Balloon-based measurements of the profile of downwelling shortwave irradiance in the troposphere

Using a helium plus reversible fluid balloon system as the observing platform, multiple profiles of shortwave irradiance between 4 and 10 km were recently obtained over the Los Angeles basin. Measurements of downwelling 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 model computations using a spectrally-resolving, plane parallel, multiple scattering model. The systematic difference between model and observations (10%) at high altitudes is discussed in terms of possible factors that affect the instrument measurement response under cold temperature conditions. The measurements and calculations of downwelling irradiance agree to better than 5% in the lower troposphere.