Solar and Thermal Radiation Measurements to 32 km at Low Solar Elevations

Abstract

The instrumentation and flight results of a balloonborne package of radiometers which measure the upward and downward fluxes of both shortwave and longwave atmospheric radiation are described. Ten flights wore conducted at Mildura and at Longreach, Australia. In most cases the balloon was launched just prior to dawn so that ascent data were obtained covering the period of sunrise, and float data (at altitudes between 21.5 and 31.5 km) were obtained covering the range of solar elevations from 4° to 45°. Composite experimental curves of the clear-sky planetary albedo, net radiation balance, and upward long-wave flux at the top of the atmosphere are presented as functions of solar elevation; the results from which these curves are constructed are remarkably consistent from one time and place to another, and can be regarded as representative for most of continental Australia. Angular intensity distributions of the short-wave radiation emerging from the top of the clear atmosphere are derived from all-Sky color photographs taken simultaneously with the flux measurements. The effect of atmospheric dust is very apparent at low sun elevations, and the derived Mie scatter polar diagram has a beam width of 48° An approximate calculation is used to show, however, that the effect of the dust on the upward shortwave flux is negligible except when the sun is close to the horizon when the increase may he of the order of 1.5%. Longwave and shortwave transmissivities of 0,23 and 0.7, respectively, are obtained for a cirrostratus cloud (visually 5/8 cover) from a flight through and above a typical deck. The reduction in out-going longwave radiation caused by this cirrus was 6 mW cm?2, twice the reduction observed on a flight over 8/8 cover of uniform altocumulus.