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Efforts to reduce errors in infrared radiation observations at the Earth's surface
by Mr. Nozomu Ohkawara


In studies of the Earth's energy budget, radiation observation data from ground-based radiation observation networks such as the baseline surface observation network (BSRN) are used for the surface radiation of the Earth. It is known that there is a large error in the infrared radiation observation in the surface radiation due to the spectral characteristics of the outer dome (i.e., silicon dome) of the pyrgeometer. In the World Meteorological Organization (WMO), experts in the meteorological and metrological fields are cooperating to establish the new infrared radiation reference traceable to the SI units, and the use of windowless radiometers as reference instruments for the new radiation reference is expected to reduce errors. However after adoption of the new radiation reference, the pyrgeometer with silicon dome will continue to be used since the infrared radiation observation in the surface network require observations is made in various weather conditions. In the calibration to deliver the new radiation reference scale to the operational instruments, the spectral characteristics of this silicon dome may cause errors due to the difference in the radiation spectrum incident on the sensing elements of the reference windowless radiometers and the operational pyrgeometers. In addition, during actual observations, the integrated water vapour (IWV) in the atmosphere varies greatly depending on the geolocation of the observation, and the observed infrared radiation spectrum may differ from that at the average amount of IWV during calibration implying in errors in the observed infrared radiation. The following efforts should be made to reduce this error in observations of infrared radiation. In the current infrared observation by the pyrgeometer, the following factors should be taken into account in the calculation formula for determining infrared radiation from the output voltage from the thermopile and the temperature of the pyrgeometer body and its silicon dome: (1) spectral characteristics of the silicon dome, and (2) IWV at the time of the observation. Furthermore, it is necessary to develop a new type of pyrgeometer with a window that has flat spectrum characteristics in the infrared region under the any observation environments.


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Topic : Theme 1: Earth Energy Balance.
Reference : T1-C13

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