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Yung Y. Chao and Hendrik L. Tolman

Abstract

Unprecedented numbers of tropical cyclones occurred in the North Atlantic Ocean and the Gulf of Mexico in 2005. This provides a unique opportunity to evaluate the performance of two operational regional wave forecasting models at the National Centers for Environmental Prediction (NCEP). This study validates model predictions of the tropical cyclone–generated maximum significant wave height, simultaneous spectral peak wave period, and the time of occurrence against available buoy measurements from the National Data Buoy Center (NDBC). The models used are third-generation operational wave models: the Western North Atlantic wave model (WNA) and the North Atlantic Hurricane wave model (NAH). These two models have identical model physics, spatial resolutions, and domains, with the latter model using specialized hurricane wind forcing. Both models provided consistent estimates of the maximum wave height and period, with random errors of typically less than 25%, and timing errors of typically less than 5 h. Compared to these random errors, systematic model biases are negligible, with a typical negative model bias of 5%. It appears that higher wave model resolutions are needed to fully utilize the specialized hurricane wind forcing, and it is shown that present routine wave observations are inadequate to accurately validate hurricane wave models.

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Hendrik L. Tolman, Jose-Henrique G. M. Alves, and Yung Y. Chao

Abstract

The accuracy of the operational wave models at the National Centers for Environmental Prediction (NCEP) for sea states generated by Hurricane Isabel is assessed. The western North Atlantic (WNA) and the North Atlantic hurricane (NAH) wave models are validated using analyzed wind fields, and wave observations from the Jason-1 altimeter and from 15 moored buoys. Both models provided excellent guidance for Isabel in the days preceding landfall of the hurricane along the east coast of the United States. However, the NAH model outperforms the WNA model in the initial stages of Isabel, when she was a category 5 hurricane. The NAH model was also more accurate in providing guidance for the swell systems arriving at the U.S. coast well before landfall of Isabel. Although major model deficiencies can be attributed to shortcomings in the driving wind fields, several areas of potential wave model improvement have been identified.

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Yung Y. Chao, Jose-Henrique G. M. Alves, and Hendrik L. Tolman

Abstract

A new wind–wave prediction model, referred to as the North Atlantic hurricane (NAH) wave model, has been developed at the National Centers for Environmental Prediction (NCEP) to produce forecasts of hurricane-generated waves during the Atlantic hurricane season. A detailed description of this model and a comparison of its performance against the operational western North Atlantic (WNA) wave model during Hurricanes Isidore and Lili, in 2002, are presented. The NAH and WNA models are identical in their physics and numerics. The NAH model uses a wind field obtained by blending data from NCEP’s operational Global Forecast System (GFS) with those from a higher-resolution hurricane prediction model, whereas the WNA wave model uses winds provided exclusively by the GFS. Relative biases of the order of 10% in the prediction of maximum wave heights up to 48 h in advance, indicate that the use of higher-resolution winds in the NAH model provides a successful framework for predicting extreme sea states generated by a hurricane. Consequently, the NAH model has been made operational at NCEP for use during the Atlantic hurricane season.

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L. C. Breaker, L. D. Burroughs, Y. Y. Chao, J. F. Culp, N. L. Guinasso Jr., R. L. Teboulle, and C. R. Wong

Abstract

Hurricane Andrew was a relatively small but intense hurricane that passed through the Bahamas, across the Florida Peninsula, and across the Gulf of Mexico between 23 and 26 August 1992. The characteristics of this hurricane primarily beyond its core are summarized using 1) marine observations from three National Data Buoy Center (NDBC) buoys and three Coastal-Marine Automated Network stations close to the storm track; 2) water levels and storm surge at 15 locations in the Bahamas, around the coast of Florida, and along the northern coast of the Gulf of Mexico; 3) currents, temperatures, and salinities at a depth of 11 m in the northern Gulf; and 4) spatial analyses of sea surface temperature (SST) before and after the passage of Andrew.

Sea level pressure, wind direction, wind speed, wind gust, air temperature, and the surface wave field were strongly influenced at locations generally within 100 km of the hurricane track. Maximum sustained winds of 75 m s−1 occurred just north of the storm track near Miami (Fowey Rocks). Significant wave height increased from 1 to 6.4 m at one NDBC buoy in the Gulf of Mexico (25.9°N, 85.9°N). A record high water level occurred at North Miami Beach. Decreases in water level occurred along the west coast of Florida with a maximum negative surge of −1.2 m at Naples. Increases in water level occurred along the Gulf coast between the Florida panhandle and Louisiana where a storm surge of +1.2 m was observed at Bay Waveland, Mississippi. Current speeds at one shallow water location along the hurricane track in the northern Gulf (28.4°N, 90.5°W) increased from ∼15 to almost 140 cm s−1 at a depth of 11 m during passage of the storm. Finally, SSTs decreased by up to 3°C at various locations along the hurricane track.

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Hendrik L. Tolman, Bhavani Balasubramaniyan, Lawrence D. Burroughs, Dmitry V. Chalikov, Yung Y. Chao, Hsuan S. Chen, and Vera M. Gerald

Abstract

A brief historical overview of numerical wind wave forecast modeling efforts at the National Centers for Environmental Prediction (NCEP) is presented, followed by an in-depth discussion of the new operational National Oceanic and Atmospheric Administration (NOAA) “WAVEWATCH III” (NWW3) wave forecast system. This discussion mainly focuses on a parallel comparison of the new NWW3 system with the previously operational Wave Model (WAM) system, using extensive buoy and European Remote Sensing Satellite-2 (ERS-2) altimeter data. The new system is shown to describe the variability of the wave height more realistically, with similar or smaller random errors and generally better correlation coefficients and regression slopes than WAM. NWW3 outperforms WAM in the Tropics and in the Southern Hemisphere, and they both show fairly similar behavior at northern high latitudes. Dissemination of NWW3 products, and plans for its further development, are briefly discussed.

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