Coherent subsiding structures in large-eddy simulations of atmospheric boundary layers

Abstract

Coherent structures are characterized in high-resolution simulations of three atmospheric boundary layers: dry convection, marine cumulus, and stratocumulus. Based on radioactive-decaying tracers emitted at different altitudes (surface, top of well-mixed layer, and cloud top), an object-oriented methodology allows individual characterization of coherent tridimensional plumes within the flow. Each boundary layer shows updraft structures surrounded by subsiding shells that have similar thermodynamical characteristics. Well-mixed downdrafts are located relatively close to updrafts and entrain dry, warm air from the free troposphere. While updrafts primarily carry the majority of heat and moisture within well-mixed layers, accounting for 62-70% of the total resolved flux, it is noteworthy that well-mixed downdrafts also contibute a significant portion, ranging from 14% to 35%. Identified in all boundary layers, these subsiding structures are triggered by air mass convergence linked to updrafts' divergence and are thus part of an overturning circulation in well-mixed layers. Close to the surface, downdrafts' divergence constrain updrafts' locations and thus shape a mesoscale cellular organization with cell sizes scaling with the boundary-layer height (aspect ratio of around 2). Active cumulus formation does not strongly perturb the spatial organization of the sub-cloud well-mixed layer. The stratocumulus-topped boundary layer also shares similarities with the overturning circulation despite having condensation and cloud-radiation diabatic effects within the mixed layer. However, the visible mesoscale organization of stratocumulus shows larger cells than the boundary-layer depth (aspect ratio (Formula presented.) 10) that suggest deviations from the clear-sky conceptual view. The boundary-layer decoupling influences mass fluxes of coherent structures and thus potentially plays a role in shaping the spatial organization. Since well-mixed downdrafts contribute to a significant part of resolved flux of heat and moisture, our results suggest that downdraft properties in well-mixed layers should be represented at the subgrid scale in climate models through non-local mass-flux parametrizations.