Experiments in numerical objective frontal analysisRenard, Robert J.; Clarke, Leo C.

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The vector motion of severe tropical cyclones (including storm, hurricane/typhoon stages) is forecasted by a numerical scheme which involves two steps: a. Numerical gcostrophic steering of the center of the cyclone using the U.S. Navy Fleet Numerical Weather Facility's (FN W F) operationally produced smoothed isobaric height fields, called SR. The tropical perturbations arc steered in I-hr. time steps up to 72 hr., using winds derivcd from the S R analysis dated closest to warning time. S R 500 mb. in thc Pacific and S R 700 mb. in the Atlantic gave the most accurate forccasts on tests of 10 northwest Pacific typhoons and all fivc north Atlantic tropical storms and hurricanes in the period Aug. 15-Nov. 1, 1965. Forecasts were made twice daily, 0600 and 1800 GMT, during this period using the best track information. b. Next, the numerical-stcering prediction is objectively modified to adjust for bias (Le., deficiency in both zonal and meridional motion) by utilizing errors made in the most recent 12-and 24-hr. numerical-steering forecasts. Scveral modes of adjustment are employed; the most recent 12-(12-and 34-) hr. numerical-steering bias yields the most accurate correction of subsequent Atlant.ic (Pacific) forccasts out to pcriods of 72 hr. The optimal Naval Postgraduate School (NPGS) technique produces forecast errors ranging from an average of 4.2 kt. for 12-hr. forecasts to 6.2 kt. for 72-hr. forecasts. The U.S. Navy's official forecast accuracy is excclled by the NPGS scheme for all time periods. Stratification of error statistics by area, trajectory, and stage of storm, intercomparison with ESSA's NHC-64 tcchnique, discussion of merits and dcficicncies of the research program relative to operational forecasts, and current experiments at F N W F are discussed.

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