Boundary Layer Cloud Transitions in North Atlantic: Lagrangian Perspective from Surface and Satellite Observations

Marine boundary layer cumulus clouds have significantly higher albedo than the underlying sea surface. Hence, these clouds have a strong effect on Earth’s climate, and differences in their representation underlies much uncertainty in estimates of climate sensitivity. Stratocumulus clouds are routinely observed over the Eastern edges of subtropical oceans in shallower boundary layers that are characterized by relatively colder sea surface temperatures and hence greater stratification between the surface boundary layer and the free atmosphere. The marine boundary layer deepens and stratocumulus break-up into more cumuliform clouds characteristic of the downwind trades as they advect with the trade winds over warmer waters. Understanding what controls the statistics of cloud amount in the trades provides a good way to test underlying mechanisms that may underpin cloud changes with warming.

We use observations from two advanced surface based remote sensing stations, one on Graciosa (39N, 28W) and another on Barbados (13N, 59W) together with satellite remote sensing to study the boundary layer cloud transitions in the North Atlantic. The Graciosa site is in the Eastern North Atlantic and is maintained as part of the Atmospheric Radiation Measurement network of stations. The Barbados site is a cooperation between the Max Planck Institute for Meteorology and the Caribbean Institute for Meteorology and Hydrology, and the Museum of Barbados. Forty-one cases (days) between October, 2015 and March, 2017 were identified using HYSPLIT model runs when the air parcel at Barbados came from within 2.5 degrees of the Graciosa site. The average transit time for an air-parcel to travel between two locations was about 7 days. The Ka-band vertically pointing Doppler cloud radar and ceilometer present at both sites gives detailed structure of cloud and precipitation features. Data from the radiosondes launches at each site and that from the Raman Lidar and wind Lidar are used to characterize the boundary layer thermodynamic and wind fields. The data collected at the two locations provide detailed conditions at the beginning and the end of the parcel transit, while satellite and reanalysis model data will provide insights on conditions during the transit. We present the average (longitude-height) evolution of cloud, thermodynamic and boundary layer properties for the transition cases as a test of understanding of trade-wind cloud controlling factors.