BALLOON-BASED MEASUREMENTS OF THE PROFILE OF DOWNWELLING SHORTWAVE IRRADIANCE IN THE TROPOSPHERE

Using a helium plus reversible fluid balloon system as the observing p latform, multiple profiles of shortwave irradiance between 4 and 10 km were recently obtained over the Los Angeles basin. Measurements of do wnwelling hemispheric broadband irradiance were made over a period of six hours in conditions that could be characterized by a midlatitude, summer model atmosphere. These data are described and compared to mode l computations using a spectrally-resolving, plane parallel, multiple scattering model. The systematic difference between model and observat ions (10%) at high altitudes is discussed in terms of possible factors that affect the instrument measurement response under cold temperatur e conditions. The measurements and calculations of downwelling irradia nce agree to better than 5% in the lower troposphere.