Emissivity measurements and theoretical model of foam-covered sea surface at C-band

Abstract

To investigate the effects of sea foam (or whitecaps) on the emissivity of sea surface, microwave radiometric experiments of foam-covered sea surface at C-band (6.8 GHz) were conducted by artificial foam during September 2012 at Tangdao bay of Qingdao. The sea surface temperatures (SST) were from 22°C to 26.4°C, and the thickness and the air bubble size of the foam layer were controlled by a compressed air pump and monitored with a video recorder. The emissivities were obtained with the measured brightness temperatures. Furthermore, based on the experimental data, a theoretical model of sea foam emissivity is proposed by the wave approach method of two-layer medium, where the foam complex permittivity is estimated by an effective medium approximation. Experimental results show that the averaged emissivity increments of both horizontal and vertical polarizations are approximately from 0.25 to 0.35 for 1 cm of foam thickness with different SST and air volume fractions (AVF). For a given thickness of the foam layer, although both AVF and SST play important roles in increasing the sea surface emissivity, the influence of AVF on the emissivity is much stronger than that of SST. The brightness temperature increases by about 113K compared to that of the foam-free surface. Moreover, the theoretical model implies that the emissivity is increased during the initial aggregation process of foam, and then fluctuates on increasing the foam thickness up to a larger saturation value. The effect of the meniscus at the foam–water boundary on foam emissivity is also discussed.

Acknowledgement

The authors would like to thank the referee’s advice for discussing the effect of the meniscus transition zone on the emissivity of the foam layer.

Funding

The work was supported by the NSFC [grant number 41276183] and National 863 Project of China [grant number 2009AA09Z102].