Het komt namelijk niet verder dan dit in de discussie van het technisch rapport over de homogenisatie. Ze vonden een exacte correctie voor de individuele dagwaarden niet belangrijk genoeg.
Bron: Technisch Rapport Homogenisatie KNMI
4 Discussion
In this report we used the percentile-matching method for homogenizing the daily temperatures of the five principal stations in the Netherlands. This statistical homogenization method preserves the statistical distribution of the temperatures. This implies that on average the correction of extreme values is taken care of. On individual days however, the real correction may differ from the one applied. Nonetheless, there is ample reason to use the homogenized series instead of the measured series. The latter are mostly not suitable for climate trend and variability studies, where the precise correction for individual days is mostly of minor importance.
Methods for homogenizing daily temperature data are only starting to be developed and are strongly dependent on the available data. The corrections presented here are statistical but flexible in the sense that they may vary with temperature and season. The current initial homogenized version is denoted version 1.0 and serves as a baseline version.
Future improvements to the current version 1.0 are possible by taking into account weather variables other than temperature. Although temperature indirectly accounts for other weather variables, it is far from perfect. For instance, the same low nighttime temperatures in winter may be caused by (a) strong advection of cold air or by (b) clear-sky and windless nights. It is known that corrections in case of (a) are mostly close to zero whereas corrections in case of (b) may become quite large. The same holds for high daytime temperatures in summer, which may be caused by advection of warm air of by clear-sky and windless days.
Temperature trends in the comparison period may affect the corrections resulting from the percentile method. This may need some attention in future studies. In the present study, however, temperature trends in the comparison periods may be considered negligible.
The study here was restricted to the most important known inhomogeneities, major relocations and a major screen change in De Bilt. There are however other known inhomogeneities that may need correction. For instance, the lowering of the thermometer screen from 2.2 m to 1.5 m around 1960, other known minor relocations and urban effects. For Tmean, the monthly CNT series and the homogenized monthly De Bilt series show that these corrections are much smaller than the corrections applied here.