TARFOX Abstracts
at AGU Special Sessions
Baltimore, MD, 5/29-30, 1997

P. Russell
NASA Ames Research Center

J. Livingston
SRI International

D. Hegg, P. Hobbs
University of Washington

T. Novakov
Lawrence Berkeley Laboratory

J. Wong
Dalhousie University

For several TARFOX days we have calculated direct aerosol radiative forcing values for comparison to results derived from aircraft and satellite radiometers. The first step in the calculations is to derive aerosol layer size distributions from optical depth spectra measured by airborne sunphotometer. These derivations use the constrained linear inversion technique and require aerosol refractive index values. For this purpose we have developed a refractive index model that attempts to take into account the broad features of the TARFOX aerosol chemical composition and single-scatter albedo results of Novakov et al. and Hegg et al. (this conference session). The model is based on published complex refractive indices for mixtures of water and sulfuric acid, but it increases the imaginary refractive index to yield single-scatter albedos in the range of dry and wet values given by Hegg et al. (~0.90 to 0.97 at wavelength 450 nm). The retrieved aerosol layer size distributions are then used to compute the layer optical properties (single- scatter albedo, asymmetry parameter, and optical depth) across the solar spectrum. These optical properties are in turn used to compute the aerosol-induced change in upwelling shortwave flux at the layer top (i.e., the dominant term in the aerosol radiative forcing at that level). The flux-change calculations are made by two techniques, each of which accounts for the effect of changing solar zenith angle on aerosol layer transmission and upward scatter: (1) the simplified expression derived by Russell et al. (JGR, in press 1997), and (2) two-stream expressions of Coakley and Chylek (J. Atmos. Sci., 1975). The calculated changes in upwelling flux vary from +9 to +40 W m-2 for midvisible optical depths between 0.06 and 0.55. These results are similar to measurements by TARFOX airborne flux radiometers (Hignett and Taylor, this session).