Linking Surf Zone to the Inner-Shelf: Parameterizing Breaking-Wave Eddy Forcing and Effects of Transient Rip Currents

Lead PI: Falk Feddersen, Scripps Institution of Oceanography
Start Year: 2015 | Duration: 3 Years
Partners: UCLA, University of Michigan, Stanford University, The University of Southern Mississippi & Office of Naval Research

The U.S. Navy’s goal is to to seamlessly forecast from the deep ocean, across the entire continental shelf to the shoreline. To accomplish this, the relevant surfzone and inner-shelf variability must be accounted for either by inclusion of the appropriate physics or by parameterization. Surfzone breaking waves generate vertical vorticity on the scales of 10-20 m, leading to surfzone eddies and transient rip current ejections on 50–100 m scales. Rip current variability at these length-scales is ubiquitous on the inner-shelf, as seen in both airborne inner-shelf dye and temperature measurements. This results in thermal and material exchange between the surfzone and inner shelf and onto the mid-shelf. Only wave-resolving models (e.g., funwaveC) represent finite-crest length wave breaking that generates surfzone eddies and transient rip currents, but do not include important shelf physics (stratification or vertically sheared currents). Wave- averaged models (e.g., Delft3D, NearCom, COAWST) include appropriate shelf physics but cannot generate surfzone eddies leading to transient rip currents. First results from direct coupling of wave-resolving and wave-averaged models (funwaveC & COAWST) demonstrate the strong effects and feedbacks that transient rip currents have on inner-shelf stratification. However, direct coupling is highly inefficient and parameterizations must be developed to allow small scale eddy generation to be represented in wave-averaged models. The overall project objectives are to develop and test parameterizations for surfzone eddy generation driven by finite-crest wave breaking (on 10–20 m scales) due to a directionally spread wave field. This will allow wave-averaged models that include stratification and vertically sheared currents (such as COAWST or Delft3D) to incorporate transient rip current effects on the inner-shelf. PI Feddersen is the funwaveC developer, which has been validated against field observations in a variety of applications including runup, alongshore currents, and surfzone eddy structure. From the wave-resolving funwaveC results, the surfzone eddy forcing mechanism will be parameterized for use in wave-averaged models.