Mass Flux Estimates and their Relationship to Cloud-base Cloudiness during the EUREC4A Campaign

Abstract:

The trade-cumulus cloud feedback in climate models is mostly driven by changes in cloud-base cloudiness, which can largely be attributed to model differences in the strength of lower-tropospheric mixing. Using observations from the recent EUREC⁴A field campaign and realistic large-eddy simulation (LES), we aim at better understanding the sensitivity of cloud-base cloudiness to changes in convective mixing (represented by the convective mass flux) and the environment. First, we ask what the day-to-day and diurnal variability of the mass flux is, and what its controls are. Second, we study how the cloud-base cloudiness varies with the mass flux.

The cloud-base massflux is estimated as the residual of the subcloud-layer mass budget by combining measurements from dropsondes intensively launched along a circle of ~200 km diameter, from ship and autonomous-system based flux measurements, and from airborne lidar. The cloud-base cloud fraction is measured with horizontally pointing lidar and radar from an aircraft flying near cloud base within the circle area.

Preliminary mass flux estimates have reasonable mean values of 13 mm/s. Hourly estimates range between 0-35 mm/s and reveal substantial day-to-day variability, which can be partly attributed to different patterns of mesoscale organization. The variability in the mass flux is equally due to variability in the large-scale vertical velocity and the entrainment rate. The substantial variability in the entrainment rate is different to previous analyses using LES and points toward the important role of the surface buoyancy flux and transition layer stability in regulating the mass flux. The relationship between the mass flux and cloud-base cloudiness will be discussed.