Chemical transport model results for a new fractional cloud cover treatment

The treatment of fractional cloud cover in global models has been a long-standing challenge in atmospheric chemistry. Clouds not only present a mixed-phase chemical environment, they also alter the photolysis rates that drive gas-phase chemistry throughout the lower atmosphere. We present the first global chemical transport model (CTM) simulations that use a new quadrature-based method for integrating over the sub-grid scale variations in photolysis rates associated with broken cloud fields. We implement the new algorithm in the tropospheric chemistry version of the UCI CTM using ECMWF forecast fields at T42L40 resolution. We examine the magnitude of the differences in the tropospheric ozone and hydroxyl radical budgets using the new cloud treatment compared to those using the linear method, in which the cloud is spread over the entire grid box and the optical depth is scaled by the cloud fraction. We also test whether specification of cloud optical properties based on continental vs. maritime or polluted vs. pristine conditions has an impact on photochemistry. We show that proper treatment of fractional cloud cover corrects a bias in CTMs by substantially increasing photochemical activity in the tropical marine boundary layer due to the presence of clear-sky fractions in each grid box, thus speeding the oxidation and removal of reactive compounds emitted from the oceans.