Effects of foam on ocean surface microwave emission inferred from radiometric observations of reproducible breaking waves

Abstract

Abstract-WindSat, the first satellite polarimetric microwave radiometer, and the NPOESS Conical Microwave Imager/Sounder both have as a key objective the retrieval of the ocean surface wind vector from radiometric brightness temperatures. Available observations and models to date show that the wind direction signal is only 1-3 K peak-to-peak at 19 and 37 GHz, much smaller than the wind speed signal. In order to obtain sufficient accuracy for reliable wind direction retrieval, uncertainties in geophysical modeling of the sea surface emission on the order of 0.2 K need to be removed. The surface roughness spectrum has been addressed by many studies, but the azimuthal signature of the microwave emission from breaking waves and foam has not been adequately addressed. Recently, a number of experiments have been conducted to quantify the increase in sea surface microwave emission due to foam. Measurements from the Floating Instrumentation Platform indicated that the increase in ocean surface emission due to breaking waves may depend on the incidence and azimuth angles of observation. The need to quantify this dependence motivated systematic measurement of the microwave emission from reproducible breaking waves as a function of incidence and azimuth angles. A number of empirical parameterizations of whitecap coverage with wind speed were used to estimate the increase in brightness temperatures measured by a satellite microwave radiometer due to wave breaking in the field of view. These results provide the first empirically based parameterization with wind speed of the effect of breaking waves and foam on satellite brightness temperatures at 10.8, 19, and 37 GHz.Index Terms-Fractional area foam coverage, microwave emissivity, microwave radiometer, microwave radiometry, ocean surface, WindSat, wind speed, wind vector.

Roughness and foam signature on SMOS-MIRAS brightness temperatures: A semi-theoretical approach

“…At microwave frequencies, the sea foam has higher surface emissivity than water (Wentz, 1997;Rose et al, 2002) and affects microwave radiometric retrievals of ocean-surface wind vector (Yueh, Wilson, Dinardo, & Li, 1999;Padmanabhan, Reising, Asher, Rose, & Gaiser, 2006) and salinity (Camps et al, 2005).…”

Emissivity Measurements of Foam-Covered Water Surface at L-Band for Low Water Temperatures

“…As an example, at 10.8 GHz and 36.5 GHz, Rose et al [8] obtained a formula of foam emissivity by means of a power-series polynomial of incidence angles for foam thickness 2.8 cm. To investigate effects of breaking waves and the foam layer on sea surface brightness temperatures, Padmanabhan et al [24] conducted an emissivity experiment of the wave-breaking surface at 10.8, 18.7, and 37 GHz. With experiments on various foam shapes and foam-water interfaces, Williams [25] showed that the meniscus of the foam-water interface can contribute a significant fraction of foam emissivity increments at 9.2 GHz.…”

Dielectric and Radiative Properties of Sea Foam at Microwave Frequencies: Conceptual Understanding of Foam Emissivity

“…1453 in [36] λ 0 free-space wavelength (6) Propagation constant (wave number) [37] Scattering parameters (7) Size parameter x [37] a bubble radius…”

“…Vertical variations of the real part of the permittivity in foam thickness invoke changes in the radiation wavelength λ f and propagation constant k f as compared to that in air λ 0 (Equations (5) and (6) in Table 1). Table 2 shows values of λ f in dry and wet foam represented with f a = 98% and 10%, respectively, for all considered frequencies.…”

“…Creating rafts of sea foam on and bubble plumes below the sea surface, breaking waves transfer momentum, heat, and mass across the air-sea interface [1][2][3][4] and alter the optical field of the water column, the surface roughness, and the ambient noise in the ocean [5][6][7]. One (among others) physical quantity that is used to evaluate these air-sea interaction processes is the whitecap fraction, defined as the fraction of the ocean surface covered by foam.…”