Annual Cycle in Aerosol and Cloud Microphysics in the Northeast Pacific: Exploring the Link Between Cloud and Aerosol Variability

Nine months of ship observations during the 2012-2013 MAGIC field campaign along a 40˚ transect between Los Angeles, California and Honolulu, Hawaii suggest an annual cycle in aerosol concentrations and cloud condensation nuclei, with peaks and minima during summer and winter, respectively. More pristine conditions during winter are consistent with stronger westerlies in the boundary layer and a weaker coastal meridional low-level jet. Interestingly, satellite climatologies also yield a cloud droplet number concentration annual cycle in phase with that from the ship-borne aerosol instruments. In this work, we will explore the causality of this annual co-variability, using in-situ observations, satellite retrievals, meteorological reanalyses, and simulations with the global 3-D chemical transport model GEOS-Chem, initialized with meteorological fields from the NASA MERRA reanalyses at a 2˚x2.5˚ horizontal resolution. We will analyze the annual variability of offshore continental aerosol transport and the contribution of different aerosol species over the Northeast Pacific. The role of the atmospheric circulation explaining the aerosol variability will be also investigated, with a particular focus on the marine boundary layer. The spatial co-variability between aerosol and cloud microphysics will be analyzed with the overall goal of determining the contributions of aerosols and atmospheric circulation to the cloud microphysical patterns. Ship-borne and satellite (CloudSat) radar data will be used to analyze the role of precipitation in regulating aerosol and cloud droplet number concentrations.