We are investigating the sensitivity of the shortwave aerosol radiative forcing and the heating rate profile to the vertical profile of aerosol optical properties (extinction, single-scattering albedo, and asymmetry parameter), solar geometry and surface albedo for clear-sky conditions. Total uncertainties in modeled local diurnally averaged direct radiative forcing (DRF) at the surface and top of the atmosphere (TOA) due to uncertainties in the input parameters mentioned above range from 0.2 to 1.3 W m-2 (42 to 20%) depending on location (from tropical to polar sites), solar zenith angle, surface reflectance, aerosol type, and aerosol optical depth. The largest contributor to total uncertainty in DRF is usually single scattering albedo; however decreasing measurement uncertainties for any property would increase accuracy in DRF. Comparison of two radiative transfer models (RRTM_SW and SBDART) suggests the contribution of modeling error is small compared to the total uncertainty although comparable to uncertainty arising from some individual properties. In this presentation we are assessing the results not only at TOA and the surface but throughout the vertical profile. As inputs, we are using slightly idealized case studies from SAFARI-2000 (Southern Africa), ACE-Asia (Sea of Japan, 2001), ARM Aerosol IOP (Oklahoma, 2003), and RADAGAST (Niger, 2006). These case studies encompass a wide range of aerosols including marine, local and transported smoke, dust, and pollution aerosols. Initial results indicate that the shape of the aerosol extinction profile has very little impact on TOA and surface forcing. However, as long as the aerosol is not purely scattering (i.e single-scattering albedo < 1), the shape of the extinction profile is important for the vertical profiles of forcing and heating rates. In contrast, the shape of the aerosol absorption profile has a slight impact on TOA and surface forcing but is the main driver for the forcing and heating rate profiles. Unfortunately, the vertical profile of aerosol absorption is a quantity that is currently measured with rather large uncertainties only. We also show actual measurements of up and down-welling spectral flux profiles from the 2003 ARM Aerosol IOP and our attempts at deriving heating rate profiles from these.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0305 Aerosols and particles (0345;
- 0360 Radiation: transmission and scattering