Resolved versus parametrized boundary-layer plumes. Part I: A parametrization-oriented conditional sampling in large-eddy simulations

A conditional sampling based on the combination of a passive tracer emitted at the surface and thermodynamic variables is proposed to characterise organized structures in large-eddy simulations of cloud-free and cloudy boundary layers. The sampling is evaluated against more traditional sampling of dry thermals or clouds. It enables the characterization of convective updrafts from the surface to the top of the boundary layer (or the top of cumulus clouds), describing in particular the transition from the sub-cloud to the cloud layer, and retrieves plume characteristics, entrainment and detrainment rates, variances and fluxes. This sampling is used to analyze the contribution of boundary-layer thermals to vertical fluxes and variances.