1. Soil moisture content at SIRTA ( m 3 /m 3 ) at different depths. SIRTA’s data has been transformed to have the same amplitude as ORCHIDEE’s simulation 2007 2008 2009 Run off Evapotranspiration Precipitation q0q0 qnqn qiqi Influence of the soil drainage boundary condition on land surface fluxes in a general circulation model Aurélien Campoy (1), Agnès Ducharne (1), Fréderic Hourdin (2), Frédérique Cheruy (2), Jan Polcher (2), Martial Mancip (2), Laurent Fairhead (2), Martial Haeffelin (2), and Jean-Charles Dupont (2) (1: UMR Sisyphe, Université Pierre et Marie Curie Paris6/CNRS ) aurelien.campoy@upmc.fr (2: Laboratoire de météorologie dynamique, IPSL/CNRS) Coupling with a GCM ORCHIDEE is coupled to a the atmospheric GCM, LMDZ, which described the evolution of the different variables X for each grid box and each time step: Zoom on the SIRTA site where horizontal resolution drops to 100x100 km² Nudging to better describe synoptic meteorology: the large-scale circulation is adjusted toward ECMWF analyses X a The power of this nudging is controlled by a time constant τ, which is maximum at the center of the zoomed area and decreases as the grid resolution decreases. Here, we nudge the model on winds and temperatures. I) Introduction The goal of this work is to assess the performances of the coupled model LMDz/ORCHIDEE, by comparison with data from the instrumented site SIRTA (located in Palaiseau, near Paris, France). We try to consider the presence of a water table near the surface. II) The land surface model ORCHIDEE It uses a classical SVAT approach for the water and energy budget. The subgrid heterogeneity of the vegetation described by the mosaic approach. ORCHIDEE has a new physically-based hydrology that resolves the Fokker-Plank equation to simulate the soil moisture variations in the first 2 meters of ground. Boundary conditions The top water flux results from the water exchange with the atmosphere. The bottom flux is a gravitational drainage modulated by a factor F. K: hydraulic permeability D: hydraulic diffusivity θ : Volumetric soil content q i : Fluxes from layer i to i+1 Equation of fluxes between soil layers VI) Conclusion ORCHIDEE-LMDZ reproduces better the soil-atmosphere continuum at the SIRTA site and at the grid cell scale using an impermeable boundary condition (F=0) The choice of F=0 is consistent with local evidences of a shallow water table The soil is wetter, and better able to meet the evaporation demand This reduces the overestimation of both sensible heat flux and atmospheric temperature, especially in summer when the bias is the most important The simulated precipitation is also slightly improved in summer. Further work is required to understand the appropriate range for F at the grid scale We will also examine the influence of F at the global scale in the coupled model LMDZ-ORCHIDEE without zoom nor nudging. F=1 F=0.1 F=0.01 F=0.001 F=0 IV) Impact of F on soil moisture content (m 3 /m 3 ) Satured 0 2 0 2 Z(m) 0 2 0 2 0 2 t t t t t Lithology of SIRTA Loam Clay Sand Clay Météo France stations III) Observations To validate the simulations, we compared it to: -surface fluxes and soil moisture data measured at the SIRTA experimental site. -The average of 8 Météo- France stations in the SIRTA grid cell Grid box Water fluxes in ORHIDEE The 3D mesh of LMDZ Time constant τ on the LMDZ’s zoomed mesh summer Diurnal cycle in 2007-2009 winter Sensible Flux (W/m²)Latent Flux (W/m²)Temperature at 2m (C) SIRTA 10 km SIRTA Diurnal cycle in 2007-2009 Sensible Flux (W/m²) Latent Flux (W/m²) Temperature at 2m (C) V) Impact of F on surface fluxes and air temperature