Impacts of Long-range Transport of Aerosols on Marine Boundary Layer Clouds in the Eastern North Atlantic

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 DOE ARM Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) aircraft field campaign near the Azores islands provided ample accurate 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 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 (ACI) require not only low-altitude aerosol preferably getting close to the marine boundary layer top, but also large cloud top height variations. Based on those criteria, the observations show the occurrence of ACI involving the transport of aerosol over the Eastern North Atlantic is about 62% in summer. For the case with noticeable long-range transport aerosol effect on MBL cloud, the susceptibilities of droplet effective radius and liquid water content are −0.11 and +0.14, respectively. When varying on the similar magnitude, aerosols originating from the boundary layer exert larger microphysical influence on MBL clouds than those entrained from free troposphere.