Detection of Entrainment Influences on Surface-Layer Measurements and Extension of Monin–Obukhov Similarity Theory
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We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution.
KeywordsBoundary-layer scaling Eddy covariance Entrainment Monin–Obukhov similarity theory Surface layer Turbulence
We greatly thank Oscar Hartogensis, Olivier de Coster, Henk Pietersen (Wageningen UR, the Netherlands) and Martin Lennefer (University Bonn, Germany) for their help with EC measurements before, during and after the BLLAST campaign. Furthermore, we thank Christine Moene for assisting in mapping the surroundings of the EC stations. We also thank Miranda Braam for her assistance with data processing and visualizing Fig. 1, and Jordi Vilà-Guerau de Arellano and Huug Ouwersloot (Wageningen UR, the Netherlands) for their critical notes on our study. We thank Fabienne Lohou (Laboratoire d’Aérologie Toulouse, France) and two anonymous reviewers for their critical comments and helpful suggestions. Furthermore, we thank Ned Patton (NCAR Boulder, US) for providing the detailed data from his LES study of 10 years ago. This work was financed by the DFG (Deutsche Forschungsgemeinschaft) project GR2687/3-1 and SCHU2350/2-1; links between local-scale measurements and catchment-scale measurements and modelling of gas exchange processes over land surfaces. The BLLAST field experiment was made possible thanks to the contribution of several institutions and supports: INSU-CNRS (Institut National des Sciences de l’Univers, Centre national de la Recherche Scientifique, LEFE-IDAO program), Météo-France, Observatoire Midi-Pyrénées (University of Toulouse), EUFAR (EUropean Facility for Airborne Research) and COST ES0802 (European Cooperation in the field of Scientific and Technical). The field experiment would not have occurred without the contribution of all participating European and American research groups, which all have contributed in a significant amount (see supports). The BLLAST field experiment was hosted by the instrumented site of Centre de Recherches Atmosphériques, Lannemezan, France (Observatoire Midi-Pyrénées, Laboratoire dAérologie). BLLAST data are managed by SEDOO, from Observatoire Midi-Pyrénées. The MODEM radiosoundings were supported by CNRS, University of Toulouse and the FEDER program (Contract nr. 34172—Development of the instrumentation of Observatoire Midi-Pyrénées—PIRENEA—ESPOIR). The 60-m tower equipment was also supported by CNRS, Université Paul Sabatier and by the European POCTEFA FluxPyr program.
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