Characteristics of Mid-Level Clouds over the Southeast Atlantic

Clouds over the southeast Atlantic are important for future climate projection. While a lot of attention has been focused on the low-level clouds due to their abundance, persistence and regional climate impact, no study has yet characterized the mid-level clouds that also occur over the southeast Atlantic. The mid-level clouds complicate the attribution of the cloudy radiation budget, as well as the space-based remote sensing retrievals. Here we characterize the mid-level clouds using observations collected during the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign, and from merged CloudSat-CALIPSO cloud retrievals. Lidar accurately places the mid-level clouds at the top of well-mixed, moist free-tropospheric aerosol layers, or between 5 and 6 km, with cloud-top temperatures ranging between 0oC and -12oC. The mid-level cloud frequency increases from July, and maximizes in October, with the highest occurrence in the morning (04-06 UTC), and the lowest in the afternoon (13-15 UTC). The mid-level clouds are also associated with synoptically-modulated continental moisture outflow that can be linked to the detrainment from the continental deep convective clouds. As such, they are most common near the coast, but can also occur far offshore. The cloud top heights are underestimated in passive remotely-sensed retrievals, and clearly impact column-retrieved cloud optical depths.