Tropical altocumulus cloud observations during the ORCESTRA/MAESTRO field campaign

During the ORCESTRA/MAESTRO field campaign, the recurrent, and somewhat unexpected, presence of mid-level clouds was documented in both in-situ and remote sensing observations. These clouds, most predominantly of the altocumulus type, are still poorly understood in terms of their formation mechanisms, and constitute a major challenge for numerical weather prediction and general circulation models, which often struggle against the representation of their mixed-phase composition. In addition, mid-level clouds can perturb the individual shortwave and longwave components of the radiation budget. However,  their role in the mesoscale organization of convection, and more generally in climate, is not established yet.

The data from the campaign, including airborne measurements from the ATR-42 research aircraft and radiosoundings conducted at Sal, Cape Verde, represent an excellent framework to help address the questions related to tropical mid-level clouds. We analyze case studies of altocumulus cloud occurrence in order to characterize the conditions of the environment in which they formed as well as their microphysical properties. Relatively deep conditionally unstable layers are found at and below the cloud level, and the cloud is often capped by a strong inversion likely driven by cloud top radiative cooling. Furthermore, there is evidence that the cloud layer is organized into sub-kilometer-scale cells composed of alternating updrafts and downdrafts. The mixed-phase nature of the cloud is confirmed, with a thin supercooled liquid layer at the top, and solid and liquid hydrometeors falling underneath. Based on preliminary results, possible mechanisms responsible for altocumulus formation and maintenance are discussed.