The Role of Precipitation in Controlling the Transition from Stratocumulus to Cumulus Clouds in a Northern Hemisphere Cold-Air Outbreak

Abstract

Aircraft observations in a cold-air outbreak to the north of the United Kingdom are used to examine the boundary layer and cloud properties in an overcast mixed-phase stratocumulus cloud layer and across the transition to more broken open-cellular convection. The stratocumulus cloud is primarily composed of liquid drops with small concentrations of ice particles and there is a switch to more glaciated conditions in the shallow cumulus clouds downwind. The rapid change in cloud morphology is accompanied by enhanced precipitation with secondary ice processes becoming active and greater thermodynamic gradients in the subcloud layer. The measurements also show a removal of boundary layer accumulation mode aerosols via precipitation processes across the transition that are similar to those observed in the subtropics in pockets of open cells. Simulations using a convection-permitting (1.5-km grid spacing) regional version of the Met Office Unified Model were able to reproduce many of the salient features of the cloud field although the liquid water path in the stratiform region was too low. Sensitivity studies showed that ice was too active at removing supercooled liquid water from the cloud layer and that improvements could be made by limiting the overlap between the liquid water and ice phases. Precipitation appears to be the key mechanism responsible for initiating the transition from closed- to open-cellular convection by decoupling the boundary layer and depleting liquid water from the stratiform cloud.

Cloud Ice Processes Enhance Spatial Scales of Organization in Arctic Stratocumulus

“…Generally, our results support findings by Abel et al (2017) in the sense that ice processes induce a cloud deck thinning and a breakup in organization. Conceptually, we show that cloud ice strengthens subcloud precipitation through sedimenting snow, ice, and graupel (as summarized in Figure 4).…”

“…However, their aerosol concentrations are much larger than observed for the Arctic environment and in deep convective clouds latent heating from condensation and turbulence are stronger than in shallow stratocumulus. Moreover, the impact of prognostic CCN, where collision-coalescence and sedimentation processes can clean out the boundary layer (Abel et al, 2017;Wood et al, 2008) should be investigated in more detail. Generally, our results agree with their study, but we find a shorter-term response in cloud base precipitation intensity and cell size induced by CCN concentration changes as opposed to ice phase processes.…”

“…Cloud types span a multitude of thermodynamic states. Cloud ice increases precipitation (Field & Heymsfield, 2015;Knight et al, 2002), which in turn is crucial for regime transitions and cloud dissipation (as shown for a cold air outbreak in model simulations by Abel et al, 2017). Even though mixed-phase clouds (MPCs) are thermodynamically unstable, they occur worldwide, mainly in areas of deep convection (Rosenfeld & Woodley, 2000), mountainous terrain (Lohmann et al, 2016), or the polar latitudes (Morrison et al, 2011;Shupe et al, 2006).…”

“…The latter has been found to be a key component of evaporative cooling, cold pool formation and a driver of cloud organization (Haerter et al, 2017;Haerter & Schlemmer, 2018;Seifert & Heus, 2013;Xue et al, 2008). In high latitudes they have so far been attributed to frontal features in low-pressure systems (Naud et al, 2016) or cold air outbreaks (Abel et al, 2017). In high latitudes they have so far been attributed to frontal features in low-pressure systems (Naud et al, 2016) or cold air outbreaks (Abel et al, 2017).…”

Relating large-scale subsidence to convection development in Arctic mixed-phase marine stratocumulus

“…S3-S5). Observational studies of Arctic mixed-phase marine Sc (Young et al, 2016) and North Atlantic CAOs (Abel et al, 2017) have previously reported precipitation as snow below cloud with little rain measured, indicating that our idealised study is in broad agreement with measurements in this region.…”