Planning the next decade of coordinated research to better understand and simulate marine low clouds

A workshop was held 27-29 January 2016 at Brookhaven National Laboratory to help define research pathways to address outstanding issues related to our understanding of marine low clouds. Discussion centered on four themes where significant progress can be made using ground-based remote sensing, in situ data, and modeling. These included aerosol indirect effects and the cloud condensation nuclei (CCN) budget, precipitation, entrainment and mixing, and mesoscale organization. The response of clouds to aerosol variations in the current climate may not accurately represent the responses of clouds and their radiative forcing to increases in aerosol concentration from anthropogenic emissions since the preindustrial era. Remote marine low cloud systems are particularly susceptible to perturbations in aerosols associated with anthropogenic emissions because of their relatively low optical thicknesses and low background aerosol concentrations. Radar observations have played a critical role in demonstrating that marine low clouds precipitate frequently with rates that are significant for the planetary boundary layer (PBL) moisture budget. Most climate models represent collision?coalescence processes using bulk representations that may result in an artificially strong sensitivity to aerosol. The estimation of entrainment rate from ground-based remote sensing and aircraft measurements to provide observational constraints for models remains a significant challenge.