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- Author or Editor: Paul H. Dobos x
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Abstract
The time consistency of three tropical cyclone track prediction aids available at the Joint Typhoon Warning Center (JTWC), Guam is examined by comparison of successive cross-track and along-track errors relative to the best track (postseason analysis) positions. The cross-track errors are divided into three groups (terciles) that are referred to as Left, Center or Right categories. Generally, the aids have more time consistency when the error in the previous forecast is in the Right category than in the Left category. A forecast verification within the Center category of 24-b path errors generally is followed by equal chances of Left, Center or Right error categories within 24–48 h. The One-way influence Tropical Cyclone Model (OTCM) provides the most consistent forecasts in time. In general, the Climatology and Persistence (CLIPER) 24-h verification provides little information as to how the subsequent CLIPER forecast wig verify. The wobble about the actual track is defined as the difference in cross-track errors between successive 12-h forecasts. The OTCM has the smallest wobble in the 24-h and 48-h forecasts and the same wobble as JTWC at 72 h. It is concluded that the time consistency of successive forecasts should be another measure of the value of tropical cyclone track aids.
Abstract
The time consistency of three tropical cyclone track prediction aids available at the Joint Typhoon Warning Center (JTWC), Guam is examined by comparison of successive cross-track and along-track errors relative to the best track (postseason analysis) positions. The cross-track errors are divided into three groups (terciles) that are referred to as Left, Center or Right categories. Generally, the aids have more time consistency when the error in the previous forecast is in the Right category than in the Left category. A forecast verification within the Center category of 24-b path errors generally is followed by equal chances of Left, Center or Right error categories within 24–48 h. The One-way influence Tropical Cyclone Model (OTCM) provides the most consistent forecasts in time. In general, the Climatology and Persistence (CLIPER) 24-h verification provides little information as to how the subsequent CLIPER forecast wig verify. The wobble about the actual track is defined as the difference in cross-track errors between successive 12-h forecasts. The OTCM has the smallest wobble in the 24-h and 48-h forecasts and the same wobble as JTWC at 72 h. It is concluded that the time consistency of successive forecasts should be another measure of the value of tropical cyclone track aids.
Abstract
The ability of three objective tropical cyclone track prediction aids and of the official forecast to indicate that recurvature will occur within 72 h is evaluated. Recurvature is defined as the change of direction from northwest through north to northeast. For the recurvers, the timing of recurvature is assessed within 12-h categories. The evaluation of a homogeneous set of 366 western North Pacific forecasts from 1979 to 1984 provides a standard of comparison for measuring the relative skill of a new objective recurvature prediction technique in Part II of this study. The one-way influence tropical cyclone model is shown to have the highest skill in predicting recurvature for this sample.
Abstract
The ability of three objective tropical cyclone track prediction aids and of the official forecast to indicate that recurvature will occur within 72 h is evaluated. Recurvature is defined as the change of direction from northwest through north to northeast. For the recurvers, the timing of recurvature is assessed within 12-h categories. The evaluation of a homogeneous set of 366 western North Pacific forecasts from 1979 to 1984 provides a standard of comparison for measuring the relative skill of a new objective recurvature prediction technique in Part II of this study. The one-way influence tropical cyclone model is shown to have the highest skill in predicting recurvature for this sample.
Abstract
The lagged-average technique of combining a set of forecasts verifying at the same time is applied to tropical cyclone track prediction. Only a 3% improvement is achieved when the 24-b one-way tropical cyclone model (OTCM) forecasts are combined with unmodified track predictions verifying at t + 24 h. After statistically modifying the previous OTCM forecasts to take into account more recent information, the mean errors of the 24-h lagged-average forecast are reduced by 15% compared to the original 24-h OTCM.
The departure of the 24-h OTCM forecast from the modified lagged-average forecast appears to be a useful predictor of whether the 24-h forecast errors will be below average, average, or above average. Thus, the lagged-average forecast approach appears to be useful for tropical cyclone track prediction.
Abstract
The lagged-average technique of combining a set of forecasts verifying at the same time is applied to tropical cyclone track prediction. Only a 3% improvement is achieved when the 24-b one-way tropical cyclone model (OTCM) forecasts are combined with unmodified track predictions verifying at t + 24 h. After statistically modifying the previous OTCM forecasts to take into account more recent information, the mean errors of the 24-h lagged-average forecast are reduced by 15% compared to the original 24-h OTCM.
The departure of the 24-h OTCM forecast from the modified lagged-average forecast appears to be a useful predictor of whether the 24-h forecast errors will be below average, average, or above average. Thus, the lagged-average forecast approach appears to be useful for tropical cyclone track prediction.
