The convective nature of Stratocumulus topped boundary layers (STBL) involves the motion of updrafts and downdrafts, driven by surface fluxes and radiative cooling, respectively. The balance between shear and buoyant forcings at the surface can determine the organization of updrafts between cellular and roll structures. We investigate the effect of varying shear at the surface and top of the STBL using LES simulations, taking DYCOMS II RF01 as a base case. We focus on spatial identification of the following features: coherent updrafts, downdrafts, and wet updrafts, and observe how they are affected by varying shear. Stronger surface shear organizes the updrafts in rolls, causes less well-mixed thermodynamic profiles, and decreases cloud fraction and LWP. Stronger top shear also decreases cloud fraction and LWP more than surface shear, by thinning the cloud from the top. Features with stronger top than surface shear are associated with a net downward momentum transport and show early signs of decoupling. Classifying updrafts and downdrafts based on their vertical span and horizontal size confirms the dominance of large objects spanning the whole STBL. Large objects occupy 14% of the volume in the STBL while smaller ones occupy less than 1%. For updraft and downdraft fluxes these large objects explain 33% of the vertical velocity variance and 53% of the buoyancy flux, on average. Stronger top shear also weakens the contribution of downdrafts to the turbulent fluxes and tilts the otherwise vertical development of updrafts.

Coastal Stratocumulus clouds (Sc) can greatly impact the available solar radiation in areas like southern California. These clouds usually dissipate during the morning, as surface fluxes increase in the day. The dissipation process begins by the thinning of the cloud layer, that then becomes a broken cloud field until the clouds completely dissipate. A better understanding of the process could improve predictions of dissipation time and solar variability for this type of clouds. We investigate the evolution of a broken cloud field using LES simulations of coastal Sc clouds over Southern California (Ghonima et al, JAS 2016). We identify and characterize coherent updrafts, downdrafts, and cloud elements using flow field variables (vertical velocity and liquid water mixing ratio). The cloud layer is observed to become a broken cloud field around hour 8 of the simulation, after which the thinning tendency strongly diminishes and the variability of cloud fraction, cloud thickness, and liquid water path suddenly increases. The few cloud elements remaining at noon are found to be located close to cloud base, unlike at night, when they are distributed throughout the whole cloud layer. The largest updrafts and downdrafts contribute to a large portion of the turbulent fluxes along the day (on average, 40% of vertical velocity variance and 48% of the buoyancy flux), and their relative contribution changes during the day as updrafts become stronger and downdrafts weaken around noon.