Deeper, Precipitating PBLs Associated With Optically Thin Veil Clouds in the Sc-Cu Transition

Abstract

Variability and vertical structure of optically thin veil clouds over the stratocumulus to cumulus transition (SCT) are investigated using spaceborne satellite observations. Optically thin veil clouds, defined as the low clouds with cloud base >1 km that do not fully attenuate Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar signal, comprise ∼30% of the low clouds over the SCT. It is found that optically thin veil clouds are geometrically thin with cloud thickness ∼200 m and commonly reside in the upper boundary layer with average cloud base >1.5 km. Satellite observations reveal pronounced relationships between optically thin veil clouds, strong precipitation, deep planetary boundary layer (PBL) height, and low-cloud droplet number concentration. The results are in agreement with the hypothesis that the low optical thickness of veil clouds over the SCT is contingent on the low-cloud droplet number concentration caused by strong precipitation scavenging occurring in active cumulus, a process of which efficiency is strongly dependent on maximum condensate amount in updrafts and thus is highly constrained by planetary boundary layer height.