Case Study in the Persistence of Low Marine Boundary Layer Aerosol Concentrations over the Eastern North Atlantic

Description

For nearly two weeks between late September and early October 2015, accumulation mode aerosol concentrations measured at the Eastern North Atlantic (ENA) ARM site in the Azores rarely exceeded 50 cm-3. Prior analysis of ARM Mobile Facility (AMF) data from the Azores found that low cloud condensation nuclei concentration (< 20 cm-3) events are almost associated with marine cold air outbreaks but typically only last on the order of hours. Thus, the persistence of such low aerosol concentrations is of particular interest. We use atmospheric reanalysis and a synergistic collection of observations from ENA to understand the large-scale, cloud and boundary layer processes at work during this time period. Aerosol depletion initiates with the arrival of a synoptic system that originated as a marine cold air outbreak and lingers in the vicinity of the Azores for several days. Heavily drizzling stratocumulus clouds are a consistent feature throughout the case study, but there appear to be distinct regimes organized by large-scale conditions and boundary layer structure. At first, soundings did not detect any significant temperature inversions capping the clouds in the cold air outbreak system. This provides a possible explanation for their depth (heights around 2 km) and the development of liquid water paths substantial enough for efficient coalescence aerosol scavenging. Subsequently, another midlatitude cyclone passes with a large area of drizzling stratocumulus following the initial front. However, this regime exhibits capping inversions around 1 km and drizzle-induced decoupling, features commonly seen with subtropical stratocumulus. We interpret the persistence of low aerosol concentrations as a confluence of synoptic conditions that initially favor deep boundary layer clouds and the positive feedback between low aerosol and efficient stratocumulus drizzle. These results provide new insight into the connections between large-scale meteorology, cloud processes and marine boundary layer structure in a region of climatological interest.