Observations of surface fluxes are in general performed at a few meters above the land surface. Such observations are representative for a relatively small area of typically 0.1x0.1 km2. When using flux observations for the evaluation of atmospheric models and satellite products fluxes on a scale of the model grid cell or the satellite pixel, typically 10x10 km2, are needed. By exploiting flux observations at higher levels in the Cabauw 200 m tower estimates on the regional scale are obtained. Additionally a scintillometer is operated over a 10 km path between the TV-tower of IJsselstein and the Cabauw tower at a height of 60 m.
Going up in the atmosphere comes with a cost. Since we are interested in surface fluxes we have to correct for the flux divergence between the surface and the measurement level. Under horizontal homogeneous conditions this can be derived from the rate of change of storage in the atmospheric layer below the instrument. However, inhomogeneities in the atmospheric or in the land surface deteriorates the rate of change and corrections are needed.
Horizontal advection is in general difficult to measure. State of the art atmospheric model/assimilation systems fed with sufficient observations contain information on horizontal advection. At least they can serve to select days were advection is relatively small. By looking at flux divergences along the tower an estimate of advection can be made provided that the measurement uncertainties are small enough.
The 200 m meteorological tower at Cabauw contains instruments that measure the state variables and turbulent fluxes of momentum, temperature, humidity and CO2. This tower flux measurement program is initiated in collaboration with the National Science Foundation of the USA and is now a consorted action of KNMI, ECN and WUR.
Application of regional flux observations are:
- Estimation of regional scale surface fluxes for the evaluation of atmospheric models and satellite derived surface fluxes.
- Estimation of horizontal advection
- Night time CO2 fluxes at low wind speed
- Surface energy budget closure
An eight days composite of total heat flux for the four turbulent flux levels at Cabauw together with available heat flux as derived from surface radiation and soil heatflux measurements. A clear heat flux imbalance is found especially in the morning time. In most surface energy balance studies only a comparison is made with turbulent instruments close tot the surface. Here we see that when going to higher levels no improvement in surface energy budget closure is obtained.
A further account on surface energy budget closure and atmospheric budgets can be found in the KNMI triennial 2007-2009 Chapter 2.
Contact: frimnk@dlevsob.de.nl
