Search Results

You are looking at 1 - 6 of 6 items for

  • Author or Editor: Robert S. Fraser x
  • Refine by Access: All Content x
Clear All Modify Search
Robert S. Fraser

Abstract

The degree of dependence among the atmospheric optical thicknesses that are measured in nonselective absorption bands is studied. The observations were made previously in many spectral bands within the range 0.36–2.4 μm from near sea level in two continents where urban and industrial pollutions were weak. The sample covariance matrices and corresponding eigenvalues and eigenvectors are computed. The two highest eigenvalues account for 90% of the total variance in 10 spectral bands within the range 0.4–1.6 μm. The linear regression of the optical thickness on the total precipitable water vapor is computed to determine the attenuation coefficient that is associated with water vapor. This coefficient decreases rapidly with wave-length as λ −3.7 in the visible spectrum and indicates that numerous water particles of radius 0.03–0.06 μm cause the attenuation.

Full access
Ziauddin Ahmad and Robert S. Fraser

Abstract

We describe the details of an iterative radiative transfer code for computing the intensity and degree of polarization of diffuse radiation in models of the ocean-atmosphere system. The present code neglects the upwelling radiation from below the ocean surface and as such is applicable to the part of the spectrum where the absorption by water is strong. TO establish the reliability of our numerical scheme as well as the computer code, we compare our results with those of Fraser and Walker (1968), Dave (1972) and Mullamaa (1964) and find them to be in excellent agreement. We also obtained reasonably good qualitative agreement with Plass et al. (1975) who utilize the Monte Carlo technique to solve the radiative transfer equation for the ocean-atmosphere system. Our computations also show that both the intensity as well as the degree of polarization of the upwelling diffuse radiation at the top of the atmosphere vary significantly, when the rough ocean at the base of the atmosphere is replaced by a Lambertian surface that reflects the same energy as the rough Ocean.

Full access
Om P. Bahethi and Robert S. Fraser

Abstract

Fluxes and intensifies of light scattered by a model atmosphere are computed by a spherical harmonics approximation and by an iterative method of solving the radiative transfer equation and are compared. The large differences in the net fluxes and intensifies reported by Dave and Armstrong (1974) for the two methods are reduced here by making a few changes in the iterative routine. Decreasing the polar angle increment from 2° to 1° in the iterative method of computing the source function does not improve the results as suggested by Dave and Armstrong.

Full access
Yoram J. Kaufman and Robert S. Fraser

Abstract

In order to utilize satellite measurements of optical thickness over land for estimating aerosol properties during air pollution episodes the optical thickness was measured from the surface and investigated. Aerosol optical thicknesses have been derived from solar transmission measurements in eight spectral bands within the band λ440–870 nm during the summers of 1980 and 1981 near Washington, DC. The optical thicknesses for the eight bands are strongly correlated. It was found that first eigenvalue of the covariance matrix of all observations accounts for 99% of the trace of the matrix. Since the measured aerosol optical thickness was closely proportional to the wavelength raised to a power, the aerosol size distribution derived from it is proportional to the diameter (d) raised to a power for the range of diameters between 0.1 to 1.0 μm. This power is insensitive to the total optical thickness. Changes in the aerosol optical thickness depend an several aerosol parameters, but it is difficult to identify the dominant one. The effects of relative humidity and accumulation mode concentration on the optical thickness are analyzed theoretically, and compared with the measurements.

Full access
Yoram J. Kaufman, Robert S. Fraser, and Robert L. Mahoney

Abstract

Emission from burning of fossil fuels and biomass (associated with deforestation) generates a radiative forcing on the atmosphere and a possible climate chaw. Emitted trace gases heat the atmosphere through their greenhouse effect, while particulates formed from emitted SO2 cause cooling by increasing cloud albedos through alteration of droplet size distributions. This paper reviews the characteristics of the cooling effect and applies Twomey's theory to cheek whether the radiative balance favors heating or cooling for the cases of fossil fuel and biomass burning. It is also shown that although coal and oil emit 120 times as many CO2 molecules as SO2 molecules, each SO2 molecule is 50–1100 times more effective in cooling the atmosphere (through the effect of aerosol particles on cloud albedo) than a CO2 molecule is in heating it. Note that this ratio accounts for the large difference in the aerosol (3–10 days) and CO2 (7–100 years) lifetimes. It is concluded, that the cooling effect from coal and oil burning may presently range from 0.4 to 8 times the heating effect. Within this large uncertainty, it is presently more likely that fossil fuel burning causes cooling of the atmosphere rather than heating. Biomass burning associated with deforestation, on the other hand, is more likely to cause heating of the atmosphere than cooling since its aerosol cooling effect is only half that from fossil fuel burning and its heating effect is twice as large. Future increases in coal and oil burning, and the resultant increase in concentration of cloud condensation nuclei, may saturate the cooling effect, allowing the heating effect to dominate. For a doubling in the C02 concentration due to fossil fuel burning, the cooling effect is expected to be 0.1 to 0.3 of the heating effect.

Full access
Yuri Mekler, Yoram J. Kaufman, and Robert S. Fraser

Abstract

Theoretical two- and three-dimensional solutions of the radiative transfer equation have been applied to the earth-atmosphere system. Such solutions have not been verified experimentally. A laboratory experiment simulates such a system to test the theory. The atmosphere was simulated by latex spheres suspended in water and the ground by a nonuniform surface, half white and half black. A stable radiation source provided uniform illumination over the hydrosol. The upward radiance along a line orthogonal to the boundary of the two-halves field was recorded for different amounts of the hydrosol. The simulation is a well-defined radiative transfer experiment to test radiative transfer models involving nonuniform surfaces. Good agreement is obtained between the measured and theoretical results.

Full access