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.

Influence of deepening and mesoscale organization of shallow convection on stratiform cloudiness in the downstream trades

“…(). Furthermore, the extent to which our findings are sensitive to the representation of microphysical processes merits further study (Stevens and Seifert, ; O et al ., ; Wood et al ., ).…”

“…Observational studies indicate that stratiform layers tend to become thinner as clouds get deeper (Lamer et al ., ; O et al ., ). O et al .…”

Boundary Layer Clouds and Convection over Subtropical Oceans in our Current and in a Warmer Climate

“…Recent field campaigns in the Pacific also report the frequent occurrence of thin inversion layer cloud in the transition from stratocumulus and cumulus, where they commonly occur along with aggregated cumulus patches of 10 km wide, and likely constitute the majority of the cloud cover within these patches [89•]. In that region, the ultraclean layers (UCLs) or veil layers are found to have very low cloud droplet concentrations, for which precipitation scavenging appears key [90]. Even when the layers are thin, they can still introduce important longwave radiative effects through their temperature difference with the underlying surface.…”

Atmospheric radiative profiles during EUREC<sup>4</sup>A

“…On average, longwave cooling is stronger than shortwave heating, such that net heating rates are largely negative from the surface up to 10 km, with a median value around -1 K/day. Additional local minima in longwave heating are observed around 3 and 5 km between the 5% and https://doi.org/10.5194/essd-2020-269 25% quantiles, which could correspond to the radiative effect of moisture reaching these higher levels, albeit less frequently (Stevens et al, 2017;Wood et al, 2018a, b;O et al, 2018;Gutleben et al, 2019).…”