By Alma Hodzic, Young-Hee Ryu, Samuel Hall, Sasha Madronich, Kirk Ullmann
The large uncertainties in the modeling of cloud fields and their optical properties often prevent chemistry-climate models from predicting accurately the radiation (actinic flux) available for photochemistry, which can result in large biases in the predicted production of secondary pollutants such as ozone or organic aerosols. To improve the accuracy in photochemical modeling in presence of clouds, we have developed a methodology for calculating the vertical distribution of tropospheric ultraviolet actinic fluxes based on satellite cloud retrievals and the NCAR TUV radiative transfer model. The satellite products employed include cloud optical depth, cloud height and cloud fraction, and are available at 4x4km2 spatial resolution routinely over the U.S.
We have applied and evaluated this approach with the NCAR actinic flux measurements collected within the 2013 Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. We showed that satellite constrained actinic fluxes can accurately reproduce airborne-measured actinic fluxes (see Fig.1), and that our product can be used quite efficiently when actinic flux measurements are not available. Our results also showed that actinic flux is reduced below optically moderate-thick clouds inversely with cloud optical depth, and can be enhanced by more than 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. This work has been submitted for publication to GRL (Ryu et al., 2016). Currently we are using the satellite-derived actinic fluxes as input to chemistry-transport models to estimate by how much we can improve the accuracy of photochemistry calculations in WRF-Chem.