The influence of longwave cloud-top cooling on marine stratocumulus cloud transitions

Abstract

The nocturnal transition (beginning around sunset) of the turbulent structure within the stratocumulus-topped boundary layer in the Southeast Pacific is simulated using near-LES model framework and sounding data from the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) as the initial conditions. In addition to the control simulation, 4 sensitivity analyses are conducted by varying the longwave radiative flux (ΔFn) across the boundary layer, thereby varying the maximum permitted radiative cooling. These radiative cooling values are constrained through a radiative-transfer calculation using all the VOCALS sounding data and liquid water path retrievals. The magnitude of radiative cooling is shown to impact boundary layer properties such as stability, cloud-top height, cloud-cover, and precipitation. For all simulations, the top-down mechanism of radiative cooling dominates over the first few hours, as downdrafts penetrate lower and lower and destabilize the boundary layer to deep-layer circulations, which in these simulations are largely surface-based cumulus-like updrafts. The simulations with stronger ΔFn undergo this transition sooner and exhibit higher cloud-top heights, stronger overall turbulence, increased precipitation, and a better-sustained cloud cover. Increases in precipitation with stronger radiative forcing arise largely through increases in precipitation area rather than intensity of drizzle cells. Simulation behavior of the transition is found to be broadly consistent with the VOCALS sounding composites and exhibits similar self-limiting drizzle behavior found in the observations.