Articles | Volume 15, issue 8
Atmos. Chem. Phys., 15, 4241–4257, 2015

Special issue: The Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST)...

Atmos. Chem. Phys., 15, 4241–4257, 2015

Research article 24 Apr 2015

Research article | 24 Apr 2015

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

H. P. Pietersen1, J. Vilà-Guerau de Arellano1, P. Augustin2, A. van de Boer1, O. de Coster1, H. Delbarre2, P. Durand3, M. Fourmentin2, B. Gioli4, O. Hartogensis1, F. Lohou3, M. Lothon3, H. G. Ouwersloot5, D. Pino6, and J. Reuder7 H. P. Pietersen et al.
  • 1Meteorology and Air Quality Section, Wageningen University, Wageningen, the Netherlands
  • 2Laboratoire de Physico-Chimie de l'Atmosphère CNRS, Université du Littoral Cote d'Opale Dunkerque, France
  • 3Laboratoire d'Aérologie, Université de Toulouse and CNRS, Lannemazan, France
  • 4Institute of Biometeorology, IBIMET CNR, Florence, Italy
  • 5Atmospheric Chemistry, Max-Planck Institute for Chemistry, Mainz, Germany
  • 6Department of Applied Physics, Universitat Politècnica de Catalunya, BarcelonaTech, and Institute for Space Studies of Catalonia (IEEC-UPC), Barcelona, Spain
  • 7Geophysical Institute, University of Bergen, Bergen, Norway

Abstract. 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.

Final-revised paper