The emissivity of foam-covered water surface at L-band: theoretical modeling and experimental results from the FROG 2003 field experiment

Abstract

Abstract-Sea surface salinity can be measured by microwave radiometry at L-band (1400-1427 MHz). This frequency is a compromise between sensitivity to the salinity, small atmospheric perturbation, and reasonable pixel resolution. The description of the ocean emission depends on two main factors: 1) the sea water permittivity, which is a function of salinity, temperature, and frequency, and 2) the sea surface state, which depends on the wind-induced wave spectrum, swell, and rain-induced roughness spectrum, and by the foam coverage and its emissivity. This study presents a simplified two-layer emission model for foam-covered water and the results of a controlled experiment to measure the foam emissivity as a function of salinity, foam thickness, incidence angle, and polarization. Experimental results are presented, and then compared to the two-layer foam emission model with the measured foam parameters used as input model parameters. At 37 psu salt water the foam-induced emissivity increase is 0.007 per millimeter of foam thickness (extrapolated to nadir), increasing with increasing incidence angles at vertical polarization, and decreasing with increasing incidence angles at horizontal polarization.

A Numerical Study on Physical Characterizations of Microwave Scattering and Emission From Ocean Foam Layer

“…The permittivities of seawater at five frequencies are listed in Table 1. As reported in [6], the bubble radius in the natural sea foam layer is a random variable and can be described using the Gamma distribution with a mean value of 0.781 mm, and the most probable radius was observed to be 0.595 mm. It follows that the side-length ℓ of Kelvin's structure was set to 0.448 mm.…”

“…The bubbles on the top level of the foam layer are polyhedral with lower water fractions, whereas the bubbles on the bottom of the foam layer tend to be small with higher water fractions and almost buried in seawater [5,6,9]. Fig.…”

“…Furthermore, the volume scattering of the foam layer is relatively weak compared to its absorption; hence, fully coherent interactions and multiple scattering effects within the foam layer should be considered for higher accuracy. As the size of the bubbles relative to wavelength is small [6] and satisfies the Rayleigh approximation condition (kr ≪ 1), the Rayleigh phase matrix [23] that includes range and size-dependent terms in the scattered field is utilized in this study.…”

“…Rose et al [5] conducted the radiometric measurements of microwave emissivity of artificial foam using radiometers operating at 10.8 GHz and 36.5 GHz. Camps et al [6] studied the geometric size distribution of bubbles and the foam emissivities at the L-band by field experiment, and developed a simple model for interpreting the measured data. Chen et al [7] applied Monte Carlo simulations by solving Maxwell's equations for densely packed coated spherical shell bubbles for evaluating the microwave absorption and scattering coefficients of a foam layer, which were then fed into dense medium radiative transfer theory (DMRT) for determining the microwave emissivities at 10.8 GHz and 36.5 GHz.…”

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

“…Salinity affects both λ f (via the salinity dependence of ε f ) and the bubble size a. Seawater with lower salinity (at fixed SST) breaks into fewer but larger bubbles (e.g., [58], Figure 5). Meanwhile, Anguelova ([24], Figure 8) shows that, as salinity decreases, the real part of foam permittivity increases, negligibly so for higher frequencies (18 GHz to 37 GHz) and more noticeably for lower (1.4 GHz to 10 GHz).…”