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J. T. Twitty


An iterative method to invert size distributions from simulated scattered radiance measurements at small angles from the sun has been investigated. The inferred size distributions were represented by piecewise linear and cubic spline functions. Various relevant characteristics were investigated and it was found that:

  1. The inverted size distribution was insensitive to the number of knots in the piecewise linear spine.
  2. Within the range of sensitivity, the choice of initial guess had little effect on the inverted size distribution.
  3. Five percent random noise in the simulated radiances appreciably deteriorated the result but variations are still tolerable when compared with other methods for determining size distributions.
  4. Ale inverted distribution was insensitive to the index of refraction used in the kernel for particle radii r>1 µm.
  5. The choice of wavelength between 0.40µm and 0.70µm has negligible effect on the inverted distribution.
  6. A range of tropospheric aerosol size distributions give acceptable inverted results.
  7. The cubic spline representation can give reasonable inverted distributions, but may become unstable.
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J. A. Weinman, J. T. Twitty, S. R. Browning, and B. M. Herman


The intensity of sunlight multiply scattered in model atmospheres is derived from the equation of radiative transfer by an analytical small-angle approximation. The approximate analytical solutions are compared to rigorous numerical solutions of the same problem. Results obtained from an aerosol-laden model atmosphere are presented. Agreement between the rigorous and the approximate solutions is found to be within a few percent.

The analytical solution to the problem which considers an aerosol-laden atmosphere is then inverted to yield a phase function which describes a single scattering event at small angles. The effect of noisy data on the derived phase function is discussed.

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J. J. DeLuisi, P. M. Furukawa, D. A. Gillette, B. G. Schuster, R. J. Charlson, W. M. Porch, R. W. Fegley, B. M. Herman, R. A. Rabinoff, J. T. Twitty, and J. A. Weinman


The experimental results in Part I are used in the theoretical interpretation of the radiation flux measurements which were taken with an aircraft. The absorption term of the complex refractive index of aerosols is estimated to be approximately 0.01 for a real part of 1.5 for the wavelength bandwidth 0.32–0.68 μm. A regional variation in the refractive index is noted.

Atmospheric heating and cooling rates due to aerosol and molecular absorption in the solar and terrestrial wavelengths are determined from the radiation flux measurements. The magnitudes of these rates are compared and their relative importance is discussed.

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J. J. Deluisi, P. M. Furukawa, D. A. Gillette, B. G. Schuster, R. J. Charlson, W. M. Porch, R. W. Fegley, B. M. Herman, R. A. Rabinoff, J. T. Twitty, and J. A. Weinman


An exploratory field experiment was undertaken to determine the practicality of a method specifically designed to obtain the optical properties of aerosols as they relate to the earth's radiation balance. The method requires a basic set of data consisting of the vertical distribution of aerosol concentrations, size distribution, optical depth, and net radiation fluxes. From these data radiation absorptions are determined, and effective aerosol refractive indices consistent with the actual absorption are deduced through the application of precision radiative transfer calculations. The results of 11 experiment episodes involving a combined aircraft and surface-based measurement system are described. The episodes took place in an arid desert region located near Blythe, California, and in a semiarid agricultural region located near Big Spring, Texas. Part I deals with the physical-numerical depiction of such aerosol properties as optical depth, size distribution, and vertical profiles of concentration. Part II will deal with the analysis of measurements of the radiation field leading to the deduction of the effective aerosol refractive index compatible with the absorption of solar radiation.

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