HR: 08:10h
AN: A21E-02 INVITED     [Abstracts]
TI: Review of Measurements of the Aerosol Global Direct Radiative Forcing
AU: * Anderson, T L
EM: tadand@u.washington.edu
AF: University of Washington, Room 408 ATG Box 35-1640, Seattle, WA 98195-1640 United States
AB: Of all the ways that anthropogenic aerosols are suspected of impacting the Earth's energy balance, direct shortwave forcing during clear-sky conditions (DF) is by far the most straightforward to measure and understand. So, how well do we know this quantity? Model-based estimates of DF have treated individual anthropogenic components, but this approach is not practical for global satellite observations, which inherently sense the total aerosol and cannot readily detect chemical composition. In conjunction with members of the "A-Train" science teams, an observational strategy is being developed that attacks the DF problem in terms of three observable parameters: mid-visible aerosol optical depth (AOD), fine-mode fraction of optical depth (FMF) - which is taken as a proxy for anthropogenic fraction - and radiative forcing efficiency per unit optical depth (RFE). This talk will assess knowledge of each parameter. AOD is rather well known, with a long history of observations by multiple satellites and an extensive validation program in place. FMF (deduced from the wavelength dependence of AOD) is routinely reported by satellites, but these data have unknown accuracy, due to the absence of a validation program, and show discontinuities at land/ocean boundaries indicative of artifacts. Many investigators have combined AOD measurements with satellite measurements of broadband flux to estimate RFE over the oceans. These studies suffer from limited coverage (a few percent of the ocean) and a lack of collocated, sub-orbital measurements sufficient to diagnose the causes of RFE variation (such as differences in ambient relative humidity or aerosol single scattering albedo). Covariation among these three parameters has not yet been assessed. Summarizing, the lower limit on uncertainty in DF is about a factor of three and the upper limit is unknown. New and enhanced satellite sensors offer the potential for greatly reduced uncertainty in the near future. However, achieving this will require that the synthesis and integration of observations be given much greater priority than at present.
UR: http://www.atmos.washington.edu/$\sim$cheeka/DAFC/DAFC.html
DE: 3359 Radiative processes
DE: 1610 Atmosphere (0315, 0325)
DE: 1640 Remote sensing
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting