Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005

From May 2003 through June 2005 a field experiment has been carried out at the observatory of KNMI in De Bilt, as part of the KNMI-program ’Hisklim’ (HIStorical CLIMate). The original objectives of the experiment were: (1) to study the representativeness of the current operational air temperature measurement site, and (2) to explore the possibility of using present-day parallel measurements to correct for inhomogeneities, caused by changes in the surroundings and a relocation of the thermometer screen in 1951. At five sites, including the operational site, temperature and wind speed were measured at a height of 1.5 m, using identical instruments and set-up. The temperature differences between the sites have been studied in connection with the local wind speed and other weather variables. Initial results for the period May 2003-April 2004 have already been published in an interimreport (Brandsma, 2004).

During the experiment, a signignifcant renovation of the area only 12 m west of the operational temperature screen took place in September/October 2004. The renovation introduced an inhomogeneity in the temperature measurements at the operational site, which was used as the reference site for the interim report. As a result, in this Final report another reference site (Test4) has been chosen and the end date of the experiment was extended from April 2005 to June 2005 to have suffcient
overlap time. This report presents the results of the whole period of the experiment (May 2003 - June 2005).

In September 2008 the operational site DB260 (06260) was relocated to the Test4 site (the reference site in this report). Both sites are considered in this report. Although this report is not meant to describe the effect of this relocation, the results presented here do give an indication of the effects of this relocation on the
temperature measurements. A detailed study of the effects of the relocation will be described in a separate publication.