Relating cirrus cloud properties to observed fluxes: a critical assessment

Abstract

The accuracy needed in cirrus cloud scattering and microphysical properties is quantified such that the radiative effect on climate can be determined. Our ability to compute and observe these properties to within needed accuracies is assessed, with the greatest attention given to those properties that most affect the fluxes. For a baseline case (defined in text), computing net shortwave fluxes at the surface to within ±5% requires accuracies in cloud ice water content that, when the optical depth is greater than 1.25, are beyond the accuracies of current measurements. Similarly, subsurface shortwave flux computations require accuracies in the asymmetry parameter that are beyond our current abilities when the optical depth is greater than four. Unless simplifications are discovered, the scattering properties needed to compute cirrus cloud fluxes cannot be obtained explicitly with existing scattering algorithms.