Aerosol variability, synoptic-scale processes, and their link to the cloud microphysics over the northeast pacific during MAGIC

Abstract

Shipborne aerosol measurements collected from October 2012 to September 2013 along 36 transects between the port of Los Angeles, California (33.7°N, 118.2°), and Honolulu, Hawaii (21.3°N, 157.8°W), during the Marine ARM GPCI (Global Energy and Water Cycle Experiment (GEWEX)-Cloud System Study (GCSS)-Pacific Cross-section Intercomparison) Investigation of Clouds campaign are analyzed to determine the circulation patterns that modulate the synoptic and monthly variability of cloud condensation nuclei (CCN) in the boundary layer. Seasonal changes in CCN are evident, with low magnitudes during autumn/winter, and high CCN during spring/summer accompaniedwith a characteristic westward decrease. CCNmonthly evolution is consistent with satellite-derived cloud droplet number concentration Ndfrom the Moderate Resolution Imaging Spectroradiometer. One-point correlation (r) analysis between the 1000 hPa zonal wind time series over a region between 125°Wand 135°W, 35°N and 45°N, and the Ndfield yields a negative r (up to -0.55) over a domain that covers a zonal extent of at least 20° from the California shoreline, indicating that Nddecreases when the zonal wind intensifies. The negative r expands southwestward as the zonal wind precedes Ndby up to 3 days, suggesting a transport mechanism from the coast of North America mediated by the California low-coastal jet, which intensifies in summer when the aerosol concentration and Ndreach a maximum. A first assessment of aerosol-cloud interaction (ACI) is performed by combining CCN and satellite Ndvalues from the Fifteenth Geostationary Operational Environmental Satellite. The CCN-Ndcorrelation is 0.66–0.69, and the ACI metric defined as ACI= ∂ln(Nd)/∂ln(CCN) is high at 0.9, similar to other aircraft-based studies and substantially greater than those inferred from satellites and climate models.