On improving storm surge forecasting using an adjoint optimal technique

•POM and its adjoint model are used to adjust IC and Cd simultaneously.•Adjusting both IC and Cd can achieve more improvements than adjusting IC/Cd only.•Adjusting IC introduces unrealistic oscillations out of the assimilation window.•The unrealistic oscillations can be suppressed by adjusting both IC and Cd.

A three-dimensional ocean model and its adjoint model are used to simultaneously optimize the initial conditions (IC) and the wind stress drag coefficient (Cd) for improving storm surge forecasting. To demonstrate the effect of this proposed method, a number of identical twin experiments (ITEs) with a prescription of different error sources and two real data assimilation experiments are performed. Results from both the idealized and real data assimilation experiments show that adjusting IC and Cd simultaneously can achieve much more improvements in storm surge forecasting than adjusting IC or Cd only. A diagnosis on the dynamical balance indicates that adjusting IC only may introduce unrealistic oscillations out of the assimilation window, which can be suppressed by the adjustment of the wind stress when simultaneously adjusting IC and Cd. Therefore, it is recommended to simultaneously adjust IC and Cd to improve storm surge forecasting using an adjoint technique.