Cabauw in-situ measurements

The Cabauw in-situ observational program aims at characterizing the physical aspects of the land surface and atmospheric boundary layer, and their interaction.

Typical use is in model evaluation, satellite retrieval validation and process studies. End products of the program are quality controlled datasets provided through the freely accessible CESAR data base, a user guide and a contact point. The user guide gives a comprehensive documentation of all relevant aspects of the site. As such it may guide the user and enable a judgement of the usability of the data for the user’s specific application. When needed further help can be obtained through the contact point.

Many of the observations run continuously for more than 25 years. These include the profiles of wind speed and wind direction, temperature and humidity along the 200 m Cabauw meteorological mast, the surface flux of precipitation, the surface radiation budget by its four components shortwave up- and downward radiation and longwave up- and downward radiation, the surface energy budget by its components sensible heat flux, latent heat flux and soil heat flux, and the momentum flux.

On top of this the scope of the program has broadened in recent years in five directions.

1) Regional scale fluxes: Atmospheric model grid cells and satellite pixel sizes typically have larger spatial scales then the characteristic scale of the observations. To overcome this scale differences a regional scale flux program is implemented. On top of the eddy correlation system at 3 m height, it consists of eddy-correlation observations of wind, temperature, humidity and CO2 at three levels (60, 100 and 180 m) in the 200 m tower, and by operating an Extra Large Scintillometer over a 10 km path. These observations are also used to study the closure of the surface energy balance and to study horizontal advection.

2) Soil heat and water: The response of the land surface to atmospheric forcings is strongly modulated by the soil water content of the soil. This pertains specifically to evaporation and to soil heat flux. The soil thermal and hydrological observations has been extended and improved in recent years. It encompass now observations of ground water table, profiles of soil water content, profiles of soil temperature and soil heat fluxes. The in-house developed  high accuracy low noise soil temperature sensors in conjunction with the soil water content observations enable now to give an independent estimate of the soil heat flux. This is very welcome for the study of the surface energy budget closure.

3) Fog and visibility: Improvement of fog forecasts is needed for safety of transport. Fog observations are at the basis of improving fog models. Inspired by a set-up of Meteo-France a fog observational program was implemented at Cabauw. It consists of visibility observations at the standard levels in the 200 m meteorological mast and shortwave and longwave incoming radiation instruments at the top of the tower. In conjunction with the surface-based radiation instruments this enables the detection of the development of the fog layer height and its radiative properties.

4) Radiative surface temperature: Surface or skin temperature is a crucial parameter that regulates the division of energy in radiation and heat, and the division of heat into soil, sensible and latent heat. By means of pyrometers which measure radiative temperature, an estimate of the skin temperature can be obtained. The accuracy of the pyrometer observations has been improved by establishing a calibration control system at KNMI. Surface temperatures are now measured locally and from the top of the 200 m tower with an accuracy that enables in-depth analysis of the representation of surface processes in models. This also relates to the fog program as it gives the temperature of the top of the fog layer.

5) Surface geostrophic wind: The geostrophic wind is proportional to the pressure gradient, and as such one of the main drivers of the atmospheric column. KNMI measures surface air pressure in a dense network over the Netherlands, with high accuracy that is maintained over long time. The country is very flat, so that height corrections contribute only marginally to the uncertainty of the air pressure observations. From these observations the surface geostrophic wind at Cabauw is derived with an unprecedented accuracy.

Less visible but certainly not less important are the improvements that have been made in recent years in maintenance. Where possible use is made of the data logging and data transfer system of the KNMI operational meteorological network. This also has the advantage of more automatic quality and warning procedures. Also where possible the operations and responsibility of a number of instruments have been moved from the research department to the operational department. A major upgrade has been made in the data logging facilities for the turbulence observations.

With these developments we expect that we can continue with the generation of high-quality datasets that remain of interest for model evaluation, satellite retrieval validation and process studies.

Turbulence instruments at the 180 m level of the meteorological mast.
Turbulence instruments at the 180 m level of the meteorological mast.