Abstract
Abstract. Large-scale subsidence, associated with high-pressure systems, is often imposed in large-eddy simulation (LES) models to maintain the height of boundary layer (BL) clouds. Previous studies have considered the influence of subsidence on warm liquid clouds in subtropical regions; however, the relationship between subsidence and mixed-phase cloud microphysics has not specifically been studied. For the first time, we investigate how widespread subsidence associated with synoptic-scale meteorological features can affect the microphysics of Arctic mixed-phase marine stratocumulus (Sc) clouds. Modelled with LES, four idealised scenarios – a stable Sc, varied droplet (Ndrop) or ice (Nice) number concentrations, and a warming surface (representing motion southwards) – were subjected to different levels of subsidence to investigate the cloud microphysical response. We find strong sensitivities to large-scale subsidence, indicating that high-pressure systems in the ocean-exposed Arctic regions have the potential to generate turbulence and changes in cloud microphysics in any resident BL mixed-phase clouds.Increased cloud convection is modelled with increased subsidence, driven by longwave radiative cooling at cloud top and rain evaporative cooling and latent heating from snow growth below cloud. Subsidence strengthens the BL temperature inversion, thus reducing entrainment and allowing the liquid- and ice-water paths (LWPs, IWPs) to increase. Through increased cloud-top radiative cooling and subsequent convective overturning, precipitation production is enhanced: rain particle number concentrations (Nrain), in-cloud rain mass production rates, and below-cloud evaporation rates increase with increased subsidence.Ice number concentrations (Nice) play an important role, as greater concentrations suppress the liquid phase; therefore, Nice acts to mediate the strength of turbulent overturning promoted by increased subsidence. With a warming surface, a lack of – or low – subsidence allows for rapid BL turbulent kinetic energy (TKE) coupling, leading to a heterogeneous cloud layer, cloud-top ascent, and cumuli formation below the Sc cloud. In these scenarios, higher levels of subsidence act to stabilise the Sc layer, where the combination of these two forcings counteract one another to produce a stable, yet dynamic, cloud layer.
Highlights
Arctic mixed-phase clouds are long-lived, and widespread single-layer stratocumulus (Sc) decks are common in the autumn, winter, and spring
We investigate the influence of subsidence on a stable cloud, precipitating clouds, and a cloud forced by a warming surface to demonstrate how subsidence can affect a variation of microphysical scenarios common to the Arctic
We suggest that the level of imposed large-scale subsidence can significantly affect the liquid phase in clean mixed-phase Sc, as Wsub positively forces the rain mass production/evaporation rates modelled in these precipitation-favouring microphysical scenarios
Summary
Arctic mixed-phase clouds are long-lived, and widespread single-layer stratocumulus (Sc) decks are common in the autumn, winter, and spring. These clouds are maintained and driven by convection caused by strong radiative cooling at the boundary layer (BL) inversion (e.g. Feingold et al, 2010; Morrison et al, 2012). G. Young et al.: Effect of large-scale subsidence on cloud microphysics issue of premature glaciation of modelled mixed-phase Sc, often concluding that the cause is an overactive ice phase and strong influence of the Wegener–Bergeron–Findeisen (WBF) mechanism. The WBF mechanism causes a constantly changing, unstable microphysical structure; these clouds have been observed to persist for long periods of time, and they have the opportunity to move geographically
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