Summertime Marine Boundary Layer Cloud, Thermodynamic, and Drizzle Morphology over the Eastern North Atlantic: A Four-Year Study

Abstract

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.

Daytime variation in the aerosol indirect effect for warm marine boundary layer clouds in the eastern North Atlantic

“…This result departs from the precipitation suppression hypothesis, in which LWP typically increases. The absence of a precipitation suppression signal is likely attributable to the relatively modest precipitation that occurs in this region during summer (e.g., Wu et al, 2020b;Zheng and Miller, 2022), resulting in a minimal precipitation suppression effect and a dominant entrainment drying effect. In terms of α c , the potential decrease in α c attributed to a LWP reduction offsets the potential increases in α c caused by the Twomey effect, resulting in a net-zero change in mean α c for clouds with N d < 40 cm −3 (Fig.…”

Investigating the sign of stratocumulus adjustments to aerosols in the global storm-resolving model ICON

“…Additionally, there is a slight underestimation of EIS in ICON compared to ERA5. It can be noted that ERA5 itself might underestimate EIS in stratocumulus regions: Although model developments led to an increasingly better representation of boundary-layer stratocumulus clouds in ERA reanalysis (Köhler et al, 2011), biases remain (Ahlgrimm et al, 2018), and the sharpness of the inversion at the top of the boundary layer is often underestimated (Kalmus et al, 2015;Zheng and Miller, 2022). Fig.…”