Near-surface Stokes drift in operational wave forecast

Near-surface Stokes drift in operational wave forecast

We apply an operational model for the mean drift of small particles near and at the surface. The drift is a sum of the Stokes drift and the current. The Stokes drift is derived from a wave model and the current is calculated by means of a 3D hydrodynamic model.

In an operational wave model setup (based on WAVEWATCH-III version 7) a parametric tail is supplied as an extension to the prognostic wave spectrum. The parametrisation is based on the modelled spectra level and the first circular moment (the spectral spread) near the highest prognostic frequency, which is typically around 0.5 Hz in operational wave modelling.

New source terms formulations has been introcuced in wave modelling (e.g. WAVEWATCH-III with effect from 2019) that reproduces the spreading and spectral level well compared to many independent observations.

The Stokes drift is calculated at a discrete number of depths down to Z = 2/Ks, where Ks is a wave number scale estimated from the prognostic spectrum. The calculation is integrated in the wave model output.

The application is evaluated in complex weather situations between January 1 and January 6 2020 in the European North West Shelf region. A set of wave model setups are compared with a variation of values of the prognostic spectral cut-off.

It is demonstrated that a the near-surface drift profile is resolved well with an order of ten discrete depths, and the parametric tail extension requires a low computational time.