An objective technique for estimating large-scale flow pattern in the tropical upper troposphere from digitized cloud brightness data.
Jacobs, Frederick T.
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An objective technique is proposed to use digitized satellite cloud brightness data to estimate large-scale flow patterns over data-void tropical regions. It is basically a diagnostic model which assumes: 1) area-averaged cloud brightness is positively correlated with large-scale divergence in the tropical upper troposphere; and 2) large-scale tropical flow is quasi-barotropic and quasi-non-divergent. It can be used at any upper level where divergence is important in determining vorticity. The model requires: 1) boundary conditions determined from surrounding wind reports, 2) a mean zonal flow determined from climatology, and 3) an equivalent divergence forcing function constructed empirically from the brightness data. The technique is tested daily over a western North Pacific region for July-August 1971. Results show that for 25% of the days tested, the model produces a flow field which accurately resembles the major features of the stream-function field analyzed by NMC. In another 30% of the days it provides some valuable information about the flow patterns which would be difficult to obtain from boundary information alone. Experiments also suggest that improvement of the model may be made when satellite infrared data is available.
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