Lead Scientist Abstract

With their extensive coverage, low clouds greatly influence global climate, but they are poorly represented in global climate models (GCMs). The response of low clouds to changes in atmospheric greenhouse gases and aerosols remains the major source of uncertainty in climate simulations. The poor representations of low clouds in GCMs are partly due to inadequate observations of their microphysical and macrophysical structures, radiative effects, and the associated aerosol distribution and budget in regions where the aerosol impact is the greatest. The Eastern North Atlantic (ENA) is a region with persistent but diverse subtropical marine boundary layer (MBL) clouds. Their albedo and precipitation are highly susceptible to perturbations in aerosol properties. Boundary layer aerosol in the ENA region is influenced by a variety of sources, leading to strong variations in cloud condensation nuclei (CCN) concentration and aerosol optical properties. The ARM Climate Research Facility has a fixed ENA atmospheric observatory on Graciosa Island in the Azores, providing invaluable information on MBL aerosols and low clouds. At the same time, the vertical structures and horizontal variabilities of aerosols, trace gases, clouds, drizzle, and atmospheric thermodynamics are critically needed for understanding and quantifying the budget of MBL aerosols, the radiative properties, precipitation efficiency, and life cycle of MBL clouds, and the cloud response to aerosol perturbations. Much of this data can be obtained only through aircraft-based measurements. In addition, the interconnected aerosol and cloud processes are best investigated by a study involving simultaneous in situ aerosol, cloud, and thermodynamic measurements. In situ measurements are also necessary for validating and improving ground-based retrieval algorithms at the ENA observatory. The Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign was motivated by the need for comprehensive in situ characterizations of boundary layer structure, and associated vertical distributions and horizontal variabilities of low clouds and aerosols over the Azores. The ARM Aerial Facility (AAF) Gulfstream-159 (G-1) research aircraft flew from Terceira Island in the Azores during two intensive operational periods: early summer 2017 (June to July) and winter 2018 (January to February). Deployments during both seasons allowed for examination of key aerosol and cloud processes under a variety of representative meteorological and cloud conditions. The science themes for ACE-ENA included:

• the budget of MBL CCN and its seasonal variation
• effects of aerosols on clouds and precipitation
• cloud microphysical and macrophysical structures, and entrainment mixing
• advancing retrievals of turbulence, clouds, and drizzle
• model evaluation and process studies.

A key advantage of the deployments is the strong synergy between the measurements aboard the G-1 and the routine measurements at the ENA observatory, including state-of-the-art profiling and scanning radars. The three-dimensional cloud structures provided by the scanning radars will put the detailed in situ measurements into mesoscale and cloud life cycle contexts. On the other hand, high-quality in situ measurements will enable validation and improvements of ground-based retrieval algorithms at the ENA observatory, leading to high-quality and statistically robust data sets from the routine measurements. The deployments, combined with the routine measurements at the ENA site, will have a long-lasting impact on the research and modeling of low clouds and aerosols in the remote marine environment. Operational Periods: --2017.06.01-2017.07.31 --2018.01.01-2018.02.28