Global Numerical Wave Model Validation Using ENVISAT ASAR Wave Mode and Radar Altimeter Data

Kurzfassung

In the present study, the German Weather Service (DWD) forecast Global Sea wave Model (GSM) is validated using simultaneous measurements of the Advanced SAR ASAR and Radar Altimeter (RA) -2 from June 2006 to May 2007. The validation is on the one hand to systematically evaluate performance of the pure forecast wave model, and on the other hand to improve the assimilation criteria of satellite observations in the numerical wave model, particularly in high sea state. The ASAR and RA -2 are both mounted on board the ENVISAT platform to provide simultaneous surface wave measurements. RA yields the nadir sea surface measurements while the right-looking SAR and ASAR (ASAR is the Advanced SAR onboard the ENVISAT) measures at a ground distance of 300 km approximately from the RA. The empirical algorithm of CWAVE_ENV, an extension of the CWAVE, is proposed to derive integral wave parameters from the ENVISAT ASAR wave mode data for full sea state. This algorithm does not need any prior information while only using the calibrated ASAR image as an input for deriving integral wave parameters, e.g., SWH and mean wave period. Accuracy of the retrieved SWH is close to in situ buoy measurement with a bias of 0.06 m in deep water. The development of CWAVE_ENV algorithm makes the ASAR sensor be another independent observation of sea surface wave parameters in addition to the RA-2. Using both sensors for measuring sea state parameters brings a significant advantage. It increases spatial sampling over the open ocean and simultaneous observations reduce uncertainties of the measurement from a single instrument. This is of particular importance to evaluate performance of numerical wave models in high sea state, such as in the tropical and extra-tropical cyclones, which often exhibit significant spatial and temporal variations. When both sensors are used, it not only increases the opportunity to capture wind storms, but also is able to investigate the spatial variations of storm wave height as the parallel ground tracks have a ground distance of 300 km. In the present study, the overall performance of the DWD forecast Global Sea wave Model (GSM) is validated using the ASAR and the RA-2 measurements from June 2006 to May 2007. Evaluation of forecast storm wave height during winter season from December 2006 to February 2007 in the North Atlantic is particularly addressed. The annual validation shows that the forecast Significant Wave Height (SWH) of the GSM wave model has a fairly good agreement with both the satellite measurements over the globe with a slight underestimation of 2% to 3%. The monthly analysis shows that in the winter season of the North and South Hemisphere, the underestimation is 7% to 9%. Special emphasis is placed on evaluation of the GSM wave model in high sea state with SWH above 6 m in the North Atlantic during the winter season from December 2006 to February 2007. Both the case study and the statistical analysis show that the GSM wave model underestimate wave height by more than 12% for high sea state.