Vertical Air Motion and Water Vapor Variability in Cumulus Topped Marine Boundary Layers

Marine boundary layer shallow cumulus clouds are ubiquitous covering vast areas from the tropics to the subtropical oceans. These clouds form due to the transport of enthalpy, moisture and momentum away from the surface due to boundary layer turbulent eddies. Shallow cumulus clouds are known to have a significant impact on local weather and climate through their role in the water cycle and radiation budget. Despite their importance, the primary mechanism by which they form, and the factors determining their size largely remain unknown. To address this issue we utilize the high-resolution observations made during shallow cumulus cloud conditions at the Atmospheric Radiation Measurement (ARM )’s Eastern North Atlantic (ENA) observatory. The ENA site is located on the Northern edge of the Graciosa Island in the Azores. The site has multiple instruments for observing cloud, thermodynamic and dynamic properties. A vertically pointing Doppler cloud radar termed as Ka-band ARM Zenith Radar (KAZR) records the full Doppler spectrum and its first three moments at a 2-second temporal and 30-m range resolution. Collocated to the KAZR is a Doppler Lidar (DL) recording the vertical air motion in the boundary layer also at a 2-second temporal and 30-m range resolution. Observations from the DL and KAZR are combined to derive vertical air motion of the entire boundary layer. A Raman Lidar (RL) is present at the site recording the photon counting rates at nitrogen (387 nm) and water vapor (408 nm) channels in two field of views at a 10 second temporal and 7.5 m range resolution. The RL data are used to derive profiles of boundary layer water vapor at a 10-second temporal and 30 m range resolution. Collocated observations from the Atmospheric Emitted Radiance Interferometer (AERI) and the Total Sky Imager (TSI) are used to derive estimates of liquid water path (LWP) and cloud sizes respectively. A total of 8 cases of shallow cumulus cloud lasting for 141 hours are examined. All the cases had northerly wind conditions, suggesting the observed clouds to be not affected by the island heating. The data will be used to 1) quantify the vertical air motion and water vapor variability below and around the shallow cumuli, 2) understand whether the observed cumuli are active or forced, and 3) understand the causes that lead to thinner or deeper clouds and narrower or wider clouds.