Journal Article

Study of a prototypical convective boundary layer observed during BLLAST: contributions by large-scale forcings

de Coster, O., Pietersen, H. P., Hartogensis, O., Augustin, P., Delbarre, H., Pino, D., Ouwersloot, H. G., Lohou, F., Vilà-Guerau de Arellano, J., Reuder, J., Fourmentin, M., Gioli, B., van de Boer, A., Durand, P., Lothon, M.

2015

Overview

We study the influence of the large-scale atmospheric contribution to the dynamics of the convective boundary layer (CBL) in a situation observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. We employ two modeling approaches, the mixed-layer theory and large-eddy simulation (LES), with a complete data set of surface and upper-air atmospheric observations, to quantify the contributions of the advection of heat and moisture, and subsidence. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70%. Constrained by surface and upper-air observations, we infer the large-scale vertical motions and horizontal advection of heat and moisture. Our findings show that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. LES results show a satisfactory correspondence of the vertical structure of turbulent variables with observations. We also find that when large-scale advection and subsidence are included in the simulation, the values for turbulent kinetic energy are lower than without these large-scale forcings. We conclude that the prototypical CBL is a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are representative for the local boundary layer.

• Publisher: Copernicus GmbH,Copernicus Publications
• Subject: Analysis / decay / Dynamic meteorology / entrainment / large-eddy simulation / radiation / transition / turbulence / Turbulence (Fluid dynamics)