Objective Analysis in the Tropics: A Proposed Scheme

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  • 1 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

The tropical wind field presents some unique problems to an assimilation scheme designed for extratropical latitudes. Foremost among these is the relative increased importance of the divergent component of the upper and lower troposphere. Examination of tropical wind field analyses from conventional assimilation schemes points to some unsatisfactory results. Because of these problems an algorithm is proposed which is intended to handle the rotational and divergent components separately. The divergent component is estimated by transforming satellite-observed outgoing longwave radiation to a velocity potential field. The full analysis is then obtained by statistical optimum interpolation (OI) using a guess field which is an addition of a rotational forecast field with the divergent field estimated above. The scheme is tested against a conventional (single-level) OI scheme for a number of cases selected from the FGGE year. The proposed scheme is shown to perform comparably to the conventional OI scheme in regions of dense conventional observed data and, by induction, to be superior in areas of insufficient data.

Abstract

The tropical wind field presents some unique problems to an assimilation scheme designed for extratropical latitudes. Foremost among these is the relative increased importance of the divergent component of the upper and lower troposphere. Examination of tropical wind field analyses from conventional assimilation schemes points to some unsatisfactory results. Because of these problems an algorithm is proposed which is intended to handle the rotational and divergent components separately. The divergent component is estimated by transforming satellite-observed outgoing longwave radiation to a velocity potential field. The full analysis is then obtained by statistical optimum interpolation (OI) using a guess field which is an addition of a rotational forecast field with the divergent field estimated above. The scheme is tested against a conventional (single-level) OI scheme for a number of cases selected from the FGGE year. The proposed scheme is shown to perform comparably to the conventional OI scheme in regions of dense conventional observed data and, by induction, to be superior in areas of insufficient data.

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