Biogenic gases are a prominent component of the summertime marine boundary layer (MBL) over the Eastern North Atlantic. One of these gases, dimethyl sulfide (DMS), can produce sulfate cloud condensation nuclei (CCN) that, in theory, can brighten clouds through photolysis, and produces a reaction product, methane sulfonic acid (MSA). It is also possible that DMS can interact with sea‐salt or other marine aerosols changing their CCN activation spectrum, which could also modify cloud microphysical structure. Data collected aboard the G1 aircraft during the Aerosol Cloud Experiment Eastern North Atlantic (ACE‐ENA) in well‐mixed and decoupled marine boundary layers (MBLs) were used to examine relationships between the cloud droplet effective radii, re ${r}_{e}$, and the concentrations of DMS and MSA in constant cloud liquid water content (LWC) bins. A weak but statistically significant negative correlation was observed between CCN concentration and re ${r}_{e}$ in most LWC bins, regardless of the source of the CCN, while a weak but statistically significant positive correlation between re ${r}_{e}$ and DMS was observed. No correlation between the cloud droplet number concentration and DMS was found. The presence of MSA indicated that DMS‐to‐sulfate photolysis was likely occurring, but data sparsity prevented a statistically significant conclusion regarding the relationship between MSA and re ${r}_{e}$. Data sparsity in decoupled conditions also prevented statistically significant conclusions. To properly address biogenic gas impacts on cloud microphysics, it is recommended that aircraft data be supplemented by long‐term biogenic gas measurements at the surface in marine locations with appropriate remote and in‐situ cloud sensing capabilities, and the analysis limited to well‐mixed MBL's.
Summertime remote sensor and in situ data from 2016-2019 collected at the ARM Eastern North Atlantic (ENA) Observatory are combined with aircraft measurements from Aerosol Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign to quantify marine boundary layer (MBL) cloud, thermodynamic, and drizzle morphology in the region. A radar reflectivity-rainfall rate relationship (Z-R) is developed from aircraft data and six-hour cloud morphological regimes are identified from ENA data using a k-means algorithm driven by three independent inputs quantifying cloud thickness, drizzle intensity, and cloud field geometric complexity. Four separate MBL structural regimes representing non- or weakly drizzling single-layer stratocumulus, drizzling stratocumulus and cumulus-coupled stratocumulus, deep convection, and broken clouds embedded in northerly flow are identified. Single-layer stratocumulus is indicated when weak subtropical anticyclones are significantly west of the ENA site, and the MBL is cooler and drier than when drizzling and cumulus-coupled stratocumulus and broken clouds are observed. Drizzling and cumulus-coupled stratocumulus clouds are observed on the eastern flank of strong subtropical anticyclones in deep warm moist air masses with windspeeds exceeding 7m s−1 and strong near surface wind shear. Broken clouds exhibit strong wind shear near the inversion, while single-layer stratocumulus clouds have lower windspeeds and minimal shear. Net latent heat fluxes in the sub-cloud layer resulting from a combination of the ocean surface heat flux and evaporating drizzle average near zero over long periods in drizzling and cumulus-coupled stratocumulus. The ECMWF Reanalysis Version 5 (ERA5) is found to accurately represent single-layer stratocumulus properties, while producing significant discrepancies when drizzling stratocumulus and cumulus-coupled stratocumulus are observed.
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