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
... [Show full abstract] 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 accompanied with a characteristic westward decrease. CCN monthly evolution is consistent with satellite-derived cloud droplet number concentration Nd from the Moderate Resolution Imaging Spectroradiometer. One-point correlation (r) analysis between the 1000 hPa zonal wind time series over a region between 125°W and 135°W, 35°N and 45°N, and the Nd field 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 Nd decreases when the zonal wind intensifies. The negative r expands southwestward as the zonal wind precedes Nd by 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 Nd reach a maximum. A first assessment of aerosol-cloud interaction (ACI) is performed by combining CCN and satellite Nd values from the Fifteenth Geostationary Operational Environmental Satellite. The CCN-Nd correlation 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.
