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

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.

Multiple vs single scattering: assessment of magnitudes.

All orders of scattering are analyzed for two artificial canopies. The SHL canopy consists of Small Horizontal Leaves that are much smaller than the leaf-to-leaf spacing. The IHL canopy consists of Infinite Horizontal Layers, where each leaf is of infinite extent (a horizontal plane). Hemispheric leaf reflectances and transmittances independent of the direction of illumination lead to exact solutions for these models. Sunlight that penetrates to a given leaf area index level is much stronger in an SHL canopy than that in IHL; but the difference becomes muted when leaf transmittance is large. Multiple scattering enhances the hemispheric canopy reflectance more strongly in SHL than it does in IHL. The enhancement depends linearly on leaf transmittance in SHL and on the transmittance squared in IHL. Comparison with measured reflectances ndicates that IHL model grossly underestimates multiple scattering in soybean canopies.

Adjacency effects on imaging by surface reflection and atmospheric scattering: cross radiance to zenith.

An analytical solution is discussed for the nadir radiance as measured from a satellite, based on a simplified single-scattering approximation in which the scattered radiation is not subject to extinction. In the solution, terms can be identified as due to a reflection from the vicinity of the object pixel, and, respectively, (1) upward scattering to zenith above the object pixel, and (2) downward scattering from the entire atmosphere to the object pixel. The first term is referred to as the cross radiance, the second as the cross irradiance. The cross radiance is proportional to the forward scattering optical thickness, as defined, and the cross irradiance to the backscattering optical thickness. The cross radiance usually constitutes the predominant effect. The effect, even at low atmospheric turbidity, can be large enough to constitute a significant fraction of the radiance registered at the satellite, thus hampering determination of spectral signature of the object pixel or identification of pixels with inherently the same spectral signature. Explicit expressions and computer solutions for the cross radiance from annular or from rectangular reflecting areas are presented. The effect depends on the height distribution and on the sharpness of the forward peak of the scattering particles.

Single-scattering solution for radiative transfer through a turbid atmosphere.

A solution is presented to the radiative transfer of the solar irradiation through a turbid atmosphere, bounded by a Lambert surface, based on the single-scattering approximation, i.e., an assumption that a photon that underwent scattering either leaves the top of the atmosphere or strikes the surface. The solution depends on idealization of the scattering phase function of the aerosols. The equations developed here are subsequently applied to analyze quantitatively (1) the enhancement of the surface irradiation and (2) the enhancement of the scattered radiant emittance as seen from above the atmosphere, caused by the surface reflectance and subsequent atmospheric scattering. An order-of-magnitude error analysis is presented.

Irradiance distribution, resolution, and size estimates in diffraction limited imagery of extended circular targets.

Various aspects of diffraction effects on imagery from distributed sources are studied. First the irradiance distribution in the focal plane from distributed, uniform, circular sources (disks) for optical systems with various obscuration ratios, was computed and presented in tables. A resolution criterion, applicable both to point sources and distributed sources, is presented and discussed. The criterion is based on an assumption that detection requires a minimum contrast. The level of this contrast was selected to be 20%. The criterion is applied to the case of two neighboring disks, and the numerical results for resolution are tabulated. The problem of size estimate, i.e., apparent diameter of a single disk on a uniform background, is treated as a question of locating points the irradiance at which is a specified level higher than the background. The real (geometric optics) diameter of a source is given in graphs in terms of its apparent size.

Baring high-albedo soils by overgrazing: a hypothesized desertification mechanism.

Observations are reported of high-albedo soils denuded by overgrazing which appear bright, in high contrast to regions covered by natural vegetation. Measurements and modeling show that the denuded surfaces are cooler, when compared under sunlit conditions. This observed "thermal depression" eflect should, on theoretical grounds, result in a decreased lifting of air necessary for cloud formation and precipitation, and thus lead to regional climatic desertification.

Diffraction-limited resolution for geoscene imagery.

The existing criteria for diffraction-limited resolution are not applicable to the problem of imaging a geoscene, that is, a scene consisting of contiguous extended sources. Using tables of annular aperture energy distribution for an extended source, recently computed by Goldberg and McCulloch, such resolution criteria are calculated and presented in forms of tables for various obscuration ratios.

Martian wave of darkening: a frost phenomenon?

A new hypothesis attributes the Martian "wave of darkening" to soil frost phenomena. Diurnal thawing and freezing of the ground, which uses moisture transported by the atmosphere from the melting polar cap, can produce various minute, frost-heaved, soil surface features. These microrelief features result in a complex porous surface structure, which causes optical darkening. The boundary at which the wave of darkening terminates on the winter hemisphere correlates with the latitude at which the diurnal peak surface temperature drops below 0 degrees C. The hypothesis is examined in terms of known properties of the Martian atmosphere and surface and the availability of water.