KNMI contribution to the WMO Laboratory Intercomparison of Rainfall Intensity Gauges

The Commission for Instruments and Methods of Observation (CIMO-XII Casablanca, 1998) of the World Meteorological Organization (WMO) discussed the need for standardization of rainfall intensity measurements and considered the need and possibility of a Rainfall Intensity Measurement Intercomparison. These issues were addressed during the Expert Meeting (EM) on Rainfall Intensity Measurements held in Bratislava, Slovak Republic, 23-25 April 2001. The meeting focused on rain intensity only. The Expert Meeting discussed the advantages and disadvantages of the various performance characteristics of different measuring techniques used for rainfall intensity measurements and concluded that only in situ catchment type gauges should be considered further. This because this type of sensors is the most practical and widely used in operational networks for rainfall intensity measurements and, in addition, laboratory and field intercomparisons of these gauges are considered to be feasible. The Expert Meeting agreed that the calibration of rainfall intensity gauges was the primary and most stringent task. Calibration techniques for catchment type gauges have been described in the literature, but at the present there is no standardized calibration equipment or procedure suitable for general application. Therefore the development and testing of a standardized calibration technique has to be developed first in well-certified laboratories. Furthermore, the laboratory tests of the rainfall intensity gauges should be performed in at least two independent certified laboratories. The Expert Meeting recognized that there was a particular need to compare gauges for high rainfall intensity rates, since the general performance characteristics of various types of rain gauges had been sufficiently tested for low intensities at various national and international intercomparisons. Taking into account the difficulties related to organizing and conducting a field intercomparison in a climatic region with the required high rainfall intensities during a comparison period and the unavailability of suitable and recognized reference instruments, it was agreed to start first with a laboratory intercomparison on rainfall intensity gauges. A decision towards a field intercomparison should then be made based on the results of the initial laboratory comparison.