Cloud climate feedbacks: What cloud-system resolving simulations tell about marine non-precipitating Sc response to wind speed

Description

Observed and projected trends in large scale wind speed over the oceans prompt the question: How might marine stratocumulus (Sc) clouds and their radiative properties respond to future changes in wind speed? Climate models do not have the resolution required to answer this question so we use high resolution cloud-system resolving simulations (up to horizontal resolutions of dx = dy = 75 m). For changes in large scale wind speed bounding the observed long-term variability in the late 20th century and forecasts for the 21st century, we find that liquid water path (LWP) exhibits a non-monotonic response to wind speed. It is free tropospheric (FT) humidity which determines whether or not the response of LWP to wind speed is monotonic: Low FT moisture allows enhanced short-wave (SW) cooling, growth, and entrainment of FT air into the boundary layer (non-monotonic response), while a moist FT suppresses SW cooling, boundary layer growth, and entrainment (monotonic response). We find that entrainment, although a small scale process, is driven by cell-scale circulation. A sufficiently large simulation domain is hence required to adequately represent entrainment, and to obtain the non-monotonic response of LWP to wind speed. The non-monotonic LWP response to wind speed is not sensitive to resolution (in the considered resolution range). For modeling cloud-climate feedbacks, depending on conditions, domain size should not be sacrificed for increased resolution, and both a sufficiently high resolution and a sufficiently large domain may be necessary to represent entrainment.