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Volume 15, issue 17 | Copyright

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

Atmos. Chem. Phys., 15, 10071-10086, 2015
https://doi.org/10.5194/acp-15-10071-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Sep 2015

Research article | 09 Sep 2015

Turbulence vertical structure of the boundary layer during the afternoon transition

C. Darbieu1, F. Lohou1, M. Lothon1, J. Vilà-Guerau de Arellano2, F. Couvreux3, P. Durand1, D. Pino4,5, E. G. Patton6, E. Nilsson1,7, E. Blay-Carreras4, and B. Gioli8 C. Darbieu et al.
  • 1Laboratoire d'Aérologie, Toulouse, CNRS UMR 5560, Université de Toulouse, Toulouse, France
  • 2Meteorology and Air Quality Section, Wageningen University, Wageningen, the Netherlands
  • 3CNRM-GAME (Météo-France and CNRS), Toulouse, France
  • 4Department of Applied Physics, Universitat Politècnica de Catalunya, BarcelonaTech, Barcelona, Spain
  • 5Institute of Space Studies of Catalonia (IEEC-UPC), Barcelona, Spain
  • 6National Center for Atmospheric Research, Boulder, Colorado, USA
  • 7Uppsala University, Uppsala, Sweden
  • 8Institute of Biometeorology, National Research Council (IBIMET-CNR), Florence, Italy

Abstract. We investigate the decay of planetary boundary layer (PBL) turbulence in the afternoon, from the time the surface buoyancy flux starts to decrease until sunset. Dense observations of mean and turbulent parameters were acquired during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment by several meteorological surface stations, sounding balloons, radars, lidars and two aircraft during the afternoon transition. We analysed a case study based on some of these observations and large-eddy simulation (LES) data focusing on the turbulent vertical structure throughout the afternoon transition.

The decay of turbulence is quantified through the temporal and vertical evolution of (1) the turbulence kinetic energy (TKE), (2) the characteristic length scales of turbulence and (3) the shape of the turbulence spectra. A spectral analysis of LES data, airborne and surface measurements is performed in order to characterize the variation in the turbulent decay with height and study the distribution of turbulence over eddy size.

This study highlights the LES ability to reproduce the turbulence evolution throughout the afternoon. LESs and observations agree that the afternoon transition can be divided in two phases: (1) a first phase during which the TKE decays at a low rate, with no significant change in turbulence characteristics, and (2) a second phase characterized by a larger TKE decay rate and a change in spectral shape, implying an evolution of eddy size distribution and energy cascade from low to high wave number.

The changes observed either in TKE decay (during the first phase) or in the vertical wind spectra shape (during the second phase of the afternoon transition) occur first in the upper region of the PBL. The higher within the PBL, the stronger the spectra shape changes.

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A case study of the BLLAST experiment is considered to explore the decay of turbulence that occurs in the convective boundary layer over land during the afternoon. Based on observations and on a large-eddy simulation, the analysis reveals two phases in the afternoon: a first quasi-stationary phase when the turbulent kinetic energy slowly decays without significant change in the turbulence structure and a second phase of more rapid decay with a change in spectral turbulence characteristics.
A case study of the BLLAST experiment is considered to explore the decay of turbulence that...
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