|Magdalena Dimitrova Anguelova
Air-sea surface fluxes of momentum, heat, and mass serve as boundary conditions of weather, wave, and climate models. Accurate observations and parameterizations of surface fluxes are required for accurate weather forecasting and wave model predictions. Breaking of ocean waves with air entrainment greatly enhances the surface fluxes. Sea foam (whitecaps) on the ocean surface is the most direct expression of such breaking wave conditions. Whitecap fraction, W, quantifies the presence of sea foam and is widely used to parameterize and evaluate the enhancement of the air-sea transfers. The high reflectivity of whitecaps at visible wavelengths has been traditionally used to estimate W from photographic data of the sea state. However, we have demonstrated that whitecap fraction can be obtained successfully at microwave frequencies due to its high emissivity. Using WindSat observations, we provide a wealth of data to study a myriad of air-sea processes.
The research on passive remote sensing of whitecap fraction continues in three areas.
The first area of research is the continuous improvement of the satellite-based whitecap retrievals. Improvements can be obtained in several ways. 1) Improve the radiative transfer models used in the retrieval algorithm, e.g., modeling of the foam emissivity and sea surface roughness, including improvement of the wave spectrum used for roughness modeling. 2) Validate the satellite-based W data with in situ observations. 3) Assess the uncertainty of the satellite-based W retrievals. 4) Expand this capability to coastal zone and polar regions. 5) Combine this microwave capability with observations at IR and visible wavelenghts.
The second area of research is the use of the satellite W data in conjunction with other meteorological and oceanographic (MetOc) data to expand our knowledge on the whitecapping in the ocean. Possible research topics include: 1) Investigate the spatial and temporal variability of W. 2) Develop new parameterizations of W that incorporate the dependence of W on various MetOc quantities, in addition to wind speed, including wave field, currents, sea surface temperature, salinity, and surfactants. 3) Assess the contribution of the whitecaps to the surface albedo in a warming climate. 4) Use W data to compare and/or validate the energy dissipation rate from wave models and field experiments.
The third area of research is to use the satellite W data either directly or through new W parameterizations to investigate air-sea processes on a global scale over long periods. These include: 1) Calculate and analyze sea spray production and its contribution to heat fluxes (thus hurricane intensification) and gas exchange. 2) Investigate the effects of the sea spray on (i) atmospheric constituents via halogen chemistry; (ii) cloud condensation nuclei (CNN) and cloud cover, thus contribution to indirect climate forcing; (iii) aerosol optical depth, thus contribution to the direct climate forcing. All these effects can also be investigated in high latitudes to address the ocean response to climate changes in Arctic and Antarctic.
In all these research areas the application of new technology and new analytical techniques are strongly encouraged.
Anguelova, M. D., and M. H. Bettenhausen, 2019, Whitecap fraction from satellite measurements: Algorithm description, J. Geophys. Res.-Oceans, 124(3), 1827-1857, doi:10.1029/2018JC014630
Albert, M.F.M.A., M. D. Anguelova, A.M.M. Manders, M. Schaap, and G. de Leeuw, 2016, Parametrization of oceanic whitecap fraction based on satellite observations, Atmos. Chem. Phys., 16, 13725-13751, doi:10.5194/acp-16-13725-2016
Anguelova, M. D., and P. A. Hwang, 2016, Using energy dissipation rate to obtain active whitecap fraction, J. Phys. Oceanogr., 46, 461481, doi:10.1175/JPO-D-15-0069.1
Air-sea interaction processes; breaking waves; whitecaps, bubbles and sea spray; whitecap fraction; sea spray production; air-sea heat transfer; air-sea gas exchange; remote sensing of air-sea processes; satellite microwave radiometers; WindSat