Several fundamental and unsolved questions should be answered or clarified before new and substantial progress can be made toward a more rigorous and complete solution of the general problem of electromagnetic scattering, and of the related problem of emission, by the ocean surface. Successively discussed are the limitations of the linear surface waves theory, the assumptions implied by and the inadequacies of the method used to obtain the spatial spectrum of the sea surface from the temporal spectrum, and the poor knowledge of the spectrum angular behavior. It is stressed that the sea surface is not uniquely determined by the wind speed and that it should be depicted by a more complete set of internal parameters. Relationships between some usual sea state parameters, such as H1/3, σ, u*a, and U(z) are discussed. On the other hand the available methods for solving the electromagnetic scattering problem are briefly discussed, and it is shown that the assumptions implied by Brown's (1978) two-scale model, based on the boundary perturbation method, are roughly satisfied when applied to the sea surface. The contribution that particular scattering mechanisms could provide to the scattered power is estimated. The azimuthal behavior of the backscattering coefficient is taken as an illustration.

Sea surface description requirements for electromagnetic scattering calculations

Given the prospect of direct assimilation of satellite microwave radiances within numerical weather prediction (NWP) schemes, this study proposes to test a nonlinear inversion for retrieving geophysical parameters from Special Sensor Microwave/Imager (SSM/I) measurements using NWP forecasts as a priori information. The inversion method relies upon an accurate analysis of the radiative transfer and simultaneously retrieves the surface wind speed, the integrated water vapor, and the cloud liquid water. The method is tested in the framework of the well-documented Atlantic Stratocumulus Transition Experiment campaign. The validation approach involves radiosondes from islands, ships, and drifting buoys, cloud liquid water measurements from instruments onboard the U.K. Meteorological Office C-130 aircraft, and the International Satellite Cloud Climatology Project cloud liquid water estimates. The retrieved geophysical parameters are in satisfactory agreement with the available in situ measurements, and the inversion process is shown to be reliable even in the detection of low liquid water contents. The SSM/I inversion scheme is also a strategy for the validation of radiative transfer models, the discrepancy between the simulated and the observed radiances at the end of the inversion process being a powerful tool for testing the radiative transfer in use.

Variational Inversion of the SSM/I Observations during the ASTEX Campaign

[1] Characteristics of spatial and temporal fluctuations of atmospheric absorption and emission at millimeter and centimeter wavelengths are presented Theoretical relations and experimental data are given on structure functions of absorption and brightness temperature at wavelengths of 08 and 135 cm It is shown that in cloudless weather the intensity of fluctuations at these wavelengths is determined mainly by random fluctuations of the humidity field Under cloudy conditions the intensity of fluctuations of microwave atmospheric radiation grows, with stratiform and cumuliform clouds differently affecting the character of the space-time dependency of fluctuations

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002RS002650

Spatial and temporal fluctuations of atmospheric microwave emission

Microwave radiometric observations were made with specially designed microwave radiometers at 6.7 and 18.6 GHz, and the results were compared with those of other investigators, over the frequency range of 1-40 GHz. Dependences of sea surface emission and reflection on wind speed, frequency, incidence angle, and polarization type are discussed in detail, following discussions of the reflective processes of sky radiation and error estimation in the retrieval of mainlobe-averaged brightness temperature. The wind speed sensitivity of brightness temperature, emissivity, and reflectivity is formulated with respect to frequency and incidence angle in each polarization. The brightness temperature, emissivity and reflectivity at arbitrary wind speed are derived employing this formulation. Based on the results obtained it is suggested that the 10-19-GHz band may be optimal for satellite microwave radiometer observations of sea-surface wind. >

Microwave emission and reflection from the wind-roughened sea surface at 6.7 and 18.6 GHz

This work summarizes the main characteristics of atmospheric and hydro-physical parameters of the water surface derived from microwave radiometric data. First, current knowledge based on experimental measurements and model calculations of emissivity of the atmosphere and water surface in the microwave spectrum is presented. Emphasis is placed on remote radio-physical methods that have the peculiarity of being related to atmospheric radio-transparency which is one of the main advantages of the microwave radiometric method compared to optical and infrared methods. A detailed presentation is attempted with specific examples of classification of water surface phenomena using software modules included in the system used for the processing of data of radio-physical experiments by the Cosmos-1500 satellite. In addition, the statistical characteristics of the “spotting” of radio-brightness temperatures obtained for the most informative thresholds are analyzed and it is argued that these characteristics for the Pacific areas can also be used to detect abnormal phenomena on the water surface of the Mediterranean Sea. Finally, it is emphasized that the results obtained from this work make it possible to rapidly evaluate various parameters such as temperature, water surface waves, foam formation areas, etc., providing predictions and allocating irregular areas.

/pdf/remote-monitoring-of-atmospheric-and-hydrophysical-27gx4yt9.pdf

Remote Monitoring of Atmospheric and Hydrophysical Characteristics of the Water Surface Based on Microwave Radiometric Measurements

A discussion of the influence of sea roughness and spatial atmospheric inhomogeneities on the precision of satellite microwave radiometric measurements of the atmospheric parameters is presented. The two-scale model for microwave radiation of the ruffled sea surface without foam and spray is confirmed by experimental measurements. Investigations of the brightness temperature fluctuations of the clear and cloudy atmosphere are made using aircraft experimental measurements at the wavelengths 0.8 and 1.35 cm. Calculations of the radiation averaged over a satellite antenna footprint are carried out on the basis of these results and of well-known results of optical and infrared measurements. It is shown that sea roughness and the nonuniformity of clouds may result in errors in the integrated atmosphere water vapor and cloud liquid water content determination by satellite two-wavelength (0.8 and 1.35 cm) radiometric measurements up to 30 and 70 percent, respectively.

Influence of sea roughness and atmospheric inhomogeneities on microwave radiation of the atmosphere-ocean system

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Influence of sea roughness and atmospheric inhomogeneities on microwave radiation of the atmosphere-ocean system