89th American Meteorological Society Annual Meeting

Tuesday, 13 January 2009: 8:30 AM
The combined effect of surface rain and wind on scatterometer observations of surface roughness
Room 128A (Phoenix Convention Center)
David E. Weissman, Hofstra University, Hempstead, NY; and M. A. Bourassa
Changes in the sea surface roughness from the combined effects of wind and rain, on scales of tens of kilometers, are being studied using the QuikSCAT scatterometer and simultaneous NEXRAD three-dimensional measurements of rain. Buoys and related data provide the additional wind information. From the remote sensing perspective, these results will show the dependence of the sea surface radar cross section, at Ku-band, as a function of the rainrate, wind speed and relative direction, and polarization. The studies of air-sea interaction, related to surface fluxes (e.g., momentum, sensible heat, and latent heat), require extended investigations because heavy rain in the boundary layer changes the surface stress and stratification, both of which alter the surface stress and turbulent heat fluxes. A new finding is that certain combinations of wind speed and rainrate can result in reduced surface roughness; implying reduced surface stress and a reduced drag coefficient. This finding naturally leads to the question ‘do such conditions occur in tropical cyclones?' The methodology described below is used to address this question.

The unique method of this study is that it combines satellite based Ku-band data with high-resolution 3-D volumetric rain measurements, from simultaneous collocated NEXRAD data. The volumetric scans of this high resolution S-Band radar are used to model the 3-dimensional Ku-band reflectivity of the volume of precipitation that the scatterometer beam passes through as it samples the sea surface. Consideration was also given to the choice of rain drop-size-distribution and the associated Z-R relation used for the NEXRAD application. The choice of a convective rain model had an appreciable effect on the correction to each measured QuikSCAT NRCS cell for rain attenuation and rain volume backscatter. The removal of these effects leaves the total contribution of the sea surface; both the wind driven and rain-impact roughness terms

One set of results to be presented were acquired during a significant rain event in the Gulf of Mexico, to the east of Corpus Christi, and just south of Houston, TX in May 2005. Preliminary results in NRCS caused by rain, relative to that in nearby regions with negligible rain shows distinct characteristics. Three regions with different wind speeds (4-6, 6-8 and 8-10 m/s) show definitive variation of this total NRCS with respect to wind magnitude, satellite-relative wind direction, polarization and rainrate. Relative changes are stronger in the lower wind region for both polarizations, with H-pol providing a more definitive signature. At higher wind speeds (e.g. 10 m/s) the relative splash induced increases in NRCS are still significant, and show distinct differences between the two polarizations. These results can be interpreted by examining the properties of rain-induced splashes, crowns, ring waves and their response to a variety of wind speeds.

Similar comparisons are being conducted for observations during Hurricane Dennis, in the Gulf of Mexico, on July 10, 2005, not far from the northwest coast of Florida. This study combine the QuikSCAT Scatterometer observations and H*WIND fields with NCEP winds. Of particular interest is the change in surface roughness, relative to what would be expected from purely wind forcing.


Supplementary URL: