Standardization of data and methods for calculating daily Tmean, Tn and Tx in the Netherlands for the 1901-1970 period

In this report we made a reconstruction of the data and methods used to calculate Tmean, 0-0Tn and 0-0Tx for the five principal stations in the Netherlands in the 1901-1970 period. We found that a large variety of approaches was followed for calculating 0-0Tn and 0-0Tx and that there was confusion about the type of data, climatological or synoptical, that had to be used in the period that both types were available.

A standard approach for calculating 0-0Tn and 0-0Tx was introduced. This approach automatically calculates 0-0Tn and 0-0Tx from all available data. Further improvement of the method by (labour-intensive) manually checking the thermograph strip charts, does not result in significantly improved 0-0Tn and 0-0Tx values. Quality flags were defined that indicate the degree of exactness of the automatically calculated 0-0Tn and 0-0Tx values. For De Bilt and Maastricht/Beek it was found that the quality of the climatological temperatures was better than that of the synoptical temperatures. The latter were used for the 1951-1970 period. For the other stations there was no indication of quality differences between both data types. It was decided to solely use climatological temperatures for calculating Tmean, 0-0Tn and 0-0Tx in the 1901-1970 period. The main reasons for this decision are: (a) improved data quality for De Bilt and Maastricht/Beek, (b) uniformity of data type for the whole 1901-1970 period, and (c) prevention of illogical combinations of both data types in the 1951-1970 period.

The standardised series of daily Tmean, 0-0Tn and 0-0Tx will serve as the new operational time series of KNMI for the five principal stations. As a results some of the record values of Tmean, Tn and Tx change. The effect of the standardization on trends is probably small but needs to be studied further in conjunction with corrections for stations relocations, which were not part of this report.