Persistent decoupling weakens marine stratocumulus, with specific humidity inversions modulating microphysical and radiative responses

Abstract. Marine stratocumulus cloud (MSC) evolution is strongly influenced by boundary-layer coupling and turbulent mixing. However, under decoupled conditions, the impacts of specific humidity inversions (SHIs) on in-cloud mixing and microphysical evolution remain poorly constrained by in situ observations. Here, 12 persistently decoupled MSC cases from six flights during VOCALS-REx, POST, and ACE-ENA are analyzed to examine cloud microphysical, mixing, and radiative evolution with and without SHIs. Persistent decoupling generally suppresses MSC maintenance, reducing cloud thickness, liquid water content (LWC), mean droplet radius (r_m), and droplet spectral width. Without SHIs, dry air entrainment drives substantial upper-cloud LWC depletion under inhomogeneous mixing (IM)-dominated conditions, characterized by preferential evaporation of small droplets and super-adiabatic droplet formation, reducing cloud optical thickness (τ), albedo, and shortwave cloud radiative forcing. Conversely, when SHIs are present, moist air entrainment suppresses evaporative losses and modifies the microphysical consequences of IM by favoring collision–coalescence growth. Consequently, droplet spectra broaden toward larger sizes, super-adiabatic droplets form more readily, cloud dissipation is mitigated, and shortwave radiative forcing can be maintained or enhanced. Under sustained cloud-top moisture supply, SHIs can even support cloud maintenance or redevelopment. However, when droplet growth becomes sufficiently strong to promote precipitation-related cloud-water loss, cloud water and optical thickness may decline despite SHIs. Collectively, the observations suggest that MSCs under persistently decoupled boundary-layer conditions may follow distinct evolutionary pathways, including evaporative dissipation, moisture-driven cloud maintenance or redevelopment, and, in some cases, precipitation-related dissipation. These pathways have important implications for low-cloud shortwave radiative feedbacks in climate models.