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ROBERT J. RENARD

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 geostrophic steering of the center of the cyclone using the U.S. Navy Fleet Numerical Weather Facility's (FN WF) operationally produced smoothed isobaric height fields, called SR. The tropical perturbations are steered in 1-hr. time steps up to 72 hr., using winds derived from the SR analysis dated closest to warning time. SR 500 mb. in the Pacific and SR 700 mb. in the Atlantic gave the most accurate forecasts on tests of 10 northwest Pacific typhoons and all five 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-steering prediction is objectively modified to adjust for bias (i.e., deficiency in both zonal and meridional motion) by utilizing errors made in the most recent. 12- and 24-hr. numerical-steering forecasts. Several modes of adjustment are employed; the most recent 12- (12- and 24-) hr. numerical-steering bias yields the most accurate correction of subsequent Atlantic (Pacific) forecasts out, to periods of 72 hr. The optimal Naval Post-graduate 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 excelled 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 technique, discussion of merits and deficiencies of the research program relative to operational forecasts, and current experiments at FNWF are discussed.

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ROBERT J. RENARD and LEO C. CLARKE

Abstract

A major area of weather analysis still requiring manual subjective determination is that of locating fronts. The experiments reported on here concern an attempt to incorporate objective frontal analysis into the operational computer routines of the U.S. Navy Fleet Numerical Weather Facility, Monterey, Calif.

The synoptically-important numerically-derived frontal zone is regarded as a hyperbaroclinic region whose boundaries may be defined as quasi first-order thermal and moisture discontinuities; the boundaries are located through use of suitably defined second derivatives of various potential temperature parameters. Application is made to 850-mb. and surface (or 1000-mb.) frontal analyses on a hemispheric basis. The analyses for 0000 and 1200 gmt January 1, 1965 are selected to exemplify results of the most promising of the experiments.

Verification against hand-derived frontal analyses, difficulties with the existing scheme, and proposed modifications to the continuing program are discussed.

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Leo C. Clarke and Robert J. Renard

Abstract

Essentials of the U. S. Navy's numerical frontal analysis model, employing special second derivatives of the potential temperature field, are reviewed. The scheme is extended to include frontal-prognosis problems, visual presentation of numerical fronts, and application of the new approach to other areas of interest in meteorology and oceanography. The difficulties of accurately depicting baroclinicity for surface frontal analysis are also discussed.

A limited evaluation of the surface, 1000- and 850-mb numerical frontal analyses relative to manually analyzed fronts for six synoptic times in February 1965 over the western part of the Northern Hemisphere is presented. Average departures of numerical frontal positions from U. S. Weather Bureau analyses are about 100 n mi and are less than 50 per cent of the mesh length used by the Fleet Numerical Weather Facility. Merits and deficiencies at each of these three levels are enumerated and discussed.

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Robert J. Renard and William H. Levings III

Abstract

Renard recently reported (Monthly Weather Review, July 1968) on the development of a numerical scheme for predicting the motion of tropical storms for periods up to three days. An extension of the forecast scheme, as presented here, may be described as a two-step process. First, numerical geostrophic steering of the cyclone center is accomplished using Fleet Numerical Weather Central's analyses and prognoses of smoothed isobaric height fields, called SR fields. Next, a statistical correction for vector bias in the numerical steering computation is used selectively in an attempt to enhance the accuracy of the forecast track of the storm. The bias modification is dependent solely on the peculiarities of recent history 12- and 24-hr forecasts in relation to the storm's actual trajectory. Forecasts for intervals up to 72 hr, generated from 1967 Atlantic operational storm positions, are compared to results from the previous experimental forecasts for 1965 using best-track positions of Atlantic storms.Results indicate that the numerical scheme shows skill in relation to the official forecast accuracy for both 1965 and 1967, as documented by Fleet Weather Facility, Jacksonville, Fla. In 1967, the average relative improvement over official forecasts, using 700-mb prognostic SR fields for steering, ranges from 52% for 12-hr forecasts (90 cases) to 9% for the 48-hr estimates (129 cases).The paper includes a discussion of various forecast modes and selective modification schemes as well as stratification of error statistics by geographical area, storm track and stage.

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Michael C. Koziara, Robert J. Renard, and William J. Thompson

Abstract

A model output statistics (MOS) scheme, using a stepwise-selection multiple linear regression approach, is used to estimate marine fog probability at 24 h intervals from 0 to 48 h, for the North Pacific Ocean (30–60°N) summer season. The predictand is uniquely determined by present and past weather, visibility and low-level cloud information in the primary synoptic report. Available predictors include 158 Fleet Numerical Oceanography Center model output parameters as well as monthly climatological fog frequencies. The regression equations, containing at most seven terms, are derived from 0000 GMT synoptic ship report data in the months of July 1976, 1977 and 1979. Evaporative heat flux and fog frequency are among the first four parameters selected for each equation. Reliability and sharpness diagrams are presented to identify specific biases in estimating fog probability. Categorical (percentage correct, threat and Heidke skill) and probabilistic (Brier) scoring methods are used to establish the credibility level of the MOS scheme in relation to climatology and FNOC's advective-fog probability program. The MOS scheme outperforms its competitors with score ranges for an independent August 1979 data set as follows: Heidke-skill, 0.47 to 0.40; threat; 0.45 to 0.42; Brier, 0.27 to 0.34; percentage correct, 78% to 70%.

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Chester W. Newton, Roy Lee, Edwin B. Fawcett, William T. Chapman, Donald E. Martin, Frederick F. Sanders, Robert J. Renard, Robert D. Fletcher, and Maurice E. Pautz
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