Forecasts Of Significant Wave Height Research Articles

Forecast Divergences of a Global Wave Model

Abstract One of the main limitations to current wave data assimilation systems is the lack of an accurate representation of the structure of the background errors. One method that may be used to determine background errors is the “NMC method.” This method examines the forecast divergence component of the background error growth by considering differences between forecasts of different ranges valid at the same time. In this paper, the NMC method is applied to global forecasts of significant wave height (SWH) and surface wind speed (U10). It is found that the isotropic correlation length scale of the SWH forecast divergence (LSWH) has considerable geographical variability, with the longest scales just to the south of the equator in the eastern Pacific Ocean, and the shortest scales at high latitudes. The isotropic correlation length scale of the U10 forecast divergence (LU10) has a similar distribution with a stronger latitudinal dependence. It is found that both LSWH and LU10 increase as the forecast period increases. The increase in LSWH is partly due to LU10 also increasing. Another explanation is that errors in the analysis or the short-range SWH forecast propagate forward in time and disperse and their scale becomes larger. It is shown that the forecast divergence component of the background error is strongly anisotropic with the longest scales perpendicular to the likely direction of propagation of swell. In addition, in regions where the swell propagation is seasonal, the forecast divergence component of the background error shows a similar strong seasonal signal. It is suggested that the results of this study provide a lower bound to the description of the total background error in global wave models.

The impact of inhomogenous background errors on a global wave data assimilation system

One of the main limitations in current wave data assimilation systems is the lack of an accurate representation of the structure of the background errors. In this work, models for the observational error variance, background error variance and background error correlations are developed based on the results of previous studies. These are tested in a global wave data assimilation system and the resulting wave forecasts are verified against independent observations from buoys. Forecasts of significant wave height show substantial improvement over the Australian Bureau of Meteorology's current operational wave forecasting system. However, forecasts of peak period are not similarly improved. The regional impacts of the new assimilation scheme are found to vary on a seasonal basis. Overall, it is shown that the inclusion of a latitudinally dependent background error, and improved specification of the background and observational error variances can reduce the root-mean-square error of 24-hour forecast Significant Wave Height by almost 10%.

Forecasting wave height probabilities with numerical weather prediction models

Operational weather forecasts now allow two week probabilistic forecasts of wave height. This paper discusses methods for generating such forecasts from numerical model output from the European Centre for Medium Range Weather Forecasting Ensemble Prediction System. The ECMWF system produces Monte Carlo style forecasts of wave statistics out to a lead time of 10 days. This ensemble of forecasts is designed to reflect uncertainty in current global weather conditions. In this paper a method for post-processing the ensemble forecasts of wave statistics is described and demonstrated using the significant wave height forecasts for four locations of interest to the offshore industry. The post-processing results in ensemble forecasts which have improved reliability, and which are better predictors of the expected error in the forecast.

Automated Wave Forecasting for the Great Lakes

Abstract The automated Great Lakes wave forecast method of the National Weather Service's Techniques Development Laboratory is described. The method, which is based on specialized wind forecasts for the Great Lakes and the wave forecast method of Bretschneider, has been used operationally since January 1975. The method of determining fetch length and duration time is explained. Forecasts of wave height are made twice daily and extend to 36 h in advance. Comparisons of 24 and 30 h forecasts of significant wave height to observations from Waverider accelerometer wave gages of the Corps of Engineers and NOAA's Great Lakes Environmental Research Laboratory are made for five storm cases during the fall of 1975. The forecasts agree fairly well with the observations.