Parameterization of clear-sky surface irradiance and its implications for estimation of aerosol direct radiative effect and aerosol optical depth

Aerosols impact clear-sky surface irradiance (E-g down arrow) through the effects of scattering and absorption. Linear or nonlinear relationships between aerosol optical depth (tau(a)) and E-g down arrow have been established to describe the aerosol direct radiative effect on E-g down arrow (ADRE). However, considerable uncertainties remain associated with ADRE due to the incorrect estimation of E-g down arrow(0) (tau(a) in the absence of aerosols). Based on data from the Aerosol Robotic Network, the effects of tau(a), water vapor content (w) and the cosine of the solar zenith angle (mu) on E-g down arrow are thoroughly considered, leading to an effective parameterization of E-g down arrow as a nonlinear function of these three quantities. The parameterization is proven able to estimate E-g down arrow(0) with a mean bias error of 0.32 W m(-2), which is one order of magnitude smaller than that derived using earlier linear or nonlinear functions. Applications of this new parameterization to estimate tau(a) from E-g down arrow, or vice versa, show that the root-mean-square errors were 0.08 and 10.0 Wm(-2), respectively. Therefore, this study establishes a straightforward method to derive E-g down arrow from tau(a) or estimate tau(a) from E-g down arrow measurements if water vapor measurements are available.