Abstract
In this study, a method for estimating two-scale roughness influences on the ocean surface emissivity is developed by solving a simplified two-scale ocean emissivity model equation. In this model, scatterings by small-scale roughness are described by the Kirchhoff approximation. For large-scale roughness, the mean local incidence angle (LIA) is introduced to describe slanted surface slope deviation from flat surface. This study focuses on the ocean state under low/moderate wind conditions in order to preclude foam and anisotropic influences within the model. Consequently, a unique pair of two-scale roughness parameters are estimated from the equation using observed ocean emissivities from AMSR2-measured radiances. The results show that the estimated small-scale roughness at 6.925 and 10.65 GHz is linearly correlated with the 10-m height wind speed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$U_{10}$ </tex-math></inline-formula> . As the frequency reaches 36.5 GHz, however, the scatters between small-scale roughness and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$U_{10}$ </tex-math></inline-formula> are increased, which suggests that the Kirchhoff bistatic scattering function is not fully suitable to describe the small-scale roughness at this frequency. The linear relationships between mean LIA and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$U_{10}$ </tex-math></inline-formula> are found with high correlation coefficients. In addition, the estimated mean LIA corresponds well with associated roughness calculated from both observed and modeled ocean wave height spectra. This evidence demonstrates that the proposed large-scale roughness parameterization is physically meaningful and, therefore, the mean LIA has a physical basis in large-scale roughness. In addition, the strong correlations between the roughness parameters and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$U_{10}$ </tex-math></inline-formula> demonstrate the possibility to estimate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$U_{10}$ </tex-math></inline-formula> from the AMSR2 data using intermediate parameters that are physically based on ocean surface characteristics.
Highlights
A FLAT sea surface emits highly polarized microwave radiances which can be theoretically described as a function of incident angle using Fresnel reflection theory with a suitableManuscript received May 4, 2021; revised July 15, 2021; accepted August 15, 2021
Since the main objective of this study is to find the relationship between the two-scale surface roughness parameters (i.e., K and ζLIA) and wind speed, the effects of foam and wind direction are ignored because those effects are small compared with the two-scale roughness effects under low and moderate wind conditions (
Spatial distributions of K s correspond closely to the wind speed distribution at 10 m above the sea surface (U10) from Advanced Microwave Scanning Radiometer 2 (AMSR2) measurements (Fig. 2), which might be expected because the K factor and U10 wind speeds are both obtained from the AMSR2 data
Summary
A FLAT sea surface emits highly polarized microwave radiances which can be theoretically described as a function of incident angle using Fresnel reflection theory with a suitableManuscript received May 4, 2021; revised July 15, 2021; accepted August 15, 2021. One is the largescale roughness by ocean wave swell that causes surface facets to become tilted, resulting in an average local incidence angle (LIA) that differs slightly from the observation angle [3]. Another is the small-scale roughness by gravity–capillary waves atop the large gravity waves which causes nonspecular scattering of microwaves [4], [5]. The relationship between wind direction and directional spectra for both large-scale and gravity–capillary waves provides the capability to measure wind direction from passive microwave emissivity (or radiance) measurements, but the dominant influence of wind on emissivity under low to moderate wind speeds (
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