On the role of large-scale forcings on the development of the atmospheric boundary layer during the BLLAST field campaign

Guided and constrained by a complete data set of surface and upper-air observations taken during the fifth Intensive Observational Period (IOP-05, 25th June 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) experiment, we reconstruct the evolution of the atmospheric boundary layer (ABL) using mixed-layer theory. The model results are corroborated by UHF radar and multiple profiles done by unmanned and manned aircraft platforms as well as by radiosondes. During the first morning hours, the ABL is mainly controlled by surface forcing, reaching a depth between 500 and 600 meter. At these levels, large-scale subsidence motions become similar in magnitude to the entrainment velocity and the ABL-growth becomes nearly zero. This equilibrium is maintained until the afternoon transition where mixed-layer model results and observations show a decrease of the boundary layer depth in spite of well mixed observed vertical profiles of heat and moisture. We discuss potential explanations to the development of subsidence motions associated to mesoscale flows driven by the proximity of the Pyrenees mountain range and to large-scale forcing. The heat and moisture budgets are also further analyzed with the mixed layer model. We find strong indications of the active role of heat and moisture advection required to reproduce the observed diurnal variability in temperature and specific humidity. By quantifying the budget terms, we are able to obtain a first estimation of the contribution of advection to the heat and moisture budgets. To complete the analysis, we extend the study to examine the turbulent characteristics of the ABL by using aircraft measurements combined with large-eddy simulations. In so doing, we are able to study the role of wind (directional) shear in the development and maintenance of the ABL. IOP05 was characterized by easterly flows within the ABL and westerly winds aloft. Our findings stress the important role of large scale forcing in understanding the ABL development at various stages during BLLAST. The proposed method can also be very useful to support the further interpretation of observations and mesoscale model experiments.