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  • Author or Editor: R. T. McMillen x
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B. B. Hicks and R. T. McMillen

Abstract

“Eddy accumulation” is a variation of standard eddy correlation techniques for determining eddy fluxes by sampling air in two separate systems depending on whether the vertical velocity is positive or negative. In concept, the corresponding eddy flux is determined directly from measurements of the pollutant concentration (or accumulation) difference between the two sampling systems. In practice, the method has not yet been demonstrated for a slowly-depositing pollutant.

A numerical simulation of the eddy accumulation technique has been used to test the sensitivity of the method to errors arising from various sources, including sensor orientation, sampling limitations and chemical resolution. These tests were conducted using artificial pollutant concentration signals derived from real meteorological data (obtained above a forest canopy), in order to avoid the possibility of injecting unwanted errors by employing a poor quality pollutant signal. To detect a pollutant deposition velocity of 0.1 cm s−1, it appears necessary to maintain linear sampling characteristics over a dynamic range corresponding to two orders of magnitude of vertical wind speed (the limits are approximately 0.05 σw and 5σw in any given condition, where σw is the standard deviation of the vertical velocity w), to maintain sampling zero offsets to less than 0.02σw of equivalent vertical velocity and to resolve chemical concentration differences amounting to about 0.4% in typical conditions.

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J. D. Bergen, B. A. Hutchison, R. T. McMillen, A. D. Ozment, and G. J. Gottfried

Abstract

The integrated albedo for solar radiation in the 0.4–0.7 μm wavelength range was measured near noon over a wet snow cover before and after a new snowfall. Observed values were compared with those estimated from measurements of surface density, air permeability, and the total-to-diffuse-flux ratio by means of five models described in the literature and by using empirical correlations to estimate grain size.

The models yield widely divergent results. The model with the best apparent performance shows an rms error of 0.02 with no particular bias. With one exception, however, the remaining models yield over-estimates.

Albedos calculated from samples taken from differing surface layers and variation of the prediction errors with the snow accumulation pattern were compared. Results suggest that a major source of error is the large depth interval sampled by the surface layer measurement technique as compared with the thickness of the surface layer of snow in which the bulk of absorption and scattering occur.

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J. C. Doran, M. L. Wesely, R. T. McMillen, and W. D. Neff

Abstract

Measurements of heat and momentum fluxes along the valley floor of Brush Creek in Colorado are described. The measurements were taken in the fall of 1984 as part of the Department of Energy's Atmospheric Studies in Complex Terrain field program. The sensible heat flux to ground decreased from approximately 40–60 W m−2 prior to midnight to about 10–25 W m−2 in the morning hours. Surface friction velocities u * ranged from approximately 20–15 cm s−1 during the corresponding time periods. Considerable site-to-site variability in flux values was found, and disturbances of the upwind flow appear to be a significant contributing cause.

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