Cloud-Top Entrainment Instability in Marine Stratocumulus Clouds: Observational Evidence From Collocated Microphysical, Turbulence, and Radiation Measurements

Abstract

Marine stratocumulus clouds (MSC) strongly influence Earth's radiation budget, yet the mechanisms governing the descent of entrainment-affected (diluted) parcels, and the relative roles of cloud-top entrainment instability (CTEI) and longwave radiative cooling (RC), remain debated. Using helicopter-borne observations from the ACORES campaign that combine high-resolution in situ vertical profiling with co-located remote sensing, we examine vertical variations of microphysics, thermodynamics, and the entrainment interfacial layer (EIL). When CTEI conditions were strongly met, inhomogeneous mixing (IM) traits appeared near cloud top and transitioned to homogeneous mixing (HM) traits deeper in the cloud layer, accompanied by localized elevations of cloud base, signatures consistent with enhanced descent of diluted parcels. We argue that these apparent HM traits arise from adiabatic warming and evaporation during descent rather than true HM. When CTEI was weakly met or not met, IM traits near the top were weaker, HM traits emerged deeper in the cloud, and cloud base elevation was not observed; these differences are explained by RC-driven buoyancy contrasts modulated by turbulence and EIL thickness. Even in such cases, diluted parcels descended, but weakly. Integrating these results with prior field studies, we provide observational evidence that sufficiently strong CTEI can dominate RC and drive diluted-parcel descent, clarifying how CTEI, RC, and EIL thickness jointly shape MSC structure and offering guidance for improved representation in weather and climate models.