The observations are used among others for evaluating land surface and boundary layer schemes in atmospheric models, validation of satellite retrieval schemes, studies for wind energy, studies of fog formation and climatology. Part of the program has an operational status, these measurements are used also by the weather service. Observed are 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 in its four components short wave up- and downward radiation and long wave up- and downward radiation, surface visibilty the components of the surface energy budget, sensible heat flux, latent heat flux and soil heat flux, the momentum flux and auxillary parameters to judge the state of the landsurface like soil water content and radiative vegetation temperature.

This measuring program couples with other observational activities within CESAR. Most noticable are: The soil hydrological program of WUR; the green house gas concentration observations of ECN; Wind profiler/RASS observations of KNMI and the Raman Lidar observations of KNMI (humidity profiles). Tower flux observations are performed in cooperation with WUR.

Other important additions to the program are: Atmospheric profiles from the radiosonde De Bilt, currently launched two times a day; for the study of representativity of Cabauw observations of the surrouding weather stations Schiphol (240), De Bilt (260), Zestienhoven (344) and Gilze Rijen (350) and for the geostrophic wind at Cabauw pressure readings from eight weather station in a radius of 75 km around Cabauw.

In 2010 KNMI started with a project to improve the quality of a number of observations. The improvements involves measurement techniques, data logging and maintenance.

In the period 1986-1996 a continuous in-situ measurement program has been performed at Cabauw. After a reconstruction period the current program started in may 2000. The number of observed parameters have increased over the years.

Here the measurements are described in categories. The categories correspond to the datasets as they are available from the CESAR web portal or in some occasions on request.

Detailed information can be found at http://www.knmi.nl/~bosveld → Observations → Documentation

Meteorological surface observations of precipitation, visibility, radiation, air pressure, wind speed, wind direction, temperature and dew point at the Cabauw meteorological observation field since May 2000. Visibility and precipitation type are available from January 2008. Instruments are the same as in the National Meteorological Network.

Surface fluxes of net radiation, momentum, sensible heat, latent heat, soil heat and carbon dioxide at Cabauw since August 2000. Turbulent fluxes are derived from measurements with a sonic anemometer/thermometer and an open path H2O/CO2 sensor at 3 or 5 m. The eddy-correlation technique is applied to arrive at fluxes. Net radiation is derived from the sum of the four radiation components. These observations are described in the section on surface radiation. Surface soil heat flux is derived from a combination of soil heat flux and soil temperature. These observations are described in the section on soil heat.

Meteorological observations of wind, temperature and humidity along the Cabauw 213 m tower since May 2000. Instruments are the same as in the National Meteorological Network.

Wind speed and wind direction is measured at six levels, 200, 140, 80, 40, 20 and 10 m. To avoid too large flow obstruction from the mast and the main building measurements are taken on booms in three different directions. At the levels 200, 140, 80 and 40 m the wind direction is measured at all three booms and wind speed is measured at two booms (South-West and North). At the levels 20 and 10 m the wind direction and wind speed are measured at two separate masts South (B-mast) and North (C-mast) of the main building. For each 10 minute interval instruments are selected that are best exposed to the undisturbed wind. Still some flow obstruction remains due to the presence of the tower and the supporting booms. Corrections are applied on the basis of a potential flow model. Corrections in the wind speed are maximal 3% and corrections in wind direction are maximal 3°.

Temperature and dew point are measured on the same levels and at 1.5 m height. In the tower measurements are only at the South-East boom. Mast interference for temperature is small. Recently we installed new sensors for humidity with wich the accuracy improves from 3.5% to 1.0% or better. This enables now a meaningful determination of the humidity profile along the 200 m tower.

For the detection of fog the profile will be augmented with visibility sensors at seven levels in 2011.

Eddy correlation fluxes are measured at three levels in the tower (60, 100 and 180m) in cooperation with WUR. At each level a sonic anemometer/thermometer and an open path H2O/CO2 sensor is installed at the South-East (130°) boom. Observations became available over time starting with the 180 m level in the course of 2002. Recently an experiment to determine the influence of mast interference on flux observations has been performed. Analysis now underway should show for which wind sector the flux observations are reliable.

Fluxes performed at these heights have large so-called footprints and therefore measure fluxes at a regional scale. To arrive at surface fluxes one has to correct for the change in storage below the turbulence sensors. For CO2 use is made of observations of ECN.

Short wave and longwave upward and downward radiation at the surface from the Cabauw meteorological field since May 2000. These observations are specifically done for the monitoring of land-atmosphere interaction at Cabauw. They differ from Cabauw BSRN observations in that the time series goes further back in time and that upward fluxes are observed. However, the quality of the observations do not match the current BSRN standard. Recently we started parallel measurements with the same type of instruments as used at the BSRN site.

Surface or skin radiation temperature is measured at two locations with pyrometers. One from a height of 2 m looking at the current soil heat plot, and one looking from the top of the 200 m tower downward and seeing a larger area. Difficulties with accuracy are present in the current dataset. Within a project at KNMI a method has been developed to monitor the reproducibility of the sensors. In the course of 2011 the old sensors will be replaced and the new sensor will be in a well established maintenance and calibration regime. Expected accuracy will then be 0.5 °C.

Soil temperatures in a profile are measured since March 2003 at 0,2,4,8,12,20,30 and 50 cm depth. In 2010 a new system has been developed in cooperation with KNMI Infrastructure Department with improved accuracy and resolution. The properties of the new system enables to derive soil heat fluxes on a time scale of 10 minutes by looking at the time rate of change of temperature in the soil.

Soil heat flux is measured with soil heat flux plates since 2002. The six plates are buried at the three vertices of an equilateral triangle with sides of 2 m at depths of 0.05 and 0.10 m. The measurements are averaged over the three plates at each depth.

Ground water table is measured since August 2000. Measurements are performed at five locations including a measurement position in a ditch, measuring the level of the managed water level in the polder. The instruments are pressure transducers which measures water pressure in soil shielded pit.

Soil water content is measured since 2003. Measurements are performed with TDR sensors at depths of 3,8 and 20 cm. Currently a project is running to assess the quality of the measurements. This includes a calibration of the sensor type with soil from the site.

In the context of CESAR the soil hydrological programme is extended by Wageningen University and Research who measures the soil water budget of a small catchments in which the site resides.

An eXtra Large Apperture Scintillometer (XLAS) operates over a 9.8 km path from the IJsselstein TV-tower to the Cabauw meteorological tower. It measures the structure parameter of temperature and from that an estimate of the path averaged sensible heat flux can be derived. This is one of the few ways to measure regional scale fluxes. From 07-jan-2002 till 14-apr-2004 the sensor path was at 40 m height. This gave rise to occasionally saturation of the scintillometer signal when sensible hat fluxes were high. From 2008 onward an XLAS is operated at an height of 60 m to avoid saturation.