FORECASTING THE MOTION OF TROPICAL CYCLONES USING A NUMERICALLY DERIVED STEERING CURRENT AND ITS BIAS^1,,2

Abstract

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.

Development and Tropical Transition of an Alpine Lee Cyclone. Part I: Case Analysis and Evaluation of Numerical Guidance

“…This inner grid is coded CMCGEMH/S, a 3.58 southward shift of the CMCGEMH grid used for the D-PHASE project designed to improve the GoG cyclone's placement within the domain. The steering flow is computed as suggested by Renard (1968), with an inner radius of 250 km and an annulus width of 500 km for the 1000-500-hPa layer. The limited-area simulations are fully nonhydrostatic and employ the single-moment version of the Milbrandt and Yau (2005) grid-scale microphysics scheme.…”

Hurricane Juan (2003). Part II: Forecasting and Numerical Simulation

“…6e,g, the 24-h forecast fields show Hurricane Juan in markedly different positions with MSLP minimum of the MC2 storm located 250 km to the west of the GHM center. The work of Renard (1968) and others suggests that the motion of a tropical cyclone is well described by a deep-layer mean wind (i.e., the steering flow). A direct comparison of the steering flow vector and track of Hurricane Juan in the MC2 and GHM forecasts is shown in Fig.…”

Tropical meteorology

“…There have been a number of efforts to improve forecasting of hurricane movements [Renard, 1968;Sdnders and Burpee, 1968;Miller et al, 1968]. These are all objective techniques, with widely differing degrees of empiricism.…”