Dr. Fred C. Bosveld
Royal Netherlands Meteorlogical Institute
3730AE, De Bilt
tel. +31 (0)30 2206911 (787)
fax +31 (0)30 2210407
1.0 DATA SET OVERVIEW
Contains the tower profile observations at the BALTEX anchor station Cabauw, lat. 51.97N lon. 4.93E, height (m.s.l.) -0.7 m.
This data set includes observations of air temperature, dew point, relative humidity, station pressure, and specific humidity at the 2.0 m height.
The 10, 20, 40, 80, 140, and 200 m heights include observations of air temperature, dew point, relative humidity, specific humidity, wind speed, wind direction, U wind component, and V wind component. The station pressure is not measured at these heights.
2.0 INSTRUMENTATION DESCRIPTION:
2.1 Wind speed and direction
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 according to Wessels (1983). Corrections in the wind speed are maximal 3% and corrections in wind direction are maximal 3 degrees.
Wind speed is measured with the KNMI cup-anemometer. Cup diameter is 105 mm and the distance between the centre of the cups to the rotation axis is 100 mm. Wind direction is measured with the KNMI wind vane. Distance between axis and the outer side of the vane is 535 mm. The azimuth of the wind vane plugs at the tip of the booms are determined with a camera relative to distant objects at close to the horizon. The instruments are logged with the KNMI wind SIAM. Wind gusts are determined from a running 3 sec mean value. Calibration of the cup anemometers is done in the wind tunnel of KNMI. Wind vanes are balanced and the direction of the vane is tested. Sensors are replaced after 26 month. The cup anemometer contains a photo-chopper with 32 slits. The accuracy is 0.5 m/s. The treshold velocity is 0.5 m/s. The resolution is 0.1 m/s. The response length is 2.5 m. The wind vane contains a code disk. Accuracy is 3o. Resolution is 1o.
2.2 Air temperature and dewpoint temperature
Air- and dewpoint temperature are measured at seven levels, 200, 140, 80, 40, 20, 10 and 1.5 m. The highest 4 levels are measured at the South-East booms of the main tower . The lowest three levels are measured at the B-mast, South of the main building. Air temperature is measured with a KNMI Pt500-element in an unventilated KNMI temperature hut. Dew point temperature is measured with a Vaisala HMP243 heated relative humidity sensor with a metal filter in a separate Vaisala unventilated hut. This hut is open in construction. The humidity data often overestimates during drying episodes after dew, fog or rain, because of a wet shielding or sensor. This may result in observed dewpoint temperatures higher than the air temperature. Heating of the sensor, the change to metal filter and the open hut improves the functioning during high humidity conditions.
Calibration is done at KNMI. Temperature sensors are replaced each 38 month. Accuracy is 0.1 oC. Resolution is 0.1 oC. Dew point sensors are subject to contamination and drift of calibration this makes it nescessary to replace them each 8 month. Accuracy is 3.5% RH. Resolution is 0.1oC. Data logging is done with the KNMI XU2-SIAM Temperatuur/Vocht HMP243.
3.0 DATA COLLECTION AND PROCESSING:
3.1 Data collection
Part of the instrumentation is the same as in the synoptical network in The Netherlands. These instruments are logged with SIAM's (Sensor Intelligent Adaptation Module). SIAM's are developed for use in an operational network. Each 12" an ASCII string is outputted, which contains status information and statistics over the last 10 minutes. Output is a-synchronious. Status information is derived from basic tests on the measured values. If severe errors are detected the measured values are set to a missing value code.
3.2 Data processing
After the data has been stored in the data base boom selection and flow corrections are performed on the wind observations. Relative humidity is derived from the air temperature and dewpoint temperature. Specific humidity is derived from the air temperature, dewpoint temperature and the surface pressure. Wind speed components are derived from the wind speed and wind direction.
These data are in the CEOP EOP-3 data format agreed to by the CEOP Scientific Steering Committee. This format is described in detail as part of the CEOP Reference Site Data Set Procedures Report which is available at the following URL: http://www.joss.ucar.edu/ghp/ceopdm/refdata_report/ceop_met_tower_format.html
4.0 QUALITY CONTROL PROCEDURES
4.1 Cabauw Quality Control Procedures
The measurements that are logged with SIAM's have an automatic quality control, which consists of tests of the electronics and of the exceedance of physical limits for the parameter at hand. After the data are stored in the database a manual (on-eye) check is performed and together with information from the logbook suspect data are rejected.
4.2 UCAR/JOSS Quality Control Procedures
UCAR/JOSS conducted two primary quality assurrance/control procedures on the reference site data. First the data has been evaluated by a detailed QA algorithm that verifies the format is correct, examines any QC flags, and conducts basic checks on data values. Second, JOSS conducts a manual inspection of time series plots of each parameter.
5.0 GAP FILLING PROCEDURES
No gap filling is applied
6.0 DATA REMARKS:
To avoid interference of the 200 m tower and its building on the wind observations, winds at the 10 and 20 m level are measured at two sides of the builing. In the postprocessing the best exposed wind sensor is selected to represent the Cabauw 10 and 20 m wind. A bug was found resulting in the selection of the sensor at the wrong (ill exposed) side of the tower. The bug was in effect for the period 20021001-20030930. The corrected version of the data set was made available from the CEOP archive in early June 2007.
The humidity data often overestimates during drying episodes after dew, fog or rain, because of a wet shielding or sensor. This may result in observed dewpoint temperatures higher than the air temperature.
7.0 REFERENCE REQUIREMENTS:
Users of the data are required to follow the CEOP data policy. Above this users of this data are kindly requested to acknowledge:
"The Royal Netherlands Meteorological Institute"
in any publication in which the data are used. Questions about the data can be directed to the e-mail adress given above. The user is kindly asked to communicate any problem in the data set to the e-mail adress given below. Users are also kindly asked to send a copy of any publication in which the data are used to the adress given above.
Wessels H. R. A. (1983). Distortion of the wind field by the cabauw meteorological tower. KNMI scientific report 83-15. De Bilt The Netherlands.