Measured infrared radiances near sea horizon and their interpretation ‐ Preliminary results

The assumption of blackbody emission (emissivity, 1.0) for a calm ocean surface can lead to significant underestimates of the sea-surface temperature (SST) derived from IR radiometric data. Taking the optical properties of the atmosphere as known, we calculate the errors stemming from the blackbody assumption for cases of a purely absorbing or a purely scattering atmosphere. It is observed that for an absorbing atmosphere the errors in SST are always reduced and are the same whether measurements are made from space or at any level in the atmosphere. As for atmospheric scattering, the SST errors are slightly reduced when one is viewing from large zenith angles but are slightly enhanced when one is viewing from the zenith. The inferred optical thickness tau of an absorbing layer can be in error under the blackbody assumption by a Deltatau of 0.01-0.08, while the inferred optical thickness of a scattering layer can be in error by a larger amount, Deltatau of 0.03-0.13. The error Deltatau depends only weakly on the actual optical thickness and on the viewing angle, but it is rather sensitive to the wavelength of the measurement. In the absence of steep slopes in the wave-slope distribution, directional emissivities are essentially unchanged by sea state when one is viewing from or near the zenith. When one is viewing from moderately large zenith angles (such as 507 degrees ), however, the departures in the directional emissivities from blackbody emission can be much larger under perturbed sea state than under calm conditions.

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Infrared radiance and solar glint at the ocean–sky horizon

Mermelstein

Shettle

Takken

et al. 1994

Appl. Opt.

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An analytic model is developed for the mean and clutter infrared radiance emitted from the ocean surface near the horizon and in the presence of solar glint. The model is based on the identification of a characteristic facet dimension over which the ocean surface is essentially flat. Fluctuations in the facet orientation generated by the water wave motion are modeled by a parameterized wave height power spectral density that provides the two orthogonal wave slope variances. The mean and root-meansquare facet radiances are calculated with Gaussian probability-density functions for the wave slopes. One can determine the number of facets within the field of view of a single detector by estimating the exposed ocean area and dividing by the facet area. This estimation takes into account shadowing effects of the swell wave, the swell wavelength, and the transverse detector field of view. The number of exposed facets together with the central-limit theorem permits computation of the radiance clutter as a function of look-down angle below the horizon. Vertical radiance profiles, parameterized by the azimuthal offset from the solar position, are calculated over a sensor look-down angle range of ±50 mrad about the horizon. The results of this analysis are compared with infrared radiance measurements of the ocean surface near the horizon and in the presence of solar glint. Agreement between the measured and calculated values of the mean and clutter radiances is good.

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Measured infrared radiances near sea horizon and their interpretation ‐ Preliminary results

Cited by 3 publications

References 6 publications

Modeling zenith-angle dependence of outgoing longwave radiation: Implication for flux measurements

Modeling zenith-angle dependence of outgoing longwave radiation: Implication for flux measurements

Effects of water-emission anisotropy on multispectral remote sensing at thermal wavelengths of ocean temperature and of cirrus clouds

Infrared radiance and solar glint at the ocean–sky horizon

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