A Comparison of Two Optical Methods for Measuring Line Averages of Thermal Exchanges Above Warm Water Surfaces

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.