Cloudiness and marine boundary-layer variability at the ARM Eastern North Atlantic site

Description

The US Department of Energy Atmospheric Radiation Measurement (ARM) Facility operates a fixed ground-based site on Graciosa Island in the Azores in the Eastern North Atlantic (ENA). The measurement record extends through two warm seasons where marine boundary-layer (MBL) clouds prevail. Here, a plethora of ground-based observations from the ARM ENA site are used to characterize the vertical and horizontal variability of the MBL and associated cloudiness. The Doppler lidar observations are used along with thermodynamic information to determine whether the MBL is coupled with the surface. The ENA site is also ideal to understand the mesoscale variability of clouds. A numerical study has shown that the MBL variability is categorized by the strength of drizzle as a result of moisture stratification in the MBL. Here, the relation between cloud scale and drizzle is studied. The cloud scale is quantified by Fourier analysis of the liquid water path (LWP) retrieved by the microwave radiometer, and compared to the horizontal variability of the sub-cloud layer, which is assessed via Fourier analysis of the vertical velocities observed by the Doppler lidar. The role of drizzle-induced evaporative cooling and moistening in modifying the MBL is examined using surface measurements, microwave radiometer, ceilometer, cloud radar, and Doppler lidar observations. Furthermore, the relationship between MBL cloudiness and subsidence is tested using reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Weather states from the International Satellite Cloud Climatology Project (ISCCP) put the results into a more general context, and provide an easy way to link them to the atmospheric situation surrounding the area.