Case studies of the vertical structure of the direct shortwave aerosol radiative forcing during TARFOX

The vertical structure of aerosol-induced radiative flux changes in the Ear th's troposphere affects local heating rates and thereby convective process es, the formation and lifetime of clouds, and hence the distribution of che mical constituents. We present observationally based estimates of the verti cal structure of direct shortwave aerosol radiative forcing for two case st udies from the Tropospheric Aerosol Radiative Forcing Observational Experim ent (TARFOX) which took place on the U.S. east coast in July 1996. The aero sol radiative forcings are computed using the Fu-Liou broadband radiative t ransfer model. The aerosol optical properties used in the radiative transfe r simulations are calculated from independent vertically resolved estimates of the complex aerosol indices of refraction in two to three distinct vert ical layers, using profiles:of in situ particle size distributions measured aboard the University of Washington research aircraft. Aerosol single-scat tering albedos at 450 nm thus determined range from 0.9 to 0.985, while the asymmetry factor varies from 0.6 to 0.8 The instantaneous shortwave aeroso l radiative forcings derived from the optical properties of the aerosols ar e of the order of -36 W m(-2) at the top of the atmosphere and about -56 W m(-2) at the surface for both case studies.