OCTOBER 2010 N C E P N O T E S 1543 NCEP NOTES Performance of NCEP Regional Wave Models in Predicting Peak Sea States during the 2005 North Atlantic Hurricane Season* YUNG Y. CHAO AND HENDRIK L. TOLMAN NOAA/NCEP/Environmental Modeling Center/Marine Modeling and Analysis Branch, Camp Springs, Maryland (Manuscript received 4 June 2009, in final form 16 April 2010) 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. 1. Introduction and production). Extensive measurements of wind and wave conditions made by the National Data Buoy Center The Atlantic hurricane season of 2005 was extraor- (NDBC) provide an excellent opportunity to validate the dinary not only for its early beginning and late ending National Centers for Environmental Prediction (NCEP) (May–December) but also for the number and the in- operational regional wave models. tensity of tropical cyclones. According to the National There are two operational regional wave models that Climatic Data Center (NCDC 2006), there was a record forecast sea states over the western North Atlantic of 27 named tropical cyclones, of which 15 were hurri- Ocean domain at NCEP. These are the Western North canes. Among these storms, seven were major hurri- Atlantic wave model (WNA) and the North Atlantic canes of category 3 or higher (i.e., hurricanes Dennis, Hurricane wave model (NAH) (Chao et al. 2003a,b, Emily, Katrina, Maria, Rita, Wilma, and Beta). Four of 2005). They are part of the National Oceanic and At- them reached category 5 (Emily, Katrina, Rita, and mosphere Administration’s global wave forecasting suite, Wilma), in which Hurricane Katrina was the most intense NOAA WAVEWATCH III (NWW3; Tolman 2002a; and destructive land-falling hurricane on record for the Tolman et al. 2002). The performance of the forecast Atlantic basin. Many of these tropical cyclones have guidance produced by the WNA and NAH models for sea created high waves disastrous to the coastal areas and states generated by Hurricane Isabel has been reviewed offshore marine activities (in particular, oil exploration by, for instance, Tolman et al. (2005). The main purpose of the present study is to assess the accuracy of these two * Marine Modeling and Analysis Branch Contribution Number 283. wave models regarding the maximum significant wave height, the associated spectral peak wave period, and the Corresponding author address: Yung Y. Chao, NCEP/EMC/ time of occurrence for each storm event at a given buoy MMAB, 5200 Auth Rd., Camp Springs, MD 20746. location. The model results used here are taken from E-mail: [email protected] the monthly hindcast data produced by NCEP. They DOI: 10.1175/2010WAF2222309.1 Unauthenticated | Downloaded 10/03/21 07:14 PM UTC 1544 WEATHER AND FORECASTING VOLUME 25 FIG. 1. Locations of NDBC buoys used in the model validation. represent operational hindcast model data consistent http://www.emc.ncep.noaa.gov/modelinfo/). The lowest with the operational real-time products of NCEP and do atmospheric level is at a pressure of 997.3 hPa. Since the not include any additional tuning or model modifications. GFDL hurricane prediction model became operational at NCEP in 1995, it has undergone substantial modifi- cations and improvements (Bender et al. 2007). The 2. Models and data 2005 GFDL model has a movable three-nest grid con- Wave model data are generated by two operational figuration. The horizontal resolution of the outermost regional wave models (WNA and NAH). These two nest is ½8, covering an area of 7583758 in latitude– models have identical model physics, spatial resolutions, longitude. The size of the inner finer mesh is 1/68, cov- and domains. The domain covers an area of 08–508N, ering an area of 1183118 in latitude–longitude. The and 308–988W, involving the North Atlantic basin, the finest center core mesh size is 1/128, covering an area of Gulf of Mexico and the Caribbean Sea. The grid reso- 58358 aligned to a single storm center. The number of lution is 0.25830.258 in latitude and longitude. Both vertical levels in the GFDL model is 42. The lowest models obtain boundary data from NCEP’s global wave sigma level is 996 hPa. model, which has a resolution of 1.00831.258 in latitude The 2005 GFS model provides forecast wind fields at and longitude. The model physics consist of the default 3-h intervals for the first 180 h and then at lower spatial model settings of WAVEWATCH III version 2.22, as and temporal resolutions for up to 16 days for each op- described in detail in Tolman (2002b). The difference erational cycle run. The GFDL model, on the other between the two models lies in their input winds. The hand, provides forecast wind fields hourly up to 126 h WNA model is driven solely with wind obtained from only. To blend with GFDL wind fields, hourly GFS wind the NCEP Global Forecast System (GFS) atmospheric fields are generated by interpolation. The required wind model, previously known as the Medium-Range Fore- field to be used in the wave models is at a 10-m height. cast (MRF) or Aviation (AVN) model (Caplan et al. Thus, the lowest sigma-level winds given by the GFS and 1997). For the NAH model, high-resolution wind fields GFDL models are converted to 10-m heights before generated hourly at NCEP by the Geophysical Fluid blending and interpolating to a uniform 0.25830.258 Dynamics Laboratory (GFDL) hurricane model are wave model grid. The blending scheme is described in blended into the GFS wind field. For the 2005 hurricane detail in Chao et al. (2005). The wave models opera- season, the GFS model’s horizontal resolution is T382, tionally run four cycles per day. Each cycle generates approximately 30 km, and the vertical resolution is 64 a 6-h hindcast that precedes the actually forecasts. The layers (see the history of upgrades to GFS online at forecasts extend up to 180 and 126 h for the WNA and Unauthenticated | Downloaded 10/03/21 07:14 PM UTC OCTOBER 2010 N C E P N O T E S 1545 FIG. 2. (bottom to top) Monthly time series of the measured (black) and predicted (NAH, red; WNA, green) spectral peak periods, significant wave heights, wind speeds, and wind directions at buoy 41002, September 2005. NAH models, respectively. In this study only hindcast Quality controlled wave data for model validation wave data are used. It should be noted that the term were obtained from the NDBC Web site (http://www. ‘‘hindcast’’ used in this paper has a slightly different ndbc.noaa.gov/historical_data.shtm). Figure 1 shows the connotation from the conventional (engineering) defi- locations of all operational NDBC buoys that provide nition. Wave hindcasts in the WNA model driven ex- measured data used in the present study. The results clusively with GFS winds are generated using 3-hourly of predictions made by the NAH and WNA wave mod- analyses from GFS’s Global Data Assimilation System els on the grid points surrounding these locations are (GDAS; see e.g., Caplan et al. 1997) for a 6-h period interpolated to these locations for validation. In the pres- preceding the current cycle’s UTC time stamp and are ent study, hourly data obtained from buoy measure- used to provide initial conditions for the wave model real- ments and model output, including the wind speed at time forecast. Unlike the GFS, the GFDL model does not 10 m above the mean sea level, the wind direction, the include a data assimilation system for the initialization of significant wave height, and the spectral peak wave pe- the model forecast. Thus, the NAH model hindcasts are riod, are used. The spectral peak wave period is the wave generated using the GFS analysis winds blended with period that corresponds to the frequency bin of maxi- GFDL forecast winds for the 0–4-h range from the pre- mum wave energy in the wave spectrum. In addition, vious cycle (26to22-h range in the current cycle). Wind the significant wave steepness fields are calculated from inputforNAHatthe21-h time of the current cycle is the NAH model for the significant wave heights greater obtained by interpolating the 22-h winds with the blended than 2 m. The significant wave steepness is defined here GFS–GFDL 0-h nowcast. Although this may seemingly as the ratio of the significant wave height to the wave- lead to lower quality winds being used for the NAH model length associated with the spectral peak wave period.