Direct Aerosol Forcing: Sensitivity to Uncertainty in Measurements of Aerosol Optical and Situational Properties
| McComiskey, Allison | CIRES / NOAA |
| Schwartz, Stephen | Brookhaven National Laboratory |
| Ricchiazzi, Paul | University of California, Santa Barbara |
| Lewis, Ernie | Brookhaven National Laboratory |
| Michalsky, Joseph | DOC/NOAA/OAR/ESRL/GMD |
| Ogren, John | NOAA/CMDL |
Category: Radiation
Understanding sources of uncertainty in estimating aerosol direct radiative forcing (DRF), the difference in a given radiative flux component with and without aerosol, is essential to local shortwave radiative closure and to quantifying changes in Earth’s radiation budget through time. We examine the uncertainty of DRF due to uncertainty in the quantities on which it depends: aerosol optical properties, i.e., single scattering albedo and asymmetry parameter, and situational variables, i.e., solar geometry and surface albedo, and the wavelength dependencies of these quantities. Sensitivity of DRF is expressed in terms of the radiative forcing efficiency (RFE), DRF per unit optical depth. Sensitivity of RFE to aerosol properties is important as RFE is often used to estimate DRF in situations where optical depth is available, from surface-based or satellite measurements, but other aerosol properties are not well known. Here RFE for net irradiance at the top of the atmosphere (TOA) and surface is computed for three base cases representing different climate regimes with distinct aerosol assemblages: Tropical Western Pacific, Southern Great Plains, and North Slope of Alaska. For each case, the RFE is presented for three scenarios (1) integrated over the shortwave and averaged over solar zenith angle for the Spring Equinox, (2) at 0.55 µm and averaged over solar zenith angle for the Spring, and (3) integrated over the shortwave at 30o, 45o, and 60o solar zenith angle. While the magnitude of the uncertainty in DRF relates directly to the magnitude of the measurement uncertainty in aerosol and situational properties, the absolute and relative effects of the several uncertainties are case dependent and can vary greatly. For example, it is found that the greatest contribution to uncertainty in estimating DRF from measurements is propagated uncertainty of 25% in surface albedo resulting in a 24-hour mean forcing at equinox of 19 W·m-2 at TOA. For aerosol optical properties, the greatest contribution to uncertainty in DRF is propagated uncertainty of 0.05 in asymmetry parameter resulting in ~8 W·m-2 at the Tropical Western Pacific at TOA and surface but <1 W·m-2 at the North Slope of Alaska at TOA and surface due to the differences in surface albedo and solar geometry. The results permit estimation of uncertainty in DRF and also identify which measurements limit accuracy in estimating DRF.
This poster will be displayed at the ARM Science Team Meeting.
