Near-infrared Albedo Research Articles

K'-band observations of the evil eye galaxy: Are the optical and near-infrared dust albedos identical?

New measurements of the reduction of the V-band surface brightness across the prominent dust feature in the galaxy NGC 4826 are compared with corresponding increases in the V-K' color within the context of radiative transfer models invoking both absorption and scattering. The K'-band surface brightness is found to be higher than expected from standard dust models. We interpret the difference as resulting from a high effective dust albedo at K', with a likely value in excess of 0.8, provided the near-IR extinction curve in NGC 4826 is identical to the Galactic one. The high effective albedo may result from scattering by dust with a maximum grain size at least twice as large as assumed by standard models, a conclusion already indirectly hinted at by recent studies of dust star-forming regions and reflection nebulae. At least part of the high effective albedo at K' may result from near-IR nonequilibrium continuum emission attributable to very small grains.

Determination of the clear-sky planetary albedo from AVHRR using a bi-directional reflectance model

Clear-sky planetary albedos are derived from NOAA-9 AVHRR data during 1986 over the U.S. Great Plains with several distinct surface types (shrubland, grassland, deciduous and coniferous forests). The cloud-free observations are selected using a combination of absolute threshold and spatial coherence cloud-screening procedures. The broadband albedo is approximated from the narrow-band visible and near-infrared albedos. Bi-directional reflectivity models are applied to account for the effect of Sun-target-sensor geometry. The corrected albedos are converted to daily means using a directional model to account for the dependence upon sun angle. The results indicate a significant reduction in variability of the observed albedos. The efficiency of the model application is demonstrated on two examples: 1) using time series of cloud-free observations over one target and 2) using one daily image of mapped data in July, 1986.

A Simple Method for Estimating Monthly Mean Albedo of Land Surfaces From AVHRR Data

It is suggested that the observed periodicity of cloud-free, visible and near-infrared data from the Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA-9 for a particular target can be efficiently used for deriving the monthly mean clear-sky planetary albedo. The broadband albedo is approximated by a linear combination of visible and near-infrared albedos. The new method of 9-day compositing distributes the clear-sky observations over the sun-target-sensor geometry combinations and weights them by the occurrence of each combination during a particular month. It is shown that the monthly mean clear-sky planetary albedo can be estimated solely from the visible and near-infrared data in a model-independent manner. The surface albedo is then obtained by applying a simple atmospheric correction to the derived clear-sky planetary albedo. A comparison of the results of the present method and those obtained by using the “minimum albedo” method indicates that the latter may lead to an underestimation of monthly values because of the angular variation in clear-sky observed albedos. The derived surface albedos are compared with those reported in climatological studies based on ground observations.

Albedos of homogeneous semi‐infinite canopies: Comparison of two‐stream analytic and numerical solutions

The albedo of plant canopies is treated as a problem in radiative transfer. We compare albedos calculated from an iterative multistream numerical model with those calculated with an analytic two‐stream solution. With the assumption of a randomly homogeneous distribution of leaf positions and orientations and isotropic scattering by individual leaves the single‐scattering albedo of the canopy can be found analytically. This single‐scattering solution is incorporated into the two‐stream solution and used to benchmark the multistream numerical model in the single‐scattering limit. Relative errors so established in the multistream model are O(0.3%) or less. The two‐stream model is also found to be remarkably accurate, with the error in multiply scattered radiation O(5%) or less, corresponding to absolute errors in visible albedo of <0.001 and near‐infrared albedo of ≤0.01. Thus the two‐stream model should be adequate for many purposes, such as climate modeling, provided the assumptions of homogeneous canopy and isotropic scattering are not too unrealistic.

Infrared reflection nebulae in Orion Molecular Cloud 2

New observations of Orion Molecular Cloud 2 have been made from 1 to 100 microns using the NASA Infrared Telescope Facility and the Kuiper Airborne Observatory. An extensive program of polarimetry, photometry, and spectrophotometry has shown that the extended emission regions associated with two of the previously known near-infrared sources, IRS 1 and IRS 4, are infrared reflection nebulae, and that the compact sources IRS 1 and IRS 4 are the main luminosity sources in the cloud. The constraints from the far-infrared observations and an analysis of the scattered light from the IRS 1 nebula show that OMC-2/IRS 1 can be characterized by L of 500 solar luminosities or less and T of roughly 1000 K. The near-infrared albedo of the grains in the IRS 1 nebula is greater than 0.08. 27 references.

Observed Perturbations of the Earth's Radiation Budget: A Response to the El Chichón Stratospheric Aerosol Layer?

The Earth Radiation Budget experiment, launched aboard the Nimbus-7 polar-orbiting spacecraft in late 1978, has now taken over seven years of measurements. The dataset, which is global in coverage, consists of the individual components of the earth's radiation budget, including longwave emission, net radiation, and both total and near-infrared albedos. Starting some six months after the 1982 eruption of the El Chichon volcano, substantial long-lived positive shortwave irradiance anomalies were observed by the experiment in both the northern and southern polar regions. Analysis of the morphology of this phenomena indicates that the cause is the global stratospheric aerosol layer which formed from the cloud of volcanic effluents. There was little change in the emitted longwave in the polar regions. At the north pole the largest anomaly was in the near-infrared, but at the south pole the near UV-visible anomaly was larger. Assuming an exponential decay, the time constant for the north polar, near-infrared anomaly was 1.2 years. At mid- and low latitudes the effect of the El Chichon aerosol layer could not be separated from the strong reflected-shortwave and emitted-longwave perturbations issuing from the El Nino/Southern Oscillation event of 1982-83.

Additional details on the near-infrared reflectivity of laboratory ice clouds

To allow the comparison between theoretical and experimental data, complementary information is given about previously published infrared reflection measurements of ice clouds. The near-infrared ice cloud albedo is (63 ± 6)%.