2:00 PM-2:00 PM
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