Abstract
A special set of radar wind profiler observations during the Tropical Cyclone Motion (TCM-90) field experiment is used to relate lower-tropospheric winds to surface sustained winds and gusts on the west coast of Okinawa. Owing to the passage of four typhoons at various separation distances, hourly comparisons are possible for lower-tropospheric wind speeds ranging from 0 to 40 m s−1. Regressions with nonzero intercepts provide more accurate estimates than simple ratios between lower-tropospheric winds and surface sustained winds and gusts. Little difference is found in use of a layer-average wind between 600 and 1800 m compared to regressions for individual levels. Stratification of the data into daytime and nighttime regression equations markedly improves surface sustained wind and gust predictions for wind speeds below 30 m s−1. It is recommended that these daytime and nighttime regression equations he used to estimate surface winds over coastal regions when lower-tropospheric wind observations are available or when a tropical island wind report is available and a lower-tropospheric wind speed estimate is required.
Abstract
A special set of radar wind profiler observations during the Tropical Cyclone Motion (TCM-90) field experiment is used to relate lower-tropospheric winds to surface sustained winds and gusts on the west coast of Okinawa. Owing to the passage of four typhoons at various separation distances, hourly comparisons are possible for lower-tropospheric wind speeds ranging from 0 to 40 m s−1. Regressions with nonzero intercepts provide more accurate estimates than simple ratios between lower-tropospheric winds and surface sustained winds and gusts. Little difference is found in use of a layer-average wind between 600 and 1800 m compared to regressions for individual levels. Stratification of the data into daytime and nighttime regression equations markedly improves surface sustained wind and gust predictions for wind speeds below 30 m s−1. It is recommended that these daytime and nighttime regression equations he used to estimate surface winds over coastal regions when lower-tropospheric wind observations are available or when a tropical island wind report is available and a lower-tropospheric wind speed estimate is required.
Abstract
Explosive cyclogenesis during the winter of the First Global GARP Experiment (January–February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb h−1 for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, kinematic vertical velocities, and the strength of the low-level baroclinity. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-tropospheric features in producing the stronger vertical motions and more rapid cyclogenesis.
Abstract
Explosive cyclogenesis during the winter of the First Global GARP Experiment (January–February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb h−1 for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, kinematic vertical velocities, and the strength of the low-level baroclinity. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-tropospheric features in producing the stronger vertical motions and more rapid cyclogenesis.
Abstract
An empirical orthogonal function (EOF) representation of relative vorticity is used to forecast recurvature (change in storm heading from west through north to cast of 360°) of western North Pacific tropical cyclones. A pattern recognition approach is adapted in which the synoptic conditions at recurvature time and each 12-h interval up to 96 h prior to recurvature are to be distinguished from the synoptic pattern for straight-mover storms. Synoptic descriptors are defined in terms of the time-dependent principal components of the vorticity fields for the individual maps. A standard discriminant analysis approach using 250-mb vorticity fields correctly identifies recurvers and straight movers in 80% and 66%, respectively, of the 782 cases. For a specific discriminant analysis that is derived to separate recurvers (74% correct) from straight movers (81% correct), the accuracy is higher than for the operational track prediction techniques and the official forecasts considered in Part I of this study. Although the accuracy of the discriminant analysis in identifying the lime to recurvature in 12-h intervals is less than desired for operational use, this new technique has higher accuracy than the techniques evaluated in Part I. Better accuracy can be achieved if the time resolution requirements are relaxed, for example, into three groups (0–24 h, 36–72 h, and greater than 72 h until recurvature).
Abstract
An empirical orthogonal function (EOF) representation of relative vorticity is used to forecast recurvature (change in storm heading from west through north to cast of 360°) of western North Pacific tropical cyclones. A pattern recognition approach is adapted in which the synoptic conditions at recurvature time and each 12-h interval up to 96 h prior to recurvature are to be distinguished from the synoptic pattern for straight-mover storms. Synoptic descriptors are defined in terms of the time-dependent principal components of the vorticity fields for the individual maps. A standard discriminant analysis approach using 250-mb vorticity fields correctly identifies recurvers and straight movers in 80% and 66%, respectively, of the 782 cases. For a specific discriminant analysis that is derived to separate recurvers (74% correct) from straight movers (81% correct), the accuracy is higher than for the operational track prediction techniques and the official forecasts considered in Part I of this study. Although the accuracy of the discriminant analysis in identifying the lime to recurvature in 12-h intervals is less than desired for operational use, this new technique has higher accuracy than the techniques evaluated in Part I. Better accuracy can be achieved if the time resolution requirements are relaxed, for example, into three groups (0–24 h, 36–72 h, and greater than 72 h until recurvature).
