Improved representation of cloud-aerosol interactions in WRF-Chem parameterized convection

A new treatment of cloud-aerosol interactions for parameterized (i.e., sub-grid) convective clouds has been implemented in WRF-Chem with the goal of improving our understanding of the aerosol lifecycle on the regional and continental scale. The new treatment uses the Kain-Fritsch cumulus parameterization, which has also been modified to better treat shallow cumuli with the Cumulus Potential trigger function. Cloud droplet number concentration (CDNC) is calculated in the modified Kain-Fritsch parameterization using the modeled aerosol distribution, and key cloud microphysical and macrophysical parameters, averaged over the population of shallow clouds or for a single deep convective cloud, are provided to a new aerosol and trace gas convective-processing routine. The convective processing routine, implemented in the WRF-Chem chemistry packages, treats vertical transport, activation/resuspension, aqueous chemistry, and wet removal of aerosol and trace gases in convective clouds.

Preliminary testing of the modified version of WRF-Chem has been completed using observations from the US Department of Energy's Cumulus Humilis Aerosol Processing Study (CHAPS) over Oklahoma in June 2007, as well as a high-resolution simulation that was completed without the need for parameterized convection. The model shows good agreement between simulated and observed aerosol mass fractions below and within the cloud layer. The WRF-Chem simulations show instances of enhanced nitrate aerosol in the clouds that are attributable to aqueous phase chemistry and which were also seen in the CHAPS observations. The simulations are used to investigate the impact of cloud-aerosol interactions on regional scale transport of black carbon (BC), organic aerosol (OA), and sulfate aerosol. Based on our simulations, changes in the column integrated BC can be as large as -50% when cloud-aerosol interactions are considered (due largely to wet removal), or as large as +40% for sulfate in non-precipitating conditions due to the sulfate production in the parameterized shallow clouds. Simulated changes in the CDNC and in the chemical composition of cloud drop residuals are also consistent with CHAPS observations.