Estimating tropical cyclone wind radii using an empirical inland wind decay model
The Tropical Prediction Center/OAR (TPC/NHC) is required to issue forecasts of the radii of 34, 50, and 64 kt winds for tropical cyclones every 6 hours as part of their forecast/advisory package. However, little objective guidance exists for such forecasts, particularly for landfalling storms. Since several recent hurricanes have produced significant damage and loss of life inland due to high winds (e.g. Andrew (1992), Opal (1995), Isabel (2003), Charlie (2004), Frances (2004), Jeanne (2004)) the need for such guidance seems quite apparent. Consequently an empirical decay model (Kaplan and DeMaria 1995, 2001) will be employed to provide estimates of the overland wind radii in the TPC/NHC forecast/advisory for both the Atlantic and Eastern Pacific basins. With financial support from the Joint Hurricane Testbed (JHT), estimates of the 34,50, and 64 kt wind radii of landfalling tropical cyclones will be provided to TPC/NHC in real-time. A gridded field depicting the maximum sustained wind along the track of the cyclone will also be provided if desired by TPC/NHC.
The decay model performance will be evaluated for landfalling hurricanes for which high quality landfall datasets exist. Figure 1 shows an example of a wind swath for Hurricane Isabel (2003). The model did a fairly good job of estimating the maximum overland winds although it tended to underestimate the wind to the north of the storm. The stronger than expected winds observed north of Isabel are likely due to the existence of a strong high-pressure system anchored to the north since the empirical decay model cannot account for the enhanced pressure gradient that would result from such a scenario. Nevertheless when the model was run using the last NHC advisory prior to landfall the average absolute error between the model predicted maximum wind and the maximum wind observed at the 80 locations shown in Fig. 1 was 7.3 kt with a bias of -2.7 kt. Furthermore, when the model was run using the best track intensity and track at landfall the average absolute error was 7.8 kt and the bias was 0.4 kt. As part of the model validation process, sensitivity tests will also be performed to determine if a recently developed version of the inland wind decay model (DeMaria et al. 2005) can be utilized to obtain estimates of the forecast wind radii. Although preliminary results suggest that this new technique provides better estimates of the overall rate of decay of the maximum wind of tropical cyclones for systems that traverse islands and peninsulas, it has not been evaluated for its capability to provide estimates of the azimuthal and radial distribution of the post-landfall wind field that are required to estimate the tropical cyclone wind radii in the TPC/NHC forecast advisory.
DeMaria, M, J.A. Knaff, and J. Kaplan, 2005: On the decay of tropical cyclone winds crossing narrow landmasses. Submitted to J. Appl. Meteor.
Kaplan, J. , and M. DeMaria, 1995: A simple empirical model for predicting the decay of tropical cyclone winds after landfall. J. Appl. Meteor., 34, 2499-2512.
Kaplan, J., and M. DeMaria, 2001: On the decay of tropical cyclone winds after landfall in the New England region. J. Appl. Meteor., 40, 280-286.
Last modified: 5/20/2005
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