Aerosol-induced radiative flux changes off the United States mid-Atlantic coast: Comparison of values calculated from sunphotometer and in situ data with those measured by airborne pyranometer

The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX ) measured a variety of aerosol radiative effects (including flux changes) while simultaneously measuring the chemical, physical, and optical properti es of the responsible aerosol particles. Here we use TARFOX-determined aero sol and surface properties to compute shortwave radiative flux changes for a variety of aerosol situations, with midvisible optical depths ranging fro m 0.06 to 0.55. We calculate flux changes by several techniques with varyin g degrees of sophistication, in part to investigate the sensitivity of resu lts to computational approach. We then compare computed flux changes to tho se determined from aircraft measurements. Calculations using several approa ches yield downward and upward flux changes that agree with measurements. T he agreement demonstrates closure (i.e., consistency) among the TARFOX-deri ved aerosol properties, modeling techniques, and radiative flux measurement s. Agreement between calculated and measured downward flux changes is best when the aerosols are modeled as moderately absorbing (midvisible single-sc attering albedos between about 0.89 and 0.93), in accord with independent m easurements of the TARFOX aerosol. The calculated values for instantaneous daytime upwelling flux changes are in the range +14 to +48 W m(-2) for midv isible optical depths between 0.2 and 0.55. These values are about 39 to 10 0 times the global-average direct forcing expected for the global-average s ulfate aerosol optical depth of 0.04. The reasons for the larger flux chang es in TARFOX include the relatively large optical depths and the focus on c loud-free, daytime conditions over the dark ocean surface. These are the co nditions that produce major aerosol radiative forcing events and contribute to any global-average climate effect.