Proceedings of the HYDRALAB III Joint User Meeting, Hannover, February 2010 INVESTIGATION OF THE VERTICAL EDDY FLUX OF MOMENTUM UNDER STABLE CONDITIONS IN THE SURFACE BOUNDARY LAYER OVER LAND USING CNRS- TOULOUSE STRATIFIED WATER FLUME Dan Dobrovolschi(1), Gert-Jan Steeneveld(2), Alexandre Paci(3), O. Eiff(4), L. Lacaze(4) (1)National Meteorological Administration, Romania, E-mail: dandobrov@gmail.com, (2)Wageningen University, The Netherlands, E-mail: Gert-Jan.Steeneveld@wur.nl, (3)CNRM-GAME (URA1357 METEO-FRANCE and CNRS), France, E-mail: Alexandre.Paci@meteo.fr, (4)Institut de Mécanique des Fluides de Toulouse, France, E-mail: eiff@imft.fr, lacaze@imft.fr This study reports on laboratory experiments in the CNRM-GAME (Toulouse) stratified water flume of a stably stratified boundary layer, in order to innovatively quantify and improve formulations for turbulent heat and momentum transfer for use in Numerical Weather Prediction and climate models. 1. INTRODUCTION Understanding and prediction of atmospheric stably stratified boundary layers (SBL) is a longstanding challenge in the field of meteorology. In the atmosphere, SBL are particularly complex due to many different processes that play a role at night. These are amongst others turbulence (sometimes intermittent) (Holtslag and Nieuwstadt, 1986), gravity waves (Nappo 2002, Steeneveld et al, 2009), radiative transport (Steeneveld et al, 2010), drainage flows, mesoscale meandering (Mahrt, 2008) etc. As a result, interpretation of field observations is not straightforward, and parameterization development is hampered. Consequently the forecasting skill of numerical weather prediction (NWP) models and climate models is rather limited for winter and nocturnal conditions. Typical errors consists of an overestimated screen level temperature and near surface wind speed. This has evident consequences for end users in transportation (Gultepe et al., 2009), agriculture (Prahba and Hoogenboom, 2008), air quality forecasting (Schaap et al., 2009) and for the current understanding of the Earths climate system (e.g. Dethloff et al, 2001). This is further illustrated in Fig 1 which shows the winter screen level temperature bias for two mainstream climate models. It is evident that both models show a bias up to 6 K in northern regions, although of opposite sign. However, observations show that especially these regions seems to be vulnerable to climate change (Bony et al, 2006), but apparently the physics are insufficiently understood to model the polar climate. To understand the SBL, consider the heat budget at the surface: G E L H Q v + + = * in which Q* is the net radiation; H the turbulent sensible heat flux, LvE the turbulent latent heat flux, and G the soil heat flux. Calculation of H is a very important component of a boundary-layer parameterization scheme in NWP and climate models (e.g. Garratt, 1992). Many different surface layer parameterizations have been proposed (e.g. Sorbjan, 1989; Garratt, 1992; Poulos and Burns 2003; Pleim 2006). Of these, the classical Louis (1979, 1982) for determining the vertical eddy fluxes of momentum and sensible heat, and its subsequent developments (e.g. Kot and Song, 1998; Poulos and Burns, 2003), are often used in these models. At present, several models of Météo-France − such as: the NWP global model ARPÈGE and its limited area model version ALADIN (Josse, 2004), the atmospheric dispersion model MEDIA (Piedelievre, 1990), the global chemistry transport model MOCAGE (Stockwell and Chipperfield, 1999) - use the classical Louis scheme. See also Chen et al. (1997) and Olivié et al. (2004) for other models using this scheme. The basic advantage of Louis scheme is computational efficiency (simplicity in formulation, and non-iterativeness). However, the scheme behaves unsatisfactorily for strong stability Troen and Mahrt (1986), e.g. in the SBL for calm conditions. This behaviour originates from the assumptions of Louis scheme: (1) the aerodynamic roughness length z0 and the heat transfer roughness length z0T are equal,