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 surface 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, specific humidity, station pressure, wind speed, wind direction, U wind component, V wind component, precipitation, skin temperature, incoming longwave radiation, incoming shortwave radiation, outgoing longwave radiation, outgoing shortwave radiation, and net radiation
The following parameters are not measured at this site: incoming PAR, outgoing PAR and snow depth.
2.0 INSTRUMENTATION DESCRIPTION:
No observations are available of snowdepth, upward PAR and downward PAR.
2.1 Surface pressure
Surface pressure is measured at the automatic weather station site, 200 m South-West of the main tower. The instrument is a Paroscientific 1016B-01. Provisions are made against dynamic pressure effects. Calibration is done at KNMI. Instruments are replaced after 26 month. Accuracy is 0.1 hPa. Resolution is 0.1 hPa. Datalogging is with the KNMI XP1-SIAM Pressure.
2.2 Air temperature and dewpoint temperature
Air- and dewpoint temperature are measured at 1.5 m. 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. Heating of the sensor, the 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.
2.3 Wind speed and direction
Wind speed and wind direction is measured at 10 m. To avoid too large flow obstruction from the mast and the main building measurements are taken 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.
Rain amount and duration is measured at the radation field South of the main tower. To surpress flow obstruction the rain gauge is positioned in a circular pit of 3 m diameter, wich is surrounded by a circular slope. Rain duration is derived from the rain gauge observations. Rain is measured with the KNMI rain gauge. Calibration is done at KNMI. Instruments are replaced after 14 month. Accuracy is 0.2 mm. Resolution is 0.1 mm. Datalogging is with the KNMI XR2-SIAM Neerslag.
2.5 Short wave radiation
Short wave upward and downward radiation are measured at the radiation field South of the tower at 1.5 m height. The downward looking sensor (albedo) is on a boom of 1 m length. The porting structure is painted black to get a wel defined radiation condition. Since December 2002 the instruments are ventilated and heated to avoid formation of dew, snow and rime. The instruments are Kipp&Zn CM11 pyranometers. Data logging is done with the KNMI XQ1/XD0/XF0-SIAM Radiation. Calibration is done at KNMI against a reference instrument which itself is calibrated at Davos (Switserland).
2.6 Long wave radiation
Long wave upward and downward radiation are measured at the radiation field South of the tower at 1.5 m height. The sensors are on a boom of xx m length. The instruments are mounted in one housing to get equal house temperatures. They are ventilated to avoid formation of dew, snow and rime and to minimise heating of the domes through irradiation. The domes are equipped with small thermistors. Corrections are applied for heating of the domes. The instruments are Eppley pyrgeometers (PIR). Data logging is done with the KNMI XL0-SIAM Eppley Radiation. Calibration is done in Davos (Switserland).
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. For the non-standard sensors (e.g. long wave radiation) a new SIAM is developed. A PC with multiplexed serial ports captures all the SIAM ASCII-strings. All 12" strings are archived. Strings send at the transition of a 10 minute interval are used to derive the reduced data like mean, maximum, minimum and standard deviation. These data are stored in the database.
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. Net radiation is derived from the the four radiation components. Skin temperature is derived from long wave upward radiation assuming black body radiation.
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_sfc_met_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:
Rain amounts were underestimated by a factor of 3 due to data processing problems. From the original 10 minute values averages instead of sums were taken to get 30 minute values. A revised dataset which fixes this problem was made available 01 June 2007.
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. A revised data set which fixes this problem was made available 01 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.
At the 16th of January 2003 the albedo instrument was moved some 10 m to the West. The reason being the presence of a small ditch in the field of view of the instrument which could reflect direct sunlight into the instrument. This became particularly obvious during a cold spell period with ice forming in the ditch. At this new location a small depression in the field close to the albedo instrument was present in which surface water could form during very wet episodes. At the 10th of February 2003 the grass around the new location was removed, soil was applied and the grass was replaced again. At the start of the growing season the surface at the albedo location rapidly became the same as its surroundings.
The instrument for short wave downward radiation have small offsets ( a few W/m2) due to long wave cooling. This results in negative values during night time. These values are set to zero.
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.