Atmospheric circulation and boundary layer processes modulating aerosol and cloud characteristics over the coastal Northeast Pacific during April to October of ARM EPCAPE field campaign

Abstract. Observations from the ARM Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) spanning April to October 2023 at Scripps Pier, La Jolla, California (32.8663° N, 117.2546° W) were used to investigate the regional-scale atmospheric factors that control the variability of marine low clouds and aerosols in the coastal boundary layer (BL). Using Self-Organizing Maps applied to ERA5 sea level pressure and near-surface winds, we classify the synoptic evolution of the subtropical anticyclone into 9 regimes, which includes: 1) patterns with a weakened subtropical anticyclone south of Scripps Pier and a midlatitude cyclone further north, 2) regimes that capture the evolution of anticyclone in terms of magnitude (strong vs weak) and location (coastal vs offshore), with their corresponding transitions in BL wind strengthening and large-scale subsidence, 3) a regime characterized by an anticyclone with its core at the northwestern edge of the domain, and 4) a regime that captures anomalies that minimally depart from the climatological mean. GOES-18 cloud retrievals reveal that regimes associated with anticyclone cores closer to Scripps Pier produce reduced low-cloud fraction, shallower clouds, and low liquid water path (LWP); whereas regimes with a west/north-westward-displaced anticyclone support extensive stratocumulus with higher LWP and elevated cloud tops. Regimes with a weak anticyclone centered adjacent to the Pier feature highest concentrations of smaller-sized particles, associated with a stable BL and stagnation under weak winds. Regimes with anticyclonic strengthening farther-offshore have lower aerosol concentrations. Partial inconsistency between cloud droplet number concentration (Nd) and aerosol concentration indicates BL turbulence critically influences aerosol activation into Nd.