Improved modeling of cloudy‐sky actinic flux using satellite cloud retrievals

Clouds play a critical role in modulating tropospheric radiation and thus photochemistry. We develop a methodology for calculating the vertical distribution of tropospheric ultraviolet (300–420 nm) actinic fluxes using satellite cloud retrievals and a radiative transfer model. We demonstrate that our approach can accurately reproduce airborne-measured actinic fluxes from the 2013 Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign as a case study. The actinic flux is reduced below optically moderate-thick clouds inversely with cloud optical depth, and can be enhanced by a factor 2 above clouds. Inside clouds, the actinic flux can be enhanced by 2–3 times in the upper part of clouds or reduced by 90% in the lower parts of clouds. Our study suggests that the use of satellite-derived actinic fluxes as input to chemistry-transport models can improve the accuracy of photochemistry calculations.