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NASA NTRS1984

About the phase functions of atmosphereless bodies in the solar system

The exhibition of an opposition effect is a phenomenon common to all atmosphereless bodies in the solar system. This effect involves a nonlinear surge in brightness for solar phase angles, alpha, less than about 7 deg, and a nonlinear drop-off in brightness at large phase angles. Except for the moon, Mercury, and close-earth-approaching asteroids, atmosphereless bodies can be observed from earth only in a limited phase angle range. Lumme and Bowell (1981) developed a radiative transfer formalism and showed that multiple scattering between the regolith particles represents an essential factor. It was found that all the phase curves can be generated by only two standard functions with two unknown parameters. The first function is due to single scattering processes in the regolith and the second function is due to multiply scattered light. Another purely empirical approach was also investigated by Lumme et al. (1984). A summary of the fit of an empirical model to 13 objects is provided in the paper.

Lumme, K.↗

NASA NTRS1985

A Monte Carlo reflectance model for soil surfaces with three-dimensional structure

A Monte Carlo soil reflectance model has been developed to study the effect of macroscopic surface irregularities larger than the wavelength of incident flux. The model treats incoherent multiple scattering from Lambertian facets distributed on a periodic surface. Resulting bidirectional reflectance distribution functions are non-Lambertian and compare well with experimental trends reported in the literature. Examples showing the coupling of the Monte Carlo soil model to an adding bidirectional canopy of reflectance model are also given.

Cooper, K. D.↗

NASA NTRS1985

Bidirectional Reflectance Modeling of Non-homogeneous Plant Canopies

The objective of this research is to develop a 3-dimensional radiative transfer model for predicting the bidirectional reflectance distribution function (BRDF) for heterogeneous vegetation canopies. The model (named BIGAR) considers the angular distribution of leaves, leaf area index, the location and size of individual subcanopies such as widely spaced rows or trees, spectral and directional properties of leaves, multiple scattering, solar position and sky condition, and characteristics of the soil. The model relates canopy biophysical attributes to down-looking radiation measurements for nadir and off-nadir viewing angles. Therefore, inversion of this model, which is difficult but practical should provide surface biophysical pattern; a fundamental goal of remote sensing. Such a model also will help to evaluate atmospheric limitations to satellite remote sensing by providing a good surface boundary condition for many different kinds of canopies. Furthermore, this model can relate estimates of nadir reflectance, which is approximated by most satellites, to hemispherical reflectance, which is necessary in the energy budget of vegetated surfaces.

Norman, J. M.↗

NASA NTRS1986

A note on Hapke's bidirectional reflectance spectroscopy. Part 3: Correction for macroscopic roughness

The multiple-scattering theory of K. Lumme and E. Bowell (1981) was criticized by B. Hapke by stating, in particular, that energy is not conserved. It is shown that Hapke's treatment is, in this respect, inferior to that of Lumme and Bowell, and itself violates the principal concepts of radiative transfer theory. Hapke's additinal claim that, in Lumme and Bowell's work, the reflectance tends to zero at the limb is also refuted. Comment is made on the deduction of surface physical properties by modeling photometric observations.

Lumme, K.↗

NASA NTRS1985

Saturn's rings - Particle size distributions for thin layer model

A model incorporating limited interaction between the incident energy and particles in the ring is considered which appears to be consistent with the multiple scattering process in Saturn's rings. The model allows for the small physical thickness of the rings and can be used to relate Voyager 1 observations of 3.6- and 13-cm wavelength microwave scatter from the rings to the ring particle size distribution function for particles with radii ranging from 0.001 to 20 m. This limited-scatter model yields solutions for particle size distribution functions for eight regions in the rings, which exhibit approximately inverse-cubic power-law behavior.

Zebker, H. A.↗

NASA NTRS1985

Vertical cloud structure of Jupiter's equatorial plumes

Multiple-scattering radiative transfer calculations were used to deduce the vertical cloud structure (VCS) of Jupiter's equatorial region. The VCS model of the equatorial plumes is obtained through an analysis of Voyager images of the 6190-A methane band and the 6000-A continuum, and ground-based 8900-A methane band images. The VCS of the equatorial plumes is found to be consistent with the hypothesis that the plumes are caused by upwelling at the ammonia condensation level produced by buoyancy due to latent heat release from the condensation of water clouds nearly three scale heights below the plumes.

