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Charles J. Neumann

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Lloyd J. Shapiro
and
Charles J. Neumann

Abstract

Statistical models for the prediction of tropical cyclone motion traditionally have been formulated in a coordinate system oriented with respect to zonal and meridional directions. An investigation is made here into the forecast error reducing potential of a grid system reoriented with respect to initial storm heading. The developmental data comprise Atlantic forecast situations from 1965 through 1980 on all storms initially north of about 25°N. Reorientation of the coordinate system reduces the total variance in 24 h storm motion by 40%, projects most of the motion onto one (along-track) component of displacement, and makes the components nearly independent of each other. For 48 and 72 h displacements, however, these advantageous effects are substantially diminished or eliminated.

Synoptic predictors derived from current deep-layer mean heights on a grid of 1700 km radius are used to forecast storm displacements. For the developmental data, grid reorientation lowers the 24 h forecast error by 13%, and reduces the slow speed bias by a factor of 2/4. For 24 h forecasts the skill in the prediction of cross-track motion is small. Empirical Orthogonal Function and Principal Estimator Patterns provide insight into the role of reorientation in the reduction of forecast error, and the position of grid-point height predictors selected by a screening technique.

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Keqin Dong
and
Charles J. Neumann

Abstract

Based on 920 cases, the relationship between Atlantic tropical cyclone motion and environmental geostrophic flows at ten levels (from 1000 to 100 mb) has been calculated and analyzed. For the average situation, it is shown that the steering relationship is considerably different between higher and lower tropospheric levels, and between easterlies and westerlies. Also, there are some differences in an optimal statistical steering function between different storm developmental states. The results of a further correlation and regression analysis of these same data show that the height of the optimum single steering level for hurricanes is higher than that for tropical storms.

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Keqin Dong
and
Charles J. Neumann

Abstract

The interaction between spatially proximate (binary) tropical cyclones is such that relative rotation in the counterclockwise sense and decreasing separation distance between the two storm centers can be expected. This is referred to as the Fujiwhara effect. This study analyzes this effect for 43 binary tropical cyclone systems which occurred over the western North Pacific, 1949–78. It is shown that most demonstrated mutual interaction according to Fujiwhara expectations. However, there were notable apparent exceptions.

Further analysis of these exceptional cases shows that environmental currents in which the storms were embedded had a significant effect on relative motion and masked the Fujiwhara effect. Additionally, it was found that storms exhibiting behavior most in accordance with Fujiwhara expectations were located in or near the Intertropical Convergence Zone. The main conclusion of the study, in confirmation of earlier studies, is that forces relative to environmental steering must be determined and filtered before one can determine forces attributable to the Fujiwhara effect alone.

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Charles J. Neumann
and
Joseph M. Pelissier

Abstract

This study provides an operational evaluation of the seven prediction models-five statistical and two dynamical-used at the National Hurricane Center. Following a brief description of the rationale for each model, various performance characteristics, including forecast error, skill. bias. dispersion, timeliness and availability are evaluated.

The conclusion of the study is that none of the models can be singled out as clearly superior or inferior, each having at least one temporal, spatial, economic or utilitarian advantage. In practice, it is difficult to combine these advantages into one all-purpose model. Accordingly. for some time to come, operational guidance will be obtained from a number of different models, both statistical and dynamical. Tropical cyclone forecasters will need to be kept aware of model attributes so that potential conflicts in the guidance can be rationally resolved.

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Charles J. Neumann
and
Miles B. Lawrence

Abstract

Current statistical models for the prediction of tropical cyclone motion use predictors derived from climatology, persistence, and observed geopotential height data. This paper describes an operational experiment conducted during the 1973 and 1974 Atlantic hurricane seasons whereby prognostic 500 mb height data from the National Meteorological Center's primitive equation model were also included as statistical predictors.

Both the “perfect-prog” and a “simulated-model-output-statistics” (SMOS) approach were utilized to introduce the prognostic height data into the statistical prediction equations. Compared to the current “state-of-the-art” of tropical cyclone forecasting, the perfect-prog technique gave relatively poor displacement forecasts for the first half of the 72 h forecast period but excellent forecasts for the latter half. The SMOS method performed well over the entire period but the 72.h displacement error was somewhat greater than that of the perfect-prog equations.

The results of the test are extremely encouraging and suggest that independent predictive information obtained from the numerical prognoses can be objectively used to improve the performance of current statistical tropical cyclone prediction models.

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Charles J. Neumann
and
Elie A. Randrianarison

Abstract

The derivation of a system of regression equations for the prediction of tropical cyclone motion over the Southwest Indian Ocean is described. The equations use the same predictors that are typically used as storm selection criteria by analog models. In this sense, the prediction model simulates an analog forecast. To complete the simulation, a method is described whereby the prediction errors (residuals) of the development data are used to construct equi-probability ellipses similar to those used by analog models. Testing the prediction equations on three years of independent data for the years 1970, 1971 and 1972 indicates a forecast accuracy closely approximating that realized by a similar system of prediction equations in operational use in the Atlantic.

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JOHN R. HOPE
and
CHARLES J. NEUMANN

Abstract

The HURRAN (hurricane analog) technique for selecting analogs for an existing tropical storm or hurricane is described. This fully computerized program examines tracks of all Atlantic tropical storms or hurricanes since 1886, and those that have designated characteristics similar to an existing storm are selected and identified. Positions of storms selected as analogs are determined at 12, 24, 36, 48, and 72 hr after the initial time. Probability ellipses are computed from the resulting arrays and plotted on an x, y (CALCOMP) offline plotter. The program also has the option of computing the probability that the storm center will be located within a fixed distance of a given point at a specific time. Operational use of HURRAN during the 1969 hurricane season, including both its utility and limitations, is described.

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CHARLES J. NEUMANN
and
JOHN R. HOPE

Abstract

The HURRAN (hurricane analog) technique, a fully computerized objective forecast aid making use of past tracks in forecasting hurricane motion, was developed prior to the 1969 hurricane season. Encouraging operational results during the 1969 and 1970 hurricane seasons suggested further evaluation of the technique. To this end, HURRAN computations were made for approximately 1,000 forecast situations. Results are stratified according to initial direction and speed of movement of the sample storms and the number of analogs selected. The utility of the technique is discussed, and the importance of position accuracy at forecast time is demonstrated. Initial indications of the value of the technique are substantiated.

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