Stratocumulus Cloud Clearings and Notable Thermodynamic and Aerosol Contrasts across the Clear–Cloudy Interface

Abstract

Data from three research flights, conducted over water near the California coast, are used to investigate the boundary between stratocumulus cloud decks and clearings of different sizes. Large clearings exhibit a diurnal cycle with growth during the day and contraction overnight and a multiday life cycle that can include oscillations between growth and decay, whereas a small coastal clearing was observed to be locally confined with a subdiurnal lifetime. Subcloud aerosol characteristics are similar on both sides of the clear–cloudy boundary in the three cases, while meteorological properties exhibit subtle, yet important, gradients, implying that dynamics, and not microphysics, is the primary driver for the clearing characteristics. Transects, made at multiple levels across the cloud boundary during one flight, highlight the importance of microscale (~1 km) structure in thermodynamic properties near the cloud edge, suggesting that dynamic forcing at length scales comparable to the convective eddy scale may be influential to the larger-scale characteristics of the clearing. These results have implications for modeling and observational studies of marine boundary layer clouds, especially in relation to aerosol–cloud interactions and scales of variability responsible for the evolution of stratocumulus clearings.

Development and characterisation of a high-efficiency, aircraft-based axial cyclone cloud water collector

“…Chloride (Cl -) and sodium (Na + ) ions are the two largest 10 constituents that were measured in the cloud water and reflect the dominant contribution of sea salt to the cloud-activated aerosol mass in the marine boundary layer, consistent with other studies (Straub et al, 2007;Benedict et al 2012;Wang et al 2016 :Na + mass ratios match reported data for sea water composition (Seinfeld and Pandis, 2016), with good agreement 15 between the probes. A contributing factor for the offset in absolute concentration could stem from differences in the collection of small droplets between the two collectors, since species concentrations often exhibit dependence on droplet size (e.g.…”

“…Although cloud water sampling for investigating aerosol activation, pollutant transport and chemical evolution now spans several decades, recent 20 measurements have shown skill in promoting linkages between cloud water chemistry and dynamic processes. Examples of this are the characterization of the coupling state of marine clouds with the ocean surface (Wang et al, 2016), the identification of above and below cloud sources (Wang et al, 2016 and the connection with physical properties, such as the presence of giant cloud condensation nuclei . In addition, progress has been made with using cloud water samples to diagnose wet scavenging in marine clouds (MacDonald et al, 2018).…”

Relationships between giant sea salt particles and clouds inferred from aircraft physicochemical data

“…Airborne data relevant to stratocumulus clouds are used from four field experiments based out of Marina, California, using the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies Twin Otter. The first Marine Stratus/Stratocumulus Experiment (MASE‐I) [ Lu et al, ] included 13 flights in July 2005, the second Marine Stratus/Stratocumulus Experiment (MASE‐II) [ Lu et al, ] included 16 research flights in July 2007, the Eastern Pacific Emitted Aerosol Cloud Experiment (E‐PEACE) [ Russell et al, ] included 30 flights between July and August in 2011, and the Nucleation in California Experiment (NiCE) [ Coggon et al, ; Crosbie et al, ] included 23 flights between July and August in 2013.…”

Aerosol characteristics in the entrainment interface layer in relation to the marine boundary layer and free troposphere

“…Navigational, dynamic, and thermodynamic data were obtained from standard instruments described in a number of previous studies (e.g., Crosbie et al, 2016;Wang et al, 2016;Dadashazar et al, 2017). Aerosol particle concentrations were measured using multiple condensation particle counters (CPCs; TSI Inc.), specifically the CPC 3010 (particle diameter, D p > 10 nm) and ultrafine CPC (UFCPC) 3025 (D p > 3 nm).…”

“…More specifically, the predominant sources in the STBL are marine sea spray and biogenic emissions as well as ship exhaust (e.g., Coggon et al, 2014;Modini et al, 2015), while the major sources impacting the FT originate from the continent, including biogenic emissions, wildfires, anthropogenic emissions, and crustal emissions (e.g., Wang et al, 2014;Crosbie et al, 2016). As it is challenging with the current dataset to separate the relative importance of the pollution type affecting the EIL, instead the focus of the subsequent discussion is on aerosol size distributions.…”