The Essential Interaction of Clouds and Sunlight: Need for More Accurate Modeling of Photochemistry and Solar Heating Rates

Cloud systems significantly alter the transport of solar radiation in the atmosphere, and the methods used to calculate both photolysis and solar heating rates in global models include a wide range of approximations. The recently developed correlated-cloud overlap algorithm for photolysis rates (Cloud-J) has been combined with a spectral extension to wavelength >0.7 microns to calculate full solar heating rates (Solar-J). With Solar-J we have been able to implement three different IR extensions (RRTMG, Chou & Suarez, Grant & Grossman), embed them in the framework of the UCI Chemistry-Transport Model (CTM), and then characterize the differences for a full range of conditions (25 years of ECMWF IFS forecasts). Both photolysis and heating codes have traditionally focused on the cross sections for gases, but we find that the broad-band intervals optimized for gaseous absorption are a poor choice for cloud absorption, and characterize those errors. Overall, the development of Solar-J has focused on assessing uncertainties and providing a balanced set of errors in the simulated heating rates. We briefly look at some new statistics from the ATom global aircraft mission (S. Hall et al.) that can provide observational constraints on modeling of photolysis rates in real cloud fields.