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TURBULENT FLUX VARIABILITIES OVER THE ARA WATERSHED. Moussa Doukouré, Sandrine Anquetin, Jean-Martial Cohard Laboratoire d’étude des Transferts en Hydrologie et Environnement (LTHE) Grenoble, France. INTRODUCTION. - PowerPoint PPT Presentation
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TURBULENT FLUX VARIABILITIES OVER THE ARA WATERSHED
Moussa Doukouré, Sandrine Anquetin, Jean-Martial Cohard
Laboratoire d’étude des Transferts en Hydrologie et Environnement (LTHE)
Grenoble, France
INTRODUCTION
The determination of turbulent fluxes is very important for the closure of energy balance equation.
In an agrometeorological point of view :
Assessment of the evapotranspiration term that quantifies water needed by plants
► Evaluate water scarcity and agricultural strategy.
In an hydrological point of view :
Close the water balance at the watershed scale
► Evaluate the water ressources avalaible for both population needs and energy production
Usual technical measurement
- Point measurement
Difficulties in measuring
Surface heterogeneities (temperature, soil humidity, topography) and atmospheric state generate secondary circulation and rolls that make difficult the accurate measurement of turbulent fluxes with conventional measurements (EC)
Simulated water vapour mixing ratio over Hamdallaye watershed (Niger)
Simulated organized turbulent structures
INTRODUCTION
(Steinfeld et al, 2006)
(Lothon et al,2007)
Problem of averaged data representative of the surfaces
Problem of flux sources control for more acurate analyses
Better estimation from the time and space average than from point measurement
Need to characterize the variability below the LAS path, wind direction and stability parameter
►Take into account the 3 dimensionnal (3D) behavior of the turbulent fluxes
► Analyse the footprint
Another concept of measurement: Large Aperture Scintillometer (LAS)
Savannah bushes
Savannah trees
Cultures and/or bare soils
Case Study : The ARA watershed (~12 km2) in Nalohou
(North part of Benin)
INTRODUCTION
Also a problem of representativeness !!!
Topography, atmospheric and ground forcings for a case study of 10/04/2006
►Use Meso-NH model (Lafore et al, 1998)
Use of atmospheric 3D model including Large-Eddy Simulation (LES) approach can resolve part of these problems.
Use of an atmospheric model coupled with land surface scheme
METHODOLOGY
Perform sensitivity studies
► Impact on the turbulent flux variabilities
1.23 2.23
10.36
9.36
1.891.399.53
10.03
1.771.44
9.91
9.58
9.83
9.611.48 1.70
Δx= Δy=18km
Cyclic boundary conditions
Δx= Δy=6km
Open boundary conditions
Δx= Δy=1km
Open boundary conditions
Δx= Δy=2km
Open boundary conditions
Δx= Δy=250m
Open boundary conditions
1.559.72
9.79
1.63
Altitude (m)
MODELING STRATEGY 5 nested domains (Two-way)
Topography - SRTM 90m
ECMWF radiation scheme
SURFEX soil-vegetation-atmospheric-transfer model (Noilhan and Planton, 1989)ECOCLIMAP (Masson et, al 2003) (1km2) for vegetation parameters
LES, sub-grid paramerization of Deardorff
Clear sky conditions
MODELING STRATEGY
Homogeneous sahelian woodland
Spot vegetation at 20m resolution ( Zin et al, 2009)
Can ECOCLIMAP be usefull for our study ?
Initialization : sounding at Parakou,10/04/2006 at 10.30 AM
MODELING STRATEGY
virtual potential temperature at 10.30 day 10/04/2009
393
1393
2393
3393
4393
5393
307 312 317 322 327
Virtual potentiail temperature (°K)
Alt
itu
de
(m)
Vertical profil of v at 10:30, 10/04/2006
10/04/2006
10.30 AM11/04/200
6 10.30 AM
Extraction of data at each time step.
20 minutesSimulation strategy
24h simulation, diurnal cycle, radiation scheme activeradiation scheme
OFF
PRELIMINARY RESULTS: WIND ROTATION
Initial South-Westerly wind forcing Final North and North-West wind
Δx= Δy=18km
Cyclic boundary conditions
Dynamic forcing no maintained
PRELIMINARY RESULTS: FLUX VARIABILITY OVER CATCHMENT SCALE
Turbulent latent heat flux
Turbulent sensible heat flux
► Highest values on the crests and advected in lower zones
W/m2
W/m2
PRELIMINARY RESULTS: IMPACT OF THE TOPOGRAPHY ON THE FLUX VARIABILITIES
m.s-1
m.s-1
Vertical wind over flat terrain
Vertical wind over the « real » topographySame meteorological forcings !
Vertical wind organized as convective rolls
Vertical motion organized according to orographic structures
► Highest values over flat terrain rather than over « real » topography.
► Vertical motion modified by the orography structure
PRELIMINARY RESULTS: SCINTILLOMETER FOOPRINT ANALYSIS
Latent heat flux over flat terrainLatent heat flux with topography
Min : 122 W/m2Max : 137 W/m2Mean : 127 W/m2Std : 4.4 W/m2
Min : 153 W/m2Max : 169 W/m2Mean : 163 W/m2Std : 4.4 W/m2
Latent Heat Flux « seen » by the scintillometer
The local wind has an impact on the shape of the footprint
CONCLUSION AND PRESPECTIVES
Improve the vegetation cover description by using satellite analysis (Zin et al., 2009)
►
- Wind rotation probably due to cyclic boundary condition
- High values of fluxes observed over crests and advected into the lower zones
- Wind circulation influenced by the horizontal gradient of turbulent fluxes
Conclusion
Perspectives
Attempt to improve simulations with OPEN boundary condition that is better to maintain wind direction
ECOCLIMAP Satelite data
Probably use of ECMWF reanalysis as forcing data !!!
Many thanks !