An Operational Coastal Wave Forecasting Model for New Jersey and Long Island Waters

Transcription

1 An Operational Coastal Wave Forecasting Model for New Jersey and Long Island Waters By Nickitas Georgas, Alan F. Blumberg, Ph.D., and Thomas O. Herrington, Ph.D. The Center for Maritime Systems Stevens Institute of Technology Castle Point on Hudson, Hoboken, NJ Page 30 Shore & Beach Vol. 75, No. 2 Spring 2007

2 ABSTRACT A high-resolution (approximately 1 km) wave forecasting model has been developed for the coastal waters of New York and New Jersey through a two-year National Weather-Service (NWS)-funded COMET project and through the collaboration of researchers from the Stevens Institute of Technology (Stevens), in Hoboken, NJ, and NWS marine forecasters from the Mount Holly, NJ, forecast office. The wave model has been continuously operational since 15 May 2006, each day producing 48-hour graphical forecasts for significant wave height, wave direction, and wave period. Marine forecasting products are available to Mount Holly NWS forecasters, as well as to the general public, through the New York Harbor Observation and Prediction System (NYHOPS) Web site designed and maintained by Stevens. ADDITIONAL KEYWORDS: Marine meteorology forecasts, Operational Coastal Oceanography. Paper submitted and accepted 1 August 2006 for the 2006 ASBPA Educational Award. The main goal of this study was to develop an operational wave forecasting system that would be used to disseminate high-resolution forecasts of the wave environment along the shore of New York and New Jersey by means of the existing New York Harbor Observation and Prediction System (NYHOPS) Web site created and maintained by the Stevens Institute of Technology (Stevens). The primary need for this system was that the current wave forecast models available from the National Weather Service (NWS) resolve wave fields at a resolution of 25 km, limiting their use in terms of understanding the complex coastal environment and the factors most important to wave propagation across the shelf waters offshore of New York and New Jersey. Another purpose for this study was to provide a testing environment for new, increasingly sophisticated, wave models. The new, high-resolution (approximately 1 km) forecasting system was successfully designed through collaborative work by NWS forecasters and Stevens engineers. WAVE FORECAST SYSTEM DESCRIPTION The NYHOPS Coastal Wave Model (NYHOPS-CWM, Figure 1) is based on the National Oceanic & Atmospheric Figure 1 (facing page). Map shows the NYHOPS Coastal Wave Modeling system: Bathymetry (m below Mean Sea Level), resolution (approximately 1 km), Surface (ETA and WRF wind) and Open Ocean (WNA WaveWatch III) boundary forcing sources, and observation network stations. Figure 2. Schematic shows the NYHOPS-CWM framework: 48-hour forecasts are initialized daily with initial conditions (IC) taken from the previous day hindcast. Model forcing is provided by winds and waves forecasts created by mesoscale NOAA and NWS models, acquired through the World Wide Web (WWW), and cached locally on Stevens computers to avoid service interruptions. Model forecast output is presently made available daily on the NYHOPS Web site. Shore & Beach Vol. 75, No. 2 Spring 2007 Page 31

3 Marine Meteorology Wind Field (m/s) 3 September :00-01:00 Marine Meteorology Composite (m) 3 September :00-01:00 Figure 3. Maps show (A) wind vectors and (B) predicted waves, posted for 3 September :00-01:00 EDT on the NYHOPS-CWM Web site, during (Tropical) Storm Ernesto. Map (B) is a composite representation of the hourlyaveraged wave field predicted by NYHOPS-CWM: The scale bar depicts significant wave height ranging from well over 10 ft (>3m) to the north to less than a foot (<0.5m) to the south; line contours show wave period ranging from seven seconds offshore to four seconds nearshore; arrows show mean wave direction. Administration (NOAA) Great Lakes Environmental Research Laboratory model (GLERL model, Donelan 1977; Schwab et al. 1984), a two-dimensional, parametric, dynamic model of windwave growth, propagation, and decay, modified to accept open boundary forcing through specification of significant wave height at the oceanic boundary. The modified GLERL model has been incorporated into the three-dimensional, curvilinear, Princeton Ocean Model (POM) framework of Blumberg and Mellor (1987), in its shallow water derivative model form, ECOMSED (Estuarine and Coastal Ocean Model with Sediment Transport HydroQual Inc. 2001). Wind forcing within the NYHOPS- CWM domain (Figure 1) is currently provided from forecasted wind fields derived from the NOAA/NWS/National Centers for Environmental Prediction (NCEP) Weather Research and Forecasting North-American Mesoscale Model (WRF-NMM), optimally interpolated to the NYHOPS-CWM high-resolution grid. Prior to 20 June 2006, when the NCEP WRF-NMM became operational, wind forcing was provided by NOAA/ NWS/NCEP s North American 12 km Eta model, on the same grid. Open ocean boundary forcing to NYHOPS-CWM is derived through optimal interpolation of forecasted wave conditions provided from the NOAA Western North Atlantic (WNA) WaveWatch III (WWIII) model at 15-min resolution. The daily prediction system is scheduled to simulate 72 hours 24 hours in the past (the hindcast mode) and then 48 hours into the future (the forecast mode). A schematic of the flow of data in the prediction system is shown in Figure 2. The hindcast part of the cycle uses observed (not forecasted) forcing functions and a file is written out that forms the basis (initial conditions) for the next cycle. The interpolated surface wind input and the 48-hour wave model forecast output are used to produce high-resolution hourly forecast plots of surface wind, significant wave height, peak wave period, mean wave direction, and a combination thereof, which become viewable online at maritimeforecast (choosing Wave Forecast on the upper right hand corner of that Web site), daily, around 06:00 EST Page 32 Shore & Beach Vol. 75, No. 2 Spring 2007

