CESAR (Cabauw) and wind

Cabauw Experimental Site for Atmospheric Research

KNMI provides high-quality meteorological measurements at CESAR (http://www.cesar-observatory.nl/ ) and has done that for over 40 years (since 1972). In February 2016 the CESAR site became a certified GRUAN station (GCOS Reference Upper Air Network: www.gruan.org). GRUAN is an international reference observing network providing long-term, high-quality climate data records from the surface up and into the stratosphere. Not more than 40 stations worldwide will achieve that status. At the CESAR site remote sensing techniques, such as LIDAR and SODAR, are tested during (inter)national measuring campaigns. Experimental studies focus on clouds, aerosols, radiation, greenhouse gases, land-surface interaction, and hydrology.

Wind measurements from the CESAR site are used in many studies into wind climatology and behaviour of wind up to elevated levels. Although the site is fairly flat, there are trees and buildings that affect the wind measurements, but the influence of these obstacles is well understood and can be corrected for. 

Wind at the CESAR-site:

1) Average wind above 200m: Since 1993 a wind profiler (phased array Doppler radar) measures from 200 m to 4 km height and in 30 minute intervals how wind speed and direction change with height, thus providing a vertical profile of the horizontal component of the wind. The wind profiler also measures the vertical profile of the vertical component of the wind and its standard deviation. Backscatter intensity measurements enable the determination of the atmospheric boundary layer height under convective conditions. The wind profiles and tall wind mast measurements (see below) have been used e.g. to derive a low level jet climatology[1].

2) Average wind below 200 m: Wind speed and direction are measured at six levels along a 213 m tall meteorological mast. To avoid interference with the mast, the wind is measured on booms that stick out into different directions at each measurement level. A flow model is used to correct for further disturbances of the wind (e.g. pressure effects upstream of the tower and booms). The corrected wind measurements are highly accurate and available from 1986-1996 and from 2000-present. The measurements have recently been used for tuning the KNMI North Sea Wind atlas (http://projects.knmi.nl/knw/ ).

3) Turbulence: Sonic anemometers are installed at 4 levels: one at 3 m height and three in the 213 m tall meteorological mast (at 60, 100 and 180 m). The measurements are on such a high spatial and temporal resolution (measuring volume 0.15 m and sample rate 10 Hz) that small structures in the turbulent wind field can be resolved. Currently the sonic measurements are used to study wind gusts at different run-times.