Radiometric measurements of the microwave emissivity of foam

Abstract

Abstract-Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45 , 45 , left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30 to 60 . Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45 , 45 , left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.

Effective medium approximation model of sea foam layer microwave emissivity of a vertical profile

“…Moreover, in Figure 6 we have calculated the emissivity of the foam layer with a foam thickness of 2.8 cm and compared it with the experimental data of E.-B. Wei Rose et al (2002) for frequencies 10.8 and 36.5 GHz, obtaining good agreement. With the above investigations, it is concluded that the uniform medium method of averaged effective permittivity is feasible to deal with the graded foam layer of vertical profile.…”

“…Microwave emissivity of a foam layer has been discussed using wind speed, foam structure, and the effective medium approximation (EMA) model (Droppleman 1970;Militskii et al 1978;Wilheit 1979). Recently, the emissivities of a foam layer were measured at 10.8, 36.5, and 1.4 GHz for different foam layer thicknesses, sea surface temperatures (SSTs), and sea surface salinities (SSSs), respectively (Rose et al 2002;Camps et al 2005). In addition, Williams (1971) conducted an experiment to discuss the effect of foam layer thickness on emissivity.…”

Application of effective medium approximation theory to ocean remote sensing under wave breaking

“…If the sea surface is completely covered by foams, in Figure 3, we have compared our results with Rose's experimental data. In this figure, the SSE changes with the incident angle at both frequencies 10.8 GHz and 36.5 GHz, where the solid line and dotted line denote the results of vertical and horizontal polarizations, respectively, and the circle and the cross are Rose's experimental values for horizontal and vertical polarization directions, respectively [24] . Clearly, comparing our results with experimental data, a good agreement is obtained.…”

“…When the value of wind speed is larger than 7 m/s, there exists a whitecap layer produced by broken waves, where the whitecaps layer consists of the bubbles and seawater droplets. For the temperature 292 K and the salinity 10 of seawater, Rose et al [24] obtained a set of SSE data by experimental measurement for foam layer thickness 2.8 cm and at microwave frequencies 10.8 GHz and 36.5 GHz, where the seawater surface is completely covered by the foam layer. In this case, they observed that the value of SSE is larger than 0.9 and will be a constant if the incident angle changes from 30° to 60° at the vertical polarization direction.…”

Passive polarimetric remote sensing of the ocean surface during the Rough Evaporation Duct experiment (RED 2001)

“…Wind roughening of the ocean in form of small scale (capillary) waves, large scale (gravity) waves, and whitecaps (with resulting foam) allow such instruments to remotely sense the wind vector. However, near-surface radiometric observations are still very much needed to improve the quantitative knowledge of the effects of changing surface conditions, including foam and roughness, on microwave emissivity [1].…”