| The effect of horizontal gradients of height-field forecast error variances upon OI forecast error statistics - :11513 | National Weather Service (NWS)
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The effect of horizontal gradients of height-field forecast error variances upon OI forecast error statistics
  • Published Date:
    1984
Filetype[PDF - 1.49 MB]


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  • Corporate Authors:
    National Meteorological Center (U.S.)
  • Description:
    In the formulation of statistical relationships among forecast errors in the optimal interpolation (OI) analysis system at NMC, it is currently assumed that horizontal gradients of the height-field forecast error standard deviation, [sigma]z are negligible. This homogeneity assumption is reasonable only in areas of uniform data density and quality. The dominant feature on the maps of [sigma]z actually produced by the OI system is in fact the rapid change of [sigma]z near boundaries between data-dense and data-sparse regions. In this note we rederive the statistical relationships among forecast errors, without assuming that the [sigma]z field is homogeneous. The resulting forecast error statistics are compared with the conventional ones, using realistic [sigma]z fields. The comparison shows that the wind-field forecast error standard deviations are increased over the entire globe, and by as much as about 30% in some regions. The wind-height and wind-wind forecast error correlations are changed even more dramatically. For example, the correlation between height and zonal wind forecast errors at a point is 0.0 if [sigma]z is constant there, but becomes as large as about ±0.6 at points where [sigma]z is changing rapidly. More generally, the wind-height and wind-wind forecast error correlations lose the homogeneity and isotropy properties they possess in conventional OI formulations, in a manner reflecting the variability of data density and quality encountered over the globe.

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