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Marvin L. Wesely

Abstract

Estimates of the global (G), diffuse (D) and direct-beam (I) irradiances at the surface of the earth can be obtained with a single instrument, the “dial” radiometer. The dial assembly intermittently shades a solid-state sensor on a continual automatic basis. This is a very simple instrument that does not require mechanical adjustments of the shade. When corrections for imperfect cosine response and excessive shading of sky radiation are performed, measurements averaged over 1 h should be accurate well within ±5%. Estimates of atmospheric turbidity or haziness can be expressed as an extinction coefficient, computed for I in reference to that obtained under cloudless clean skies for the same solar zenith angle. The uneven spectral response of silicon-cell and PAR (photosynthetically active radiation) sensors should be considered when comparing estimates of G, D or I to measurements of these components by a wide-band sensor. Linear relationships seem adequate for a variety of cloud conditions. This allows the use of a single dial silicon-cell radiometer, for example, to estimate quite accurately the values of G, D and I that would be seen by wide-band or PAR radiometers. An alternative, but less exact, means of obtaining estimates of hourly averages of D and I is to measure only G and use the ratio of G to that which would be obtained under clean, cloudless conditions as the sole determining factor.

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Marvin L. Wesely

Abstract

Depending on whether the radiation under consideration is acoustic, visible or microwave, either temperature or humidity fluctuations are ordinarily assumed to be an insignificant source of refractive index fluctuations. For applications in the atmospheric surface layer or in a free convection layer, the value of the local Bowen ratio β, which is the ratio of sensible to latent heat flux densities, can be used to determine when variations in both temperature T and water vapor pressure e are important considerations. When |β| < 0.3 for applications involving visible radiation, |β| < 0.6 for acoustic radiation, and |β|>0.32 for microwave radiation, both T and e fluctuations have at least a 10% effect on the amplitudes of refractive index fluctuations, provided T and e are highly correlated. If T and e are uncorrelated, this 10% level is |β| < 0.067, |β| < 0.13 and |β| > 1.45 for acoustic, visible and microwave radiation, respectively. With knowledge of β and the extent of the T and e correlation, refractive index “fluxes” and structure function coefficients can be calculated from (or inversely, can be used to calculate) the corresponding parameters for temperature and humidity.

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Marvin L. Wesely

Abstract

Line averages of the vertical transports of sensible and latent heats in the surface boundary layer are determined above a warm water surface by the use of two optical, line-of-sight, remote-sensing techniques. In one method, the amount of atmospherically-induced blurring of images is observed visually with the aid of a small portable astronomical telescope. The other method utilizes unaided observations of the formation of inferior mirages, in particular the heights of distant objects that appear to be partially hidden behind the mirage. In the present application above industrial cooling ponds, lines of sight are within 1 m of the water surface and 0.4 to 1.5 km in length. The evaluation of the heat fluxes also requires estimates of the surface friction velocity and the ratio of the sensible to latent heat fluxes, each of which can be obtained with sufficient accuracy over a warm water surface from relatively simple measurements of temperatures and wind speeds. The direct visual measurements are highly reproducible and, since the equipment is easily deployed in the field, the thermal performances of different sections of the same pond can be evaluated rapidly. Agreement within 10% is found between values of the heat fluxes estimated by these methods and those obtained from low-level bulk-aerodynamic procedures, but only if lines of sight are chosen to ensure that fetch is adequate, particularly for the mirage observations.

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Marvin L. Wesely

Abstract

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Marvin L. Wesely
and
Bruce B. Hicks

Abstract

Temperature and humidity fluctuations at frequencies within the inertial subrange are found experimentally to be partially correlated in the surface boundary layer over warm wet surfaces. The spectral correlation coefficient, deduced from variances and covariances computed by analog electronics, is near unity in the flux-carrying eddies and decreases with increasing frequency, approximately as n½ . As a result, optical refractive index fluctuations may have the false appearance of being strongly anisotropic in the inertial subrange.

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