Stoker, C. R.↗

NASA NTRS1986

Radiative transfer in a sphere illuminated by a parallel beam - An integral equation approach

The problem of multiple scattering of nonpolarized light in a planetary body of arbitrary shape illuminated by a parallel beam is formulated using the integral equation approach. There exists a simple functional whose stationarity condition is equivalent to solving the equation of radiative transfer and whose value at the stationary point is proportional to the differential cross section. The analysis reveals a direct relation between the microscopic symmetry of the phase function for each scattering event and the macroscopic symmetry of the differential cross section for the entire planetary body, and the interconnection of these symmetry relations and the variational principle. The case of a homogeneous sphere containing isotropic scatterers is investigated in detail. It is shown that the solution can be expanded in a multipole series such that the general spherical problem is reduced to solving a set of decoupled integral equations in one dimension. Computations have been performed for a range of parameters of interest, and illustrative examples of applications to planetary problems as provided.

Shia, R.-L.↗

NASA NTRS1986

Thirteenth International Laser Radar Conference

One hundred fifteen papers were presented in both oral and poster sessions. The topics of the conference sessions were: spaceborne lidar applications; extinction/visibility; differential absorption lidar; winds and tropospheric studies; middle atmosphere; clouds and multiple scattering; pollution studies; and new systems.

Source record↗

NASA NTRS1986

Near-infrared observations of IRAS sources in and near dense cores

We report 0.4 to 20 micron photometry of 27 Infrared Astronomy Satellite (IRAS) point sources associated with dense cores in nearby dark clouds. It is found that these objects have a bimodal distribution of spectral slopes. The objects in the group with steep spectral slopes are typically within one half power radius of the core peak and are not visible on the POSS. The remainder of the objects are typically further from their cores, are optically visible and have shallow spectral slopes (s is approximately 0.6). Most of the sources in this group which have been previously identified are T Tauri stars. Both the groups of objects have essentially the same luminosity function with median luminosity of 1 to 2 L(solar), which is similiar to the luminosity function for T Tauri stars. The near-infrared (J,H,K,L) colors of the objects have been used to estimate the visual extinction to the stars. This indicates that the typical steep spectrum source has extinction A(upsilon) of approximately 30 magnitudes, which is larger by a factor of 3 to 4 than a uniform core can provide, thus the density must rise steeply in the vicinity of the star. For those objects where optical estimates of the extinction are available the optical estimates are typically smaller than our IR estimates by a factor of 1.6. For multiple scattering by grains that scatter primarily in the forward direction, this implies a grain albedo of 0.4 at V, consistent with theoretical predictions for bare grains. Using this derived extinction, the FIR emission from a spherical shell with an inverse power law density which contributes the correct amount of extinction has been modelled. It is shown that to be consistent with both the observations of the density of the core at a scale of 1.5 x 10(17) cm and the extinction derived from the near-IR colors there must be a circumstellar hole of radius 10-100 A.U. for all density laws with exponents greater than 1.2. It is also shown that a component in addition to the star and circumstellar shell is necessary to match the observed spectrum of steep spectrum sources.

Fuller, G. A.↗

NASA NTRS1986

Calibration of NOAA-7 AVHRR, GOES-5 and GOES-6 VISSR/VAS solar channels

The NOAA-7, GOES-5 and GOES-6 Visible Infrared Spin Scan Radiometer/Vertical Atmospheric Sounder (VISSR/VAS) solar channels were calibrated. The White Sands Monument area in New Mexico, whose reflectance properties are well known, and space are used as calibration targets. The shortwave reflected terrestrial irradiance that is measured at satellite altitude is computed using a fairly accurate radiative transfer model which accounts for multiple scattering and bidirectional effects. The ground target reflectance and relevant characteristics of the overlying atmosphere are estimated from climatological data and observation at the nearest meteorological sites. The approach is believed to produce accuracies of 8 to 13% depending on the channel considered.

Frouin, R.↗

NASA NTRS1986

Temperature retrievals by Rayleigh backscatter lidar signals

Differences between retrieved temperatures from lidar and atmospheric model temperatures are calculated for four problems: (1) the effect of a transient thin aerosol layer at any altitude on temperature retrieved by a single-wavelength lidar; (2) the effect of residual aerosols on temperature retrieved by a two-wavelength lidar; (3) the errors due to splicing two backscatter signals which are obtained separately; and (4) the effect of multiple scattering. The results show that temperatures can be retrieved for + or - 3 K if Rayleigh backscatter can be measured for + or - 1.5 percent. The LITE lidar in a 240 km orbit aboard the Shuttle should be able to make a + or -3 K measurements from 10-40 km with vertical resolutions of 1 km and horizontal resolutions of 300 km.

