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

摘要

Aerosols impact clear-sky surface irradiance ( ) through the effects of scattering and absorption. Linear or nonlinear relationships between aerosol optical depth ( τ _a) and have been established to describe the aerosol direct radiative effect on (ADRE). However, considerable uncertainties remain associated with ADRE due to the incorrect estimation of ( 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 are thoroughly considered, leading to an effective parameterization of as a nonlinear function of these three quantities. The parameterization is proven able to estimate 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 , 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 from τ _a or estimate τ _a from measurements if water vapor measurements are available.