A consistent picture of the emission characteristics of sea foam at microwave frequencies has emerged from a survey of published radiometric measurements. The results are summarized, as functions of frequency and angle, by means of simple equations. Available data on the reflection properties of foam are also examined and shown to be qualitatively, but not quantitatively, in agreement with the radiometric results. Presently available evidence indicates that the two primary features of the ocean surface that are responsible for microwave brightness temperature departures from those characteristic of a specular surface are waves and foam. A theoretical treatment of wave effects has been given by Stogryn [1967] and has received partial confirmation in the experimental work of Hollinger [1970, 1971]. Further work on the effect of waves is in progress and will be discussed elsewhere. The significance of foam on the water's surface seems to have first been recognized by Williams [1969] and several subsequent experiments performed by various groups have verified its importance. Although an interesting attempt at a theoretical description of the emissivity of foam was made by Droppleman [1970], it is clear that the complexity of the electromagnetic boundary value problem has precluded the construction of a physically and mathematically convincing theoretical model. Thus, at least for the present, complete reliance must be placed on experimental data in studies relating to the effects of foam.To date, the published data on the microwave radiometric properties of foam have been quite sparse and no attempt seems to have been made to correlate the various measurements that are available. In view of the importance of this problem in connection with the interpretation of many proposed remote sensing experiments, a synthesis of known data is attempted in this work. Figure 1) may be expressed as T •(v, O) = •(v, 0)T,. q-T•'(,, O) (1) where E, (r, 0) is the emissivity in the direction 0, T• is the thermal temperature of the water and foam, and Tp • (r, 0) is the temperature of the reflected sky radiation. To write equation 1, it is assumed that the brightness temperature is measured near the surface so that certain additional atmospheric contributions to Tp (•, 0), which are sometimes of importance, may be ignored. Further, an implicit assumption is made that azimuthal variations in the incident skybrightness temperatures and emissivity can be ignored. This hypothesis seems to be quite reasonable for sea foam since foam generally has a statistically isotropic structure and, for the data to be discussed below, atmospheric conditions also appeared to be isotropic. As is customary in microwave radiometric studies, the two independent polarization directions p will be taken to be horizontal (h) and vertical (v). Peake [1959] has shown that the term T,' (•, 0) in (1) may be expressed by means of a set of bistatic scattering coefficients 7,J (r, 0, 0', •o') (i, j --h or v) in the form if 0)
SOME BASIC RELATIONSHIPS
The brightness temp...