4 Figure 4. Figure shows hourly time series comparisons of (A) significant wave height, (B) significant wave period, and (C) wind data collected at the offshore NOAA Buoy 44014, versus NYHOPS-CWM predictions for the same location. Model predictions are a mosaic of daily 48-hour forecast simulations from 2 August 2006 to 13 September Gray areas show days when the forecast ran without WNA wave boundary information, due to unavailability of WNA wave data. Note the influences of Tropical Storm Ernesto (around -12 days) and Hurricane Florence (around 0 days). (07:00 EDT). Figure 3 shows a screen capture of such a composite plot forecasted for 3 September :30 EDT, after the passing of Tropical Storm Ernesto, as well as the interpolated wind field at that time. The wave model output can also be visually compared to actual data taken from four offshore (NOAA) and two nearshore (Stevensmaintained) stations (locations shown in Figure 1), continuously transmitted to Stevens through the NYHOPS wireless network (in the abovementioned Web site, select one of the six wave stations shown on Figure 1 by scrolling to the bottom of the Choose Time Series dropdown list). WAVE MODEL RESULTS An example of time series data, collected at NOAA Buoy and the coastal station at Avalon, NJ (Figure 1), are provided in Figures 4 and 5, as compared to a mosaic of NYHOPS-CWM forecasts for the period of 2 August 2006 to 15 September 2006; forecast day zero in these graphs is 13 September As can be seen in Figures 4 and 5, the model is able to simulate local wind waves surprisingly well. The inclusion of the open boundary condition to the GLERL numerical scheme increased the capacity of the model to simulate wave conditions, as seen for example in Figure 5. During forecast day -19 (25 August 2006) the unavailability of open ocean boundary wave information from Shore & Beach Vol. 75, No. 2 Spring 2007 Page 33

5 Figure 5. Figure shows hourly time series comparison of (A) significant wave height and (B) wind data collected at the coastal station in Avalon, NJ, versus NYHOPS-CWM predictions for the same location. Model predictions are a mosaic of daily 48-hour forecast simulations from 2 August 2006 to 13 September Gray areas show days when the forecast ran without WNA wave boundary information, due to unavailability of WNA wave data. the NOAA WNA model, significantly reduced the capability of NYHOPS- CWM to forecast wave conditions. The model does not do as well when the measured winds are weak below approximately 5 knots (2.5 m/s). An example can be seen in Figures 4 and 5 during forecast day -30 (14 August 2006). Under these circumstances, the wave field tends to become swell-dominated as manifested by an increase in the significant wave period (decrease in wave frequency). During these low wind low wave height conditions, the model still appears to have some limitations in capturing the dynamics of swell generated outside the model domain. Figure 6 shows the correlation of significant wave heights to forecasted ones by NYHOPS-CWM, at the six station locations shown in Figure 1, for the 44- day period starting 2 August 2006 and ending 15 September The correlation coefficients range from 0.72 at NOAA Buoy to 0.88 at NOAA Buoys and It is important to note that the day for which WNA data were unavailable is included in these correlations, to provide a measure of the total skill of the wave forecasting system, and not of the model itself. As mentioned earlier, were WNA data actually available for all days, the system s predicting capability would have been even higher. CONCLUSIONS AND FUTURE RESEARCH The high resolution wave model described in this work has been continuously operational and available to the general public through the NYHOPS Web site since 15 May The wave forecasts created are of high quality, especially during wind-wave-dominated periods, with some loss of predictive capability observed during swell-dominated days, or when WNA data are not available from NOAA at the open boundary. One of the benefits of having an operational wave forecasting system is that it can provide a testing ground for the next generation of wave models. Research is ongoing in ways to improve the forecast of swell as well as shallow water wave propagation, through testing of a new wave model by Mellor and Donelan (Mellor 2003). ACKNOWLEDGEMENTS The authors wish to acknowledge the significant contributions to this research by the following: Dr. Jon K. Miller and Dr. Michael S. Bruno from Stevens; Dr. Shejun Fan, formerly at Stevens, now at Fugro GEOS; Al Cope from the National Weather Service Mount Holly, NJ, office; and Genevieve Boehm from the New Jersey Department of Transportation. This effort was funded by NWS/ COMET grant S The principal author received the 2006 ASBPA Educational Award for this work, and is grateful to the association for the honor. Page 34 Shore & Beach Vol. 75, No. 2 Spring 2007

6 REFERENCES Blumberg, A.F. and G.L. Mellor A Description of a Three-Dimensional Coastal Ocean Circulation Model, Three-Dimensional Coastal Ocean Models, American Geophysical Union, N. Heaps, Ed., Donelan, M.A., A Simple Numerical Model for Wave and Wind Stress Application. Rept., National Water Research Institute, Burlington, Ontario, Canada, 28 pp. HydroQual Inc A primer for ECOMSED, Version 1.3 users manual. HydroQual Inc., One Lethbridge Plaza, Mahwah, NJ, 196 pp. Mellor, G.L., The three dimensional, current and surface wave equations. J. Phys. Oceanography, 33, Schwab, D.J., M.A. Donelan, J.R. Bennett, and P.C. Liu Application of a simple numerical wave prediction model to Lake Erie. J. Geophys. Res., 89, Figure 6. Forecasted (x axis) versus measured (y axis) significant wave heights (meters) at the six stations shown on Figure 1 for the period 2 August 2006 to 13 September Correlation coefficients are listed in parentheses next to the name of each gage. NOTE: (*) Limited availability of data: These NOAA buoys were not operational for a few days within this period. Shore & Beach Vol. 75, No. 2 Spring 2007 Page 35