Soil moisture content at SIRTA ( m 3 /m 3 ) at different depths.

1. Influence of the soil drainage boundary condition on land surface fluxes in a general circulation model AurélienCampoy (1), AgnèsDucharne (1), FrédericHourdin (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 météorologiedynamique, IPSL/CNRS) Satured Lithology of SIRTA Loam Clay Sand Clay I) IntroductionThe 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. Soil moisture content at SIRTA (m3/m3 ) at different depths. SIRTA’s data has been transformed to have the same amplitude as ORCHIDEE’s simulation 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. Water fluxes in ORHIDEE Precipitation Evapotranspiration IV) Impact of F on soil moisture content (m3/m3 ) Run off 0 F=1 q0 2 t F=0.1 Equation of fluxes between soil layers Z(m) 0 qi K: hydraulic permeability D: hydraulic diffusivity θ : Volumetric soil content qi: Fluxes from layer i to i+1 2007 2008 2009 2 t F=0.01 qn Z(m) 0 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 Xa 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. 2 t Z(m) F=0.001 0 2 t F=0 The 3D mesh of LMDZ Z(m) 0 2 t Time constant τ on the LMDZ’s zoomed mesh 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 V) Impact of F on surface fluxes and air temperature • 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. Diurnal cycle in 2007-2009 Latent Flux (W/m²) Sensible Flux (W/m²) Temperature at 2m (C) Grid box SIRTA Météo France stations Diurnal cycle in 2007-2009 Temperature at 2m (C) Latent Flux (W/m²) Sensible Flux (W/m²) winter SIRTA summer 10 km