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Volume 15, issue 15
Atmos. Chem. Phys., 15, 9031–9047, 2015
https://doi.org/10.5194/acp-15-9031-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

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

Atmos. Chem. Phys., 15, 9031–9047, 2015
https://doi.org/10.5194/acp-15-9031-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Aug 2015

Research article | 14 Aug 2015

Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study

C. Román-Cascón1, C. Yagüe1, L. Mahrt2, M. Sastre1, G.-J. Steeneveld3, E. Pardyjak4, A. van de Boer3, and O. Hartogensis3 C. Román-Cascón et al.
  • 1Dept. de Geofísica y Meteorología, Universidad Complutense de Madrid, Madrid, Spain
  • 2NorthWest Research Associates, Corvallis, OR, USA
  • 3Meteorology and Air Quality Section, Wageningen University, Wageningen, the Netherlands
  • 4Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, USA

Abstract. The interactions among several stable-boundary-layer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.

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Short summary
Stable-boundary-layer processes have been analysed using BLLAST data. Shallow drainage flows were formed at some locations after the near calm stage of the late afternoon. This stage ended with the arrival of a deeper wind associated with the mountain-plain circulation. At the same time, gravity waves were detected with an array of microbarometers. The interaction of these processes with turbulence was studied through multi-resolution flux decomposition at different sites and heights.
Stable-boundary-layer processes have been analysed using BLLAST data. Shallow drainage flows...
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