Abstract
Abstract. Vertical profiles of aerosols are inadequately observed and poorly represented in climate models, contributing to the current large uncertainty associated with aerosol–cloud interactions. The US Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) aircraft field campaign near the Azores islands provided ample observations of vertical distributions of aerosol and cloud properties. Here we utilize the in situ aircraft measurements from the ACE-ENA and ground-based remote-sensing data along with an aerosol-aware Weather Research and Forecast (WRF) model to characterize the aerosols due to long-range transport over a remote region and to assess their possible influence on marine-boundary-layer (MBL) clouds. The vertical profiles of aerosol and cloud properties measured via aircraft during the ACE-ENA campaign provide detailed information revealing the physical contact between transported aerosols and MBL clouds. The European Centre for Medium-Range Weather Forecasts Copernicus Atmosphere Monitoring Service (ECMWF-CAMS) aerosol reanalysis data can reproduce the key features of aerosol vertical profiles in the remote region. The cloud-resolving WRF sensitivity experiments with distinctive aerosol profiles suggest that the transported aerosols and MBL cloud interactions (ACIs) require not only aerosol plumes to get close to the marine-boundary-layer top but also large cloud top height variations. Based on those criteria, the observations show that the occurrence of ACIs involving the transport of aerosol over the eastern North Atlantic (ENA) is about 62 % in summer. For the case with noticeable long-range-transport aerosol effects on MBL clouds, the susceptibilities of droplet effective radius and liquid water content are −0.11 and +0.14, respectively. When varying by a similar magnitude, aerosols originating from the boundary layer exert larger microphysical influence on MBL clouds than those entrained from the free troposphere.
Highlights
It has been long hypothesized that increased high concentrations of aerosols serving as cloud condensation nuclei (CCNs) can reduce cloud droplet effective radius, enhance cloud albedo, suppress drizzle formation, and change cloud lifetime and fraction – the so-called aerosol indirect effects (AIEs; Twomey, 1977; Seinfeld et al, 2016)
We find that 5 d (28 June, 30 June, 6 July, 12 July and 15 July) out of 8 during the summer phase of the ACEENA field campaign clearly show the interactions between aerosols from long-range transport and local MBL clouds, corresponding to a 62.5 % occurrence frequency
Our back-trajectory analyses confirm that anthropogenic and/or biomass-burning aerosols were mainly from the US continent during the summer phase of ACE-eastern North Atlantic (ENA), while the dust plumes mainly originated from the Sahara
Summary
It has been long hypothesized that increased high concentrations of aerosols serving as cloud condensation nuclei (CCNs) can reduce cloud droplet effective radius, enhance cloud albedo, suppress drizzle formation, and change cloud lifetime and fraction – the so-called aerosol indirect effects (AIEs; Twomey, 1977; Seinfeld et al, 2016). In this study, we aim to characterize long-range transport of aerosols and to assess their impacts on MBL clouds by combining in situ aircraft measurements with cloud-resolving model simulations. For those aerosols resulting from long-range transport, one of the most important aspects pertinent to aerosol–cloud interactions (ACIs) is their vertical distribution, or in other words, their position relative to cloud layers. This study leverages the airborne measurements of aerosol vertical profiles for different chemical species to understand aerosols and their influence on MBL cloud microphysical properties over the Azores, with the ultimate goal of providing observational constraints on the global climate model simulations.
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