Bias in modeled solar radiation by non-resolved intra-daily AOD variability

Abstract

Attenuation of broadband solar irradiance by aerosols has been traditionally evaluated in wide areas using aerosol optical depth (AOD) discretized at monthly time steps. This fact is known to induce a bias in the long-term solar irradiance evaluations. Nowadays weather models allow for a spatially-continuous description of AOD at sub-daily steps (e.g., hourly or 3-hourly). But considering the still high uncertainty of their modeled AODs in relation with the natural sub-daily AOD variability, a key question is whether the attenuation of solar irradiance by aerosols could simply be accounted for from daily AOD means. Hence, the bias induced by the non-resolved sub-daily AOD variability should be investigated. This work proposes a mathematical methodology to understand the roots of such a bias and uses AOD observations from 213 ground sites to investigate it. Overall, the mean and standard deviation of the bias at evaluating the daily mean global horizontal solar irradiance stay at about −0.2 and 1.2 W/m2, respectively, or −2.0 and 5.5 W/m2 for direct normal irradiance. The mean of the daily bias nearly vanishes using the proposed methodology while the standard deviation reduces to only 1.0 (3.3) W/m2 in global horizontal (direct normal) solar irradiance.