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 ([Image: see text]) through the effects of scattering and absorption. Linear or nonlinear relationships between aerosol optical depth (τ(a)) and [Image: see text] have been established to describe the aerosol direct radiative effect on [Image: see text] (ADRE). However, considerable uncertainties remain associated with ADRE due to the incorrect estimation of [Image: see text] (τ(a) in the absence of aerosols). Based on data from the Aerosol Robotic Network, the effects of τ(a), water vapor content (w) and the cosine of the solar zenith angle (μ) on [Image: see text] are thoroughly considered, leading to an effective parameterization of [Image: see text] as a nonlinear function of these three quantities. The parameterization is proven able to estimate [Image: see text] 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 τ(a) from [Image: see text], 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 [Image: see text] from τ(a) or estimate τ(a) from [Image: see text] measurements if water vapor measurements are available.