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The effect of initial uncertainty in topical analysis upon 5-day forecasts with MNC's global spectral model
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The  effect of initial uncertainty in topical analysis upon 5-day forecasts with MNC's global spectral model
  • Description:
    The effect of initial perturbations in the tropical portions of the global optimum interpolation (OI) analysis scheme used at the National Meteorological Center (NMC) upon 5-day forecasts in both the tropics and in middle latitudes is examined. Systematic tropical mass and wind field differences at 850 and 200 mb between analyses produced at NMC and the European Center for Medium-range Weather Forecasting (ECMWF) during the FGGE year are first shown to exist and to persist during February and June of 1979. Subsets of the ECMWF mass and wind field analyses at all levels in regions where the systematic difference or "uncertainty" is largest between the analysis are used to construct initial fields which are perturbed in isolated portions of the tropics. Five-day forecasts are then run from these initial data as well as from unperturbed "control" NMC analyses, and the results compared. The response to initial tropical perturbations takes 2 forms: a local response and a teleconnection response to middle latitudes. Pertubations propagate in the tropics fairly slowly via Kelvin modes and mixed Rossby gravity modes. Propagation to middle latitudes occurs rapidly via ultra long wave length barotropic Rossby modes. The middle latitude response is similar regardless of the location of the initial perturbation, and the response is strongest when the initial perturbation lies near regions of tropical westerlies, suggesting that an initial tropical disturbance escapes to middle latitudes through these westerly wind ducts.
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