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

2007 2008 2009

Run off

Evap

otra

nspi

ratio

n

q0

qn

qi

Influence of the soil drainage boundary condition on land surface fluxes in a general circulation modelAurélien 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 GCMORCHIDEE 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.

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 ORCHIDEEIt 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 conditionsThe 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 diffusivityθ : Volumetric soil contentqi: 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.

IV) Impact of F on soil moisture content (m3/m3 )

0

2

0

2Z(m)

0

2Z(m)

0

2Z(m)

0

2Z(m)

t

t

t

t

t

Lithology of SIRTA Loam Clay Sand Clay

Météo France stationsIII) ObservationsTo 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

Diurnal cycle in 2007-2009Sensible Flux (W/m²)Latent Flux (W/m²)Temperature at 2m (C)

SIRTA

10 km

SIRTA

Diurnal cycle in 2007-2009Sensible Flux (W/m²)Latent Flux (W/m²)Temperature at 2m (C)

V) Impact of F on surface fluxes and air temperature