(Too) Hot or not? Field experiment with the KNMI thermometer screen

The design and first results of a field experiment are described to assess the overall uncertainty of the air temperature observation in the Dutch surface network. In recent years, increased attention has been paid at KNMI to achieve an integrated approach for the determination of the quality of each measurement chain. For air temperature, it is known that several important error sources exist when operating temperature sensors in the field. These are mainly related to siting and the radiation screen used with the sensor. In order to quantify the screen effects such as the radiation error and wetting effect, a field experiment was prepared and started in the summer of 2013. Prior to the test, a number of laboratory experiments were conducted in order to study the thermal response and flow characteristics of the screen.
The field experiment (ONATIS) is conducted at the test site in De Bilt and will last for at least 1 year to cover all possible meteorological conditions. The main purpose is to investigate the impact of the screen and sensor setup under field conditions and to compare the non-aspirated, naturally ventilated KNMI multi-plate screen with an updated, artificially ventilated screen. The experiment is supported by a variety of ancillary measurements of temperature and other relevant meteorological variables. For example, the screens are monitored continuously by a thermal imaging camera and equipped with a dense network of thermistors to obtain the temperature of the lamellas.