EMS2011 September 12-16, Berlin 1

Regional scale flux observations at CabauwFred C. Bosveld

Royal Netherlands Meteorological Institute

With contributions of Miranda Braam, Kasper Gerritsen, Claudia Brauwer (Wageningen University) and Jerome Schalkwijk (Delft

University)

Content:• Motivation• Observations• Fetch Conditions • Corrections• Surface Energy Budget• Advection• Conclusions

EMS2011 September 12-16, Berlin 2

http://www.cesar-observatory.nlhttp://www.cesar-database.nl

Consortium van 8 instituten:

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Regional scale fluxes -> Motivation

• Surface fluxes at the scale of atmospheric model

gridcel

• Surface fluxes at the scale of satellite pixel

• Surface Energy budget closure studies

• Evaluation of advection in models

• Cabauw serves as reference case since it is relatively

homogeneous and very flat

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180 m

100 m

60 m

3 m

Flux levels

Extra LArge Scintillometer (XLAS)over a 10 km path at 60 m height

Regional scale fluxes -> Observation

Gill R3 sonicanemo/thermometer

Licor 7500H2O/CO2 sensor

Cabauw 213 m meteorologival towerT,q, CO2 fluxes andconcentrations

Water budgetof the polder

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Regional scale fluxes -> Fetch conditions

100m

60m

3m

180m

Water Budget

XLAS 10 km

Tower

2 km

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- Mast interference

N

Sonic E Sonic B

Regional scale fluxes->Corrections

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Regional scale fluxes -> Corrections

- Standard corrections for eddy correlation techniqueDensity, Flow distortion, sensor seperation

- High accuracy neededCalibration issues

- Low frequency flux contributionExtention of surface layer relations

180 m100 m

60 m

3 m

U = 5 m/s T=600 s

Based on Kaimal (1972) surface layer spectraExtended by Schalkwijk et al (2010)

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Regional scale fluxes -> Surface fluxes

zz

dzz

CW

x

CUdzzC

tzwcwc

00

)()()()0(

Surface flux Local flux Storage flux Advective flux

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Regional scale fluxes -> Surface energy budget

Mean diurnal variation of 8 comparable days

Surface total heat flux (THF=H+LE) estimated from different heights. Available Energy (Qav=Qnet-G)

Imbalance is independentof height of fluxobservation.

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Deriving fluxes from XLASat 60 m height and 10 km path

No constant flux layer in the morning? Does CT2 at 60 m scales with:

• Surface sensible heat flux (s)• Local sensible heat flux at 60 m (l)

Braam et al. (subm. BLM)

Surface and 60 msensible heat fluxesNo constant flux layer

CT2 from similarity functions with

surface and local scaling,compared with CT

2 derived directly

from XLAS and from sonic at 60 m.

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Regional scale fluxes -> Advection (Diurnal variation)

April-June 2008

Difference in surface flux between60 – 100 m and 100 – 180 m

Some indication of higher local latent heat flux.

CO2 Latent heat flux

Sensible heat flux

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Regional scale fluxes -> Advection(Wind direction)

Difference in surface flux between60 – 100 m and 100 – 180 m

April-June 2008daytime 30 min valuesandwind direction class mean values

Some signature visible

Sensible heat flux

Latent heat flux

CO260-100m 100-180m

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Conclusions

• The Cabauw 213 m meteorological tower gives a unique opportunity to perform tower based regional scale flux observations.

• The site is flat and rather homogeneous.

• Special care for low frequency flux loss correction

• Surface energy imbalance independent of measuring height

• Scaling of scintillometer at 60 m (outside constant flux layer) investigated

• Slightly significant signals in advection found both in time of day and in wind direction.