TURBULENT FLUX VARIABILITIES OVER THE ARA WATERSHED
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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.

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TURBULENT FLUX VARIABILITIES OVER THE ARA WATERSHED

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Turbulent flux variabilities over the ara watershed

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


Turbulent flux variabilities over the ara watershed

INTRODUCTION

The determination of turbulent fluxes is very important for the closure of energy balance equation.

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

In an agrometeorological point of view :

Assessment of the evapotranspiration term that quantifies water needed by plants

► Evaluate water scarcity and agricultural strategy.

Usual technical measurement

- Point measurement


Turbulent flux variabilities over the ara watershed

INTRODUCTION

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 organized turbulent structures

Simulated water vapour mixing ratio over Hamdallaye watershed (Niger)

(Steinfeld et al, 2006)

Problem of averaged data representative of the surfaces

Problem of flux sources control for more acurate analyses

(Lothon et al,2007)


Turbulent flux variabilities over the ara watershed

Savannah trees

Savannah bushes

Cultures and/or bare soils

INTRODUCTION

Another concept of measurement: Large Aperture Scintillometer(LAS)

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

Case Study : The ARA watershed (~12 km2) in Nalohou

(North part of Benin)

Also a problem of representativeness !!!


Turbulent flux variabilities over the ara watershed

METHODOLOGY

Use of an atmospheric model coupled with land surface scheme

Use of atmospheric 3D model including Large-Eddy Simulation (LES) approach can resolve part of these problems.

►Use Meso-NH model (Lafore et al, 1998)

Perform sensitivity studies

Topography, atmospheric and ground forcings for a case study of 10/04/2006

► Impact on the turbulent flux variabilities


Turbulent flux variabilities over the ara watershed

MODELING STRATEGY  5 nested domains (Two-way)

10.36

9.83

9.79

9.36

9.61

2.23

1.23

1.48

1.70

10.03

9.91

9.72

1.63

1.55

9.53

9.58

1.89

1.77

1.39

1.44

Topography - SRTM 90m

Δx= Δy=18km

Cyclic boundary conditions

Δx= Δy=1km

Open boundary conditions

Altitude (m)

Δx= Δy=250m

Open boundary conditions

Δx= Δy=6km

Open boundary conditions

Δx= Δy=2km

Open boundary conditions


Turbulent flux variabilities over the ara watershed

MODELING STRATEGY

ECMWF radiation scheme

LES, sub-grid paramerization of Deardorff

Clear sky conditions

SURFEX soil-vegetation-atmospheric-transfer model (Noilhan and Planton, 1989)

ECOCLIMAP (Masson et, al 2003) (1km2) for vegetation parameters

Can ECOCLIMAP be usefull for our study ?

Homogeneous sahelian woodland

Spot vegetation at 20m resolution ( Zin et al, 2009)


Turbulent flux variabilities over the ara watershed

Vertical profil of qv at 10:30, 10/04/2006

MODELING STRATEGY

Initialization : sounding atParakou,10/04/2006 at 10.30 AM

Extraction of data at each time step.

Simulation strategy

20 minutes

radiation scheme

OFF

24h simulation, diurnal cycle, radiation scheme active

10/04/2006

10.30 AM

11/04/2006 10.30 AM


Turbulent flux variabilities over the ara watershed

PRELIMINARY RESULTS: WIND ROTATION

Δx= Δy=18km

Cyclic boundary conditions

Final North and North-West wind

Initial South-Westerly wind forcing

Dynamic forcing no maintained


Turbulent flux variabilities over the ara watershed

PRELIMINARY RESULTS: FLUX VARIABILITY OVER CATCHMENT SCALE

Turbulent latent heat flux

W/m2

Turbulent sensible heat flux

W/m2

► Highest values on the crests and advected in lower zones


Turbulent flux variabilities over the ara watershed

PRELIMINARY RESULTS: IMPACT OF THE TOPOGRAPHY ON THE FLUX VARIABILITIES

Same meteorological forcings !

Vertical wind over the « real » topography

m.s-1

Vertical wind organized as convective rolls

Vertical wind over flat terrain

m.s-1

Vertical motion organized according to orographic structures

► Highest values over flat terrain rather than over « real » topography.

► Vertical motion modified by the orography structure


Turbulent flux variabilities over the ara watershed

PRELIMINARY RESULTS: SCINTILLOMETER FOOPRINT ANALYSIS

Latent heat flux over flat terrain

Latent heat flux with topography

The local wind has an impact on the shape of the footprint

Latent Heat Flux « seen » by the scintillometer

Min : 153 W/m2

Max :  169 W/m2

Mean : 163 W/m2

Std : 4.4 W/m2

Min : 122 W/m2

Max : 137 W/m2

Mean : 127 W/m2

Std : 4.4 W/m2


Conclusion and prespectives

CONCLUSION AND PRESPECTIVES

Conclusion

  • 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

Perspectives

Attempt to improve simulations with OPEN boundary condition that is better to maintain wind direction

Improve the vegetation cover description by using satellite analysis (Zin et al., 2009)

Satelite data

ECOCLIMAP

Probably use of ECMWF reanalysis as forcing data !!!


Turbulent flux variabilities over the ara watershed

Many thanks !


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