Comparison study of SEASAT scatterometer and conventional wind fields
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LocationWestern North Atlantic
A demonstrated need exists for better wind field information over the open ocean, especially as a forcing function for ocean circulation models. Microwave scatterometry, as a means of remotely sensing surface wind information, developed in response to this requirement for a surface wind field with global coverage and improved spatial and temporal resolution. This development led to the 1978 deployment of the SEASAT Satellite Scatterometer (SASS). Evaluations of the three months of SEASAT data have established the consistency of SASS winds with high quality surface wind data from field experiments over limited areas and time periods. The directional ambiguity of the original SASS vectors has been removed by Atlas et al. (1987) for the entire data set, and the resulting SASS winds provide a unique set of scatterometer wind information for a global comparison with winds from conventional sources. A one-month (12 August to 9 September 1978) subset of these dealiased winds, in the western North Atlantic, is compared here with a conventional, pressure-derived wind field from the 6-hourly surface wind analyses of the Fleet Numerical Oceanographic Center (FNOC), Monterey, CA. Through an objective mapping procedure, the irregularly spaced SASS winds are regridded to a latitude-longitude grid, facilitating statistical comparisons with the regularly spaced FNOC wind vectors and wind stress curl calculations. The study includes qualitative comparisons to synoptic weather maps; calculations of field statistics and boxed mean differences; scatter plots of wind speed, direction, and standard deviation; statistical descriptions of the SASS-FNOC difference field, and wind stress curl calculations. The SASS and FNOC fields are consistent with each other in a broad statistical sense, with wide scatter of individual values about a pattern of general agreement. The FNOC wind variances are slightly smaller than the SASS values, reflecting smoothing on larger spatial scales than the SASS winds, and the SASS mean values tend to be slightly higher than the FNOC means, though the increase is frequently lost in the large scatter. Exceptions to the pattern of relatively small consistent variations between the two fields are the pronounced differences associated with extremely strong winds, especially during Hurricane Ella, which traveled up the East Coast of the United States during the latter part of the study period. These large differences are attributed mainly to differences in the inferred positions of the pressure centers and in the response at the highest wind speeds (> 20m/s). The large statistical differences between the SASS and FNOC fields, present under high wind conditions, may yield significantly different ocean forcing, especially when the strong winds persist over longer periods of time. Under less intense wind conditions, usually prevailing over the ocean, the two fields correspond well statistically and the ocean responses forced by each should be similar.
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1988This thesis is not subject to U.S. copyright.
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