It is therefore not an overstatement to say that understanding of the climate system begins with the understanding of radiation and its interactions with the atmospheric constituents such as clouds, aerosols, and greenhouse gases.
Because of the important role of radiation in the climate system, radiation measurements are indispensable for climate research. They provide the best check for the theory of radiative transfer in the Earth’s atmosphere and can be used for the evaluation and improvement of models designed for weather and climate prediction. Furthermore, long-term measurements of surface radiation provide an opportunity for the detection of climate change.
The importance of accurate and precise radiation measurements for climate research was the primary reason for the joint scientific committee of the World Climate Research Programme (WCRP) to establish in 1988 the international Baseline Surface Radiation Network (BSRN)1). The goal was to establish a world-wide network of radiation measurements adhering to the highest achievable standards. Currently there are 39 fully operational and 8 candidate stations (Figure 1). In 2004 BSRN was designated as the global baseline network for surface radiation for the Global Climate Observing System (GCOS).
The idea for KNMI to join BSRN existed in the early 1990s but it was not until 2001 that the necessity for accurate and precise radiation measurements became urgent. A radiative closure study2) revealed that the radiation measurements were not accurate enough to draw firm conclusions on the significance of differences between model calculations and measurements of surface radiation. After an exploratory visit to the BSRN station of Payerne, Switzerland, in 2002 it was decided to initiate the construction of a radiation site in Cabauw according to the BSRN requirements.
The field constructions for the BSRN station in Cabauw were completed by the end of 2004 (Figure 2). The data acquisition system, designed to meet the stringent BSRN requirements, and a basic set of radiation instruments were operational by early 2005. The first consistent monthly dataset of basic radiation measurements (global, direct, diffuse and downward longwave radiation) was obtained in February 2005. Since then, the station has been extended with various spectral radiation measurements (both direct and diffuse) made at different solar wavelengths. A total sky imager has been installed as well as instruments for measuring ultraviolet and photosynthetically active radiation and sunshine duration. The formal status of Cabauw as BSRN station was announced at the 9th BSRN Workshop and Scientific Review in Lindenberg, Germany, May 20063).
Besides the installation of constructions and instruments, considerable effort has been put into the implementation of quality control procedures and the development of a web-based system for the access to quick looks and measurements. Some of the applied quality control procedures involve the use of radiative transfer models. An example for a selection of downward longwave irradiances measured at BSRN Cabauw is shown in Fig. 3. The same quantity was calculated using the radiative transfer model MODTRAN 4. The small bias of 7 ± 4 W m-2 between model and measurements gives confidence in the measurements and excludes serious instrument malfunction.
From 26 September until 14 October 2005 KNMI participated in the 10th International Pyrheliometer Comparison (IPC-X), Davos, Switzerland4). This 5-yearly recurring comparison of instruments for the measurement of direct solar radiation provides the basis for the worldwide dissemination of the World Radiometric Reference5) (WRR), the measurement standard representing the SI unit of irradiance (W m-2). For BSRN stations, representing the highest standard of radiation measurements, it is essential to participate in IPCs so as to guarantee direct traceability of solar radiation measurements to the WRR.
KNMI took part in IPC-X with an Eppley HF cavity radiometer (Figure 4), which is one of the most accurate instruments for measuring direct solar irradiance. Favourable weather conditions resulted in a record number of calibration measurements. Figure 3b shows the performance of the KNMI instrument relative to the WRR. On average the difference appeared to be as small as 0.20 ± 0.08%. The excellent agreement with the WRR will allow us to perform on site calibrations of the solar radiation instruments of the BSRN site in Cabauw, that are directly traceable to the WRR.
One of the first applications of the radiation measurements made in Cabauw was a detailed evaluation of different methods for the determination of sunshine duration from measurements of global radiation6,7). The true sunshine duration was derived by application of the formal WMO definition of sunshine duration, which says that ‘the sun shines’ when the direct solar irradiance exceeds 120 W m-2. The evaluation showed that relatively accurate estimates of daily sunshine duration can be made from 10-min mean observations of global irradiance, using simple correlation techniques (Figure 5). The uncertainties for daily and yearly sums appear to be typically 0.5 h day-1 and 0.5%, respectively.
Slob and Monna8) developed a more complicated method for the determination of sunshine duration from global radiation, which relies on parameterized estimates of direct and diffuse radiation. An adjusted version of the method has been operational since 1992 for nation-wide estimates of sunshine duration9). Using the BSRN measurements for Cabauw, Hinssen and Knap6,7) showed that the method is potentially accurate but requires extensive tuning of model parameters. They also showed that the operational algorithm significantly overestimates the sunshine duration; the cumulative number of sunshine hours for 2005 appears to be 13% too high. This is caused by the fact that the operational method is tuned to the traditional way of determining sunshine duration, which is based on the use of the Campbell-Stokes sunshine recorder (a glass sphere that burns a trace in a paper card). This instrument tends to overestimate the sunshine duration, especially during broken-cloud conditions. Hinssen6) gives recommendations on how to improve the operational sunshine duration product.
The BSRN radiation measurements made in Cabauw, together with other measurements made within the framework of the Cabauw Experimental Site for Atmospheric Research (CESAR) allow us to perform detailed studies of the interaction between clouds, aerosols, and radiation. An example of the effect of aerosols on the diffuse sky radiation is shown in Figure 6, which contains cloudless measurements made in 2006 on a day with low aerosol load (9 September) and high aerosol load (12 September). The aerosol optical thickness (AOT) at 500 nm, measured with the SPUV sunphotometer10,11), ranges from 0.06 on the first day to 0.41 on the second day, which largely spans the spectrum of AOT values occurring in the Netherlands. At noon, the diffuse sky radiation on 12 September (high AOT) is about 60 W m-2 higher than on 9 September (low AOT), which is caused by enhanced scattering of sunlight by aerosols. Since aerosols scatter visible sunlight without a clear preference for a certain wavelength, the human eye observes this increase in diffuse sky radiation as a change in sky colour; from deep blue to whitish.
To further analyse the relation between aerosols and radiation, radiative transfer calculations were made for a Rayleigh atmosphere (containing only the permanent atmospheric gases) with added water vapour (dotted lines in Figure 6). By substracting these calculations from the measurements of diffuse sky radiation, one is left with the direct aerosol effect for the diffuse sky radiation. On the two days mentioned above the effect appears to be at noon 40 W m-2(low AOT) and as much as 100 W m-2 (high AOT).