Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 Theo Brandsma De Bilt, 2011 | Scientific report; WR 2011-01 Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 Version 1.0 Date March 14, 2011 Status Final final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Colofon Title Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 Authors Theo Brandsma — T +3130 220 66 93 Page 5 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Page 6 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Table of contents Foreword — 9 Summary — 11 1 Introduction — 13 1.1 Problem description — 13 1.2 Scope and objectives of the report — 14 2 Data and methods — 17 2.1 Site description and instrumentation — 17 2.1.1 Site description — 17 2.1.2 Instrumentation — 20 2.2 Methodology — 21 3 Results — 23 3.1 Climatic conditions during the experiment — 23 3.2 Monthly mean temperature differences — 23 3.3 Daily temperature differences — 24 3.4 Diurnal temperature cycle differences — 29 3.5 Diurnal wind speed cycle differences — 31 3.6 Wind direction dependent differences — 33 3.7 Vapor pressure differences — 37 3.8 Comparison of DB260 with neighboring stations — 38 4 Discussion — 41 5 Conclusions and recommendations — 45 Acknowledgements — 47 References — 49 A Experimental details and calibration — 51 A.1 Sites and data collection details — 51 A.2 Instrument calibration — 51 Temperature sensors — 51 Anemometers — 54 B Effect of tent placement — 55 Page 7 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Page 8 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Foreword 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 interim report (Brandsma, 2004). During the experiment, a significant 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 sufficient 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. Page 9 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Page 10 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 Summary Air temperature measurements at the KNMI-observatory in De Bilt are important mainly because the observatory has a long and relatively homogeneous record and because its observations often serve as an indicator of changes in climate for the Netherlands as a whole. Among others, relocations of the temperature measurement sites and (gradual) changes in surroundings influence the measurements. To improve the homogeneity of the long-term temperature record and to study the representativeness of the current measurements, a parallel experiment was carried out at the observatory of KNMI in De Bilt from May 2003 through June 2005. Five sites at the KNMI-observatory, including the (at that time) operational site WMO 06 260 (further denoted as DB260), were equipped with identical (operational) instruments for measuring temperature and wind speed at a height of 1.5 m (see for an overview of the sites Figure 1.1). The instruments were calibrated each half-year and the calibrations curves were used to correct the data to minimize instrumental errors. With the measurements at the Test4 site (operational site since 25 September 2008) as a reference, the temperature differences between the sites were studied in connection with the local wind speed and its differences and operationally measured weather variables at the KNMI-observatory. In September/October 2004 the area west of the operational site DB260 was renovated and made into a landscaped park. From 1999 onwards that area slowly transformed from grassland into a neglected area with bushes (wasteland). The parallel measurements provided the opportunity to study the impact of this new inhomogeneity in detail. The results show that changes in surroundings complicate or impede the use of present-day parallel measurements for correcting for site changes in the past. For instance, the (vertical) growth of the bushes in the wasteland area west of DB260, caused increasing temperature differences between the operational site DB260 and four neighboring stations. The effects were most clearly visible in the dry summer of 2003, when the mean monthly maximum temperatures at DB260 were up to about 0.4◦C larger than those at the reference Test4. This increase was more than counteracted by a decrease in the mean monthly minimum temperature of up to 0.6◦C. After the renovation of the wasteland area, the temperature differences between DB260 and Test4 became close to zero (< 0.1◦C). The comparison of DB260 with four neighboring stations showed that the renovation restored to some extent the temperatures of the old situation of before the year 1999. However, the land use west of the DB260 has been changed permanently (no longer grassland as in the period 1951–1999, but landscaped park land with ponds). Therefore, operational measurements at DB260 became problematic and KNMI decided to move the operational site to the Test4 site in September 2008. The Test4 site is the most open of five sites studied in the report. The results increase our understanding of inter-site temperature differences. One of the most important causes of these differences is the difference in sheltering between sites. Sheltering stimulates the build up of a night-time stable boundary layer, decreases the outgoing long-wave radiation, causes a screen to be in the shade in the hours just after sunrise and before sunset, and increases the radiation error of screens due to decreased natural ventilation. Depending on the degree and nature of the sheltering, the net effect of sheltering on temperatures may be a temperature increase or decrease. DB260 is a sheltered site where the net effect is a decrease of the mean temperature (before the renovation). The former historical site Test1 is an example of a site where the net effect is a temperature increase. The Page 11 of 56 final j Parallel air temperature measurements at the KNMI observatory in De Bilt (the Netherlands) May 2003 - June 2005 j March 14, 2011 monthly mean minimum temperature at Test1 is up to 1.2◦C higher than the reference and the maximum temperature is up to 0.5◦C higher than that at Test4. The mean temperature at Test1 is, however, only slightly higher than the mean at Test4. This is caused by the relatively low temperatures in the hours after sunrise and before sunset, when the screen at Test1 is in the shade. Both the Test1 and Test4 location are probably not affected by the renovation. The renovation of the wasteland area causes not only a shift of the location of the pdf of the daily temperature differences but also a change in the shape. This means that for the homogenization of daily temperature series it is not sufficient to correct only the mean. We showed that the magnitude of the inter-site temperature differences strongly depends on wind speed and cloudiness. In general the temperature differences increase with decreasing wind speed and decreasing cloudiness. Site changes directly affect wind speed because they are usually accompanied by changes in sheltering. Some effects, like the built up and (partly) breaking down of the stable boundary layer near the surface, are highly non-linear processes and therefore difficult to model. The fact that these processes are mostly active at low wind speeds (< 1.0 m/s at 1.5 m) further complicates the modeling.