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

Abstract

AbstractCoherent 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 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.