In-cloud motions shape warm-rain production in marine clouds: Evidence from multiyear observations.

Abstract

Despite years of research, the processes controlling warm-rain formation remain uncertain. Leveraging multiple years of observations collected across four oceanic regions, we systematically reassess the properties of precipitation just beneath cloud base. The observations show that warm-rain characteristics are remarkably consistent within cloud regimes yet differ substantially between stratocumulus and cumulus, indicating that cloud-regime properties exert a stronger influence on warm precipitation than regional environmental variability. Precipitation is observed beneath marine stratocumulus in over 56% of cloudy columns with liquid water path (LWP) <130 g m-2, rising to more than 84% in columns with LWP beyond 130 g m-2. This indicates that marine stratocumulus routinely support warm-rain formation and suggests that barriers to precipitation onset in this regime are weaker than commonly assumed. For the same LWP, oceanic cumulus precipitate less frequently than stratocumulus but on average produce two times larger and one to two orders of magnitude fewer precipitation drops. This contrast is consistent with cumulus presenting two times stronger updrafts and lower adiabaticity than stratocumulus, both of which can affect drop growth histories. Whereas the precipitation characteristics of stratocumulus are generally consistent with growth by collision-coalescence during gravitational free fall in low LWP clouds, those of cumulus cannot be explained without invoking additional processes. Together, these findings suggest that warm-precipitation parameterizations should more explicitly account for the influence of small-scale dynamical processes on precipitation growth. The apparent importance of drop growth histories further motivates continued exploration of Lagrangian particle-tracking approaches in both models and observations.