Research For The Microwave Radiation Mechanism Of Foam-covered Sea Surface

Generally, the sea surface microwave brightness temperature and scattering coefficient are the functions of sea surface physical and geometric parameters. The measurement principle for ocean passive remote sensing is based on the response of sea surface brightness temperature to sea surface physical parameters, such as sea surface temperature(SST), salinity(SSS) and wind speed. The active remote sensing measurements utilize the relationship of sea surface backscattering coefficient and sea surface geometric parameters to retrieve these parameters, such as significant wave height, the slope of wave surface and wind speed. However, the sea foam produced by waves breaking under high wind speed can change the dielectric and geometric properties of sea surface, and then influence scattering and emission characteristics of sea surface. This paper investigates the mechanism of sea surface brightness temperature increments induced by sea foam with theory and experiments, aiming to eliminate the brightness temperature influence of foam(whitecap) on theoretical and empirical models, and obtain the sea surface parameters retrieval models with high accuracy.With the effective medium approximation(EMA) theory, the foam layer is regarded as an isotropic dielectric medium, in which spherical air bubbles coated with sea water shell distributes randomly in the air background medium. Subsequently, the Rayleigh method, which exactly considers the influence of interactions between dense particles, is developed to calculate the effective dielectric constant of this kind of composite medium. Furthermore, in order to calculate conveniencely, a simple formula for foam effective dielectric constant at 1.4GHz is derived with Pade approximation method and Rayleigh results. Through the comparison of computed results with Rayleigh method and experimental measured emissivity, we find that the agreement is better at lower frequency. This phenomenon owes to the EMA theory which requires microwave wavelength larger than particle size.To investigate the microwave radiation mechanism of foam layer, we perform emissivity experiments at C-band and L-band(low SST) with a completely foam-covered surface. The data of microwave emissivity and emissivity increments are derived from the measured brightness temperature of foam-covered sea surface. Based on these measured data and cost function method, a microwave emissivity model of foam layer is determined to analyze the influence of foam parameters on microwave emissivity. Experimental and theoretical results indicate that the effects of air volume fraction and foam thickness on foam emissivity are stronger than those of SSS and SST. Besides, we estimate the SSS and SST retrieval error induced by foam layer with measured emissivity increments. When wind speed is 10.0 m/s, the foam coverage fraction on the sea surface is about 1%, the SSS retrieval errors(incidence angle 45?) for SST= 1.5?C and 19?C are about 0.7-1.0 psu and 0.6 psu, respectively; the induced SST retrieval error is about 3?C for SST =22?C. The effect of foam layer on emissivity should be considered on retrieving sea surface parameters.Besides, with the method of simulated data(model results+Gaussian noise) replacing measured data, L band passive and active SSS retrieving model is simulated to analyze the influence of equipment accuracy, SST, SSS, wind field and incidence angle on retrieval results of salinity and wind speed. Statistical results show that: 1) Salinity and wind speed retrieval errors decrease with the increase of incidence angle; 2) The retrieval accuracy at downwind(180?) and upwind(0?) is higher than that at crosswind(90?, 270?); 3) Salinity retrieval error is smaller for high SST, SSS and low wind speed; 4) Wind speed retrieval error is strongly affected by wind speed, the higher the speed is, the bigger the retrieval error will be; 5) the equipment noise standard deviation of brightness temperature and scattering coefficient need be less than 0.2K and 0.1db for monthly average, and less than 0.3K and 0.1d B for quarterly average, so that the salinity retrieval accuracy reaches to 0.2psu.Actually, the real sea surface state is complex and fast-changing under high wind speeds, and the physical parameters of sea surface have interactions. Except foam layer, there are many factors can change dielectric properties and emission characteristics of sea surface, such as surface roughness, surface current and sea-air temperature difference. These complicated influences on remote sensing measurements need to be further studied by combining experiments and remote sensing data.