| Global forecast-error correlation. Part 1, Isobaric wind and geopotential - :11458 | National Weather Service (NWS)
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Global forecast-error correlation. Part 1, Isobaric wind and geopotential
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  • Description:
    Results of a thorough study of the correlation structure of observation-minus-forecast increments for mandatory pressure level radiosonde observations of zonal and meridional wind components and geopotential, differenced with NMC's 6-hour global forecasts, are reported. Our work focused on the selection of a representation for spatial lag-correlations to be used in updating the multivariate statistical objective analysis algorithm of the global data assimilation system, with attention given to regional and seasonal dependence of the correlation structure, and on the degree to which the increments are in the same geostrophic balance as the signal and forecast fields individually. We compare the performance of several candidates for representing geopotential autocorrelations, on the one hand, and the auto- and cross-correlations of the wind components, on the other, for five mandatory pressure levels, for four regions of the Northern Hemisphere and for the Southern Hemisphere; and we identify one functional form as optimal for global objective analysis. The parameters are shown to vary with level and season. Furthermore, the geopotential and wind correlation fits have identified important differences in corresponding parameter values. A single algorithm which covers the primary candidates in one fitting operation, for future semi-automatic updating, has been developed in the course of this work. Results of its use are presented and discussed. Part 2 will add the vertical component of three-dimensional correlation structure, and present analysis and forecast impact test results of use of this structure in the global optimal interpolation algorithm.

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