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Abstract

A wave ensemble forecast system is being developed based on the NOAA WAVEWATCH III (NWW3) two nesting multigrid model over Taiwan area. The ensemble system consisted of 20 ensemble members and was set with spatial resolutions of 0.25° and 0.1°. The wind forcing is coming from the WRFbased ensemble forecast system (WEPS) 10 m wind field of Central Weather Bureau (CWB). The cycle initial condition of each wave ensemble member from the previous run of the same ensemble member is applied to generate a history perturbation of swell. The objectives of this work are to verify the impact of different wind forcing formulas, to find the better composition of ensemble members, and to evaluate the forecast capacity of resulting ensemble forecast system. We proposed the combination of using two built-in wind forcing formulas to form the ensemble members, which can reserve the advantages of different formulas under various wind fields (monsoon and typhoon period), increase the average ensemble spread (SPRD) and decrease the difference between the root mean square error (RMSE) and average SPRD based on the truth value at open seas. With Reliability diagram, Brier Skill Score and Relative Operating Characteristic analyses, the ensemble system has better forecast skill than the operational deterministic forecast and can discriminate between the events and non-events. Nevertheless the overestimation near the coast could be improved by increasing the grid resolution and resolving nearshore wave simulation to reduce RMSE. For the underestimation of SPRD we intend to add perturbation at low frequency swell as initial condition to increase SPRD in the near future.

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