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SASS Wind Ambiguity Removal by Direct Minimization

Ross N. HoffmanGoddard Laboratory for Atmospheric Sciences, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

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Abstract

A gridded surface wind analysis is obtained by minimizing an objective function, the magnitude of which measures the error made by the gridded analysis in fitting the SASS wind data, the conventional surface wind observations and the forecast surface wind field. The ambiguity of the SASS winds is then removed by choosing the alias closest to the analyzed wind. Because minimizing the objective function is a problem of nonlinear least-squares, the minimizing gridded analysis is not unique and a good first guess is necessary to assure convergence to a reasonable solution. A good first guess may be generated by performing the analysis in stages.

Illustrative results are shown for a limited region in the North Atlantic containing the QE II storm and for a limited amount (∼12 min) of data observed near the synoptic time 1200 GMT 10 September 1978. Within the SASS data swath the resulting gridded analysis is a reasonable representation of the surface wind and is not sensitive to the forecast surface wind field. The analysis is sensitive to wind directions reported by ships near the center of the storm. The wind circulation center present in the forecast is moved by the analysis. The resulting dealiased SASS wind directions are noisy.

Abstract

A gridded surface wind analysis is obtained by minimizing an objective function, the magnitude of which measures the error made by the gridded analysis in fitting the SASS wind data, the conventional surface wind observations and the forecast surface wind field. The ambiguity of the SASS winds is then removed by choosing the alias closest to the analyzed wind. Because minimizing the objective function is a problem of nonlinear least-squares, the minimizing gridded analysis is not unique and a good first guess is necessary to assure convergence to a reasonable solution. A good first guess may be generated by performing the analysis in stages.

Illustrative results are shown for a limited region in the North Atlantic containing the QE II storm and for a limited amount (∼12 min) of data observed near the synoptic time 1200 GMT 10 September 1978. Within the SASS data swath the resulting gridded analysis is a reasonable representation of the surface wind and is not sensitive to the forecast surface wind field. The analysis is sensitive to wind directions reported by ships near the center of the storm. The wind circulation center present in the forecast is moved by the analysis. The resulting dealiased SASS wind directions are noisy.

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