Modelling Solar Radiation Transmission in Cloudy Atmospheres

Abstract

<p>The transmission of solar radiation through cloudy atmospheres was examined using nine years of continuous hourly radiation and meteorological records. Transmittances of clouds were empirically determined using data from five stations in southern Canada. Statistical parameters were evaluated for exponential, linear and constant expressions for transmittance of global irradiances after correcting for multiple reflection between the surface and atmosphere. Exponential and linear uncorrected results compared well with previous work, however marked differences were noted in comparisons of constant transmittance values for Canada with those calculated for Hamburg, Germany and those for Blue Hill, Massachusetts. Multiple reflection effects were shown to enhance the surface irradiance by as much as 30%. Results of regression analysis indicated transmittance to be effectively independent of zenith angle.</p> <p>Several expressions for estimating direct beam transmittance were tested in a numerical model using data from three stations in eastern Canada. Results showed the present form, the product of global radiation and one minus the total cloud opacity, performed best thereby justifying its further usage.</p> <p>Results of direct beam and cloud transmittance analyses were combined to estimate direct, diffuse and global surface irradiances. Results were compared with measured fluxes for hourly, daily, monthly and monthly mean hourly time periods. Correcting for multiple reflection underestimated surface irradiances. No improvement in performance was obtained in models using uncorrected Canadian-derived parameters over existing parameters determined for Blue Hill. Underestimation of transmittance by the corrected parameters was attributed to the presence of undetected overlying cloud above the overcast deck. Although mean bias error values are better for global irradiances determined using the original Blue Hill parameters, it is shown that differences in model estimates which used Canadian data are within the range of uncertainties on the calculated values of the solar constant, aerosol transmission and surface albedo.</p>