| 06 Mar 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Summary
This study focuses on the aerosol, cloud, and boundary layer properties during the ARM EPCAPE field campaign by contextualizing the analysis in the different synoptic patterns, classified using self-organizing maps. Overall, this paper presents an interesting perspective by linking the in situ and remote sensing data of aerosol and clouds, and thermodynamics data collected during the field campaign with the background synoptic pattern, which is key in determining the cloud types and the aerosol transport. However, I believe further clarification is needed regarding the analysis. I recommend accepting this paper with the revisions suggested below.
Major comments
1. The writing and structure.
2. Equations for deriving Nd and LWP based on satellite data
Minor Comments
Line 20. It seems confusing to have “9 regimes”, but 4 points listed.
Line 30. Please be more specific about what “smaller-sized particles” mean.
Line 99-100. I suggest that the authors note the dominant cloud type in the abstract as well.
Line 137. To avoid confusion with the minus sign before the numbers for frequency, I suggest using parentheses here instead of hyphens.
Line 143. Please clarify how these rain flags are derived. Does this mean that the LWP results presented later are for non-precipitating clouds only? If so, this should be noted in the figure captions and main text.
Line 149-151. If LWP data have been filtered to exclude raining time periods, does the Nd derived here only apply to non-precipitating clouds?
Line 207. Are the times here in UTC or local time?
Line 287-290. Do the SOM maps based on data for all the months, or only for data in April-Oct of these 11 year period? If so, please clarify in the text and also in the caption of Figure 1.
Line 306. Please explain briefly what the silhouette score is and how one should interpret the values.
Line 411. Please double-check the temperatures given in the text, which are inconsistent with Figure 4a.
Line 502. Are the low clouds below 2km?
Line 586, Figure 9. MWR3 data has been filtered based on rain flags. Are there similar filters for the satellite data?
Line 596. If the aerosol data is a merged product based on both SMPS and APS, should the larger sizes also include coarse mode?
Line 606. Please clarify why only these two levels are chosen for analysis, given that EPCAPE has CCN data measured at 6 supersaturation levels.
Line 624. Perhaps just revise the order of Figure 11 and Figure 12, given that Figure 12 is presented first.
Line 624-630. The results in Figure 12 are quite interesting, but some more explanation is needed. Line 626 states, “enhanced turbulence promotes aerosol growth into accumulation mode aerosols”. Is there a decrease in Aitken mode aerosol concentration with increasing TKE to suggest that the enhancement in the aerosols larger than 100nm is due to enhanced aerosol growth? Or is that just because stronger TKE is associated with enhanced aerosol production, such as sea spray, or aerosol transport from the coast?