Linking Surfzone to the Inner-shelf: Parameterzing Breaking-Wave eddy forcing and effects of transient rip currents
Abstract
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., PI developed 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). Waveaveraged 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. A key element in developing these parameterizations is understanding the space-time statistics (e.g., alongshore wavenumber spectra) of eddies (vorticity) and breaking-wave eddy forcing. PI preliminary results with funwaveC simulations highlight the role of wave directional spread in setting the magnitude and alongshore length-scales of surfzone eddies. To quantify these effects, funwaveC simulations are required over a wide range of wave conditions and bathymetries. 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. Specifically, we will: Develop a SMP parallelized version of funwaveC increasing the model’s computational efficiency and allowing many more simulations to be performed in a particular period of time. Conduct 500 funwaveC simulations across a wide variety of wave conditions and bathymetries. Quantify the space-time structure and statistics of the breaking-wave surfzone eddy generation mechanism from the funwaveC simulations and develop parameterizations for it. This will lead to the development of a COAWST-ROMS subroutine enabling proper surfzone eddy generation. We will subsequently test this wave-averaged model parameterization to ensure that transient rip currents are correctly generated and evolving. Develop tested parameterizations for the the effects of 50–100 m scale rip current variability on larger scales for coastal models with coarser resolution. The project results will be improved models and predictive capability that will enable accurate simulation of ocean conditions across the continental shelf.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512607
Entities
People
- Falk Feddersen
Organizations
- Office of Naval Research
- United States Navy
- University of California, San Diego