Uchino, O.↗

NASA NTRS1987

Calibration of NOAA-7 AVHRR, GOES-5, and GOES-6 VISSR/VAS solar channels

Frouin, Robert↗

NASA NTRS1987

Optical observations of lightning from a high-altitude airplane

Near-infrared optical measurements of about 1300 optical pulses produced by 79 lightning flashes were obtained by a NASA U-2 high-altitude airplane. Ninety percent of the flashes produced peak radiant energy densities of 4.7 microJ/sq m per sr or greater relative to the full field of view of the instrument. A median pulse rise time of 240 microsec and FWHM of 370 microsec are found, and these slow optical rise times and broad pulse widths are suggested to be due to multiple scattering within the cloud. The spectral characteristics in the near-infrared of the neutral emission lines noted above the clouds are shown to be similar to those found with ground-based measurements.

Christian, H. J.↗

NASA NTRS1988

Thin cirrus clouds - Seasonal distribution over oceans deduced from Nimbus-4 IRIS

Spectral differences in the extinction of the 10.8- and 12.6-micron bands of the IR window region, due to optically thin clouds, were found in the measurements made by both an airborne broadband IR radiometer and the IR interferometer spectrometer (IRIS) aboard the Nimbus-4 satellite; the extinction at 12.6 microns was significantly larger than that at 10.8 microns; both water and ice particles in the clouds can account for such spectral difference in extinction. Multiple scattering radiative transfer calculations of IRIS data revealed this spectral feature about 100 to 20 km away from the high-altitude cold clouds; it is assumed that this feature is related to the spreading of cirrus clouds. Based on this assumption, mean seasonal maps of the distribution of thin cirrus clouds over the oceans were deduced from the IRIS data. The maps show that such clouds are often present over the convectively active areas, such as ITCZ, SPCZ, and the Bay of Bengal during the summer monsoon.

Prabhakara, C.↗

NASA NTRS1988

Enceladus - Implications of its unusual photometric properties

Voyager 1 and 2 images are used to ascertain the photometric properties of Enceladus up to a 43-deg phase angle. The lack of correlation noted between spectrophotometric properties and terrain type indicates that optical characteristics result from a recently-deposited, ubiquitous surface layer that may have originated on the E-ring of Saturn. Enceladus' distinctive phase and photometric functions can be accounted for by the high degree of surface multiple scattering; observations to date indicate that the Enceladus regolith's textural characteristics are similar to those of other icy satellites of Saturn.

Buratti, Bonnie J.↗

NASA NTRS1988

Heating rates in tropical anvils

The interaction of infrared and solar radiation with tropical cirrus anvils is addressed. Optical properties of the anvils are inferred from satellite observations and from high-altitude aircraft measurements. An infrared multiple-scattering model is used to compute heating rates in tropical anvils. Layer-average heating rates in 2 km thick anvils were found to be on the order of 20 to 30 K/day. The difference between heating rates at cloud bottom and cloud top ranges from 30 to 200 K/day, leading to convective instability in the anvil. The calculations are most sensitive to the assumed ice water content, but also are affected by the vertical distribution of ice water content and by the anvil thickness. Solar heating in anvils is shown to be less important than infrared heating but not negligible. The dynamical implications of the computed heating rates are also explored and it is concluded that the heating may have important consequences for upward mass transport in the tropics. The potential impact of tropical cirrus on the tropical energy balance and cloud forcing are discussed.

Ackerman, Thomas P.↗

NASA NTRS1988

Radar observations of basaltic lava flows, Craters of the Moon, Idaho

Radar images of Craters of the Moon, Idaho were used to study the backscatter characteristics of basaltic lava flows of predominantly pahoehoe textures and to determine the ability to detct fissure vents. Four images were obtained: X-band HH, X-band HV, L-band HH, and L-band HV. Hummocky pahoehoe flows were found to have strong backscatter in all four of these images. Aa lava flows showed the greatest variation in backscatter intensities, due to an increase in multiple scattering at the L-band scale. Eruptive fissures are detectable in the radar images by virtue of associated parallel spatter ramparts which have diagnostic, strong backscatter in the X-band images that are in contrast to the weak backscatter of the surrounding shelly pahoehoe lava. The importance of look direction in the use of radar images to characterize terrains is emphasized.

Greeley, Ronald↗

NASA NTRS1988

CSDUST3 - A radiation transport code for a dusty medium with 1-D planar, spherical or cylindrical geometry

The program solves the radiation transport problem in a dusty medium with one-dimensional planar, spherical or cylindrical geometry. It determines self-consistently the effects of multiple scattering, absorption, and re-emission of photons on the temperature of dust grains and the characteristics of the internal radiation field. The program can treat radiation field anisotropy, linear anisotropic scattering, and multi-grain components. The program output consists of the dust-temperature distribution, flux spectrum, surface brightness at each frequency and the observed intensities (involving a convolution with a telescope beam pattern).

Egan, Michael P.↗