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WMO Laboratory Intercomparisons of Rainfall Intensity (RI) Gauges

The WMO Laboratory Intercomparison of Rainfall Intensity (RI) Gauges was launched, simultaneously, in September 2004, in the laboratories of the Royal Netherlands Meteorological Institute, Météo‑France and the Department of Environmental Engineering (University of Genoa).
No intercomparisons of instruments for the measurement of RI had been organized earlier. Following the recommendations of the Expert Meeting on Rainfall Intensity Measurements, Bratislava, Slovakia, April 2001, it was considered as the first and necessary step to organize an intercomparison of such instruments in the laboratory. Some laboratory tests of rain gauges were done and reported in the literature, however no systematic intercomparison of a large number of instruments, in one or several laboratories, had yet been conducted.
The objective of the Intercomparison was to test the performance of catchment type rainfall intensity gauges of different measuring principles under documented conditions. Other important tasks were to define a standardized procedure for laboratory calibration of catchment type rain gauges, and to provide information relevant to improving the homogeneity of rainfall time series with special consideration given to high rainfall intensities. Finally, a comment on the need to proceed with a field intercomparison of catchment type of rainfall intensity gauges was required as well as to identify and recommend the most suitable method and equipment for reference purposes within the field intercomparison of catching and non-catching types of gauges.
The Final Report of the Intercomparison is available here, and is published as IOM Report n. 84 (TD 1304) by WMO/CIMO.
The CIMO Project Team, consisted of ET/IOC Chair, the Project Leader and Site Managers coordinated the work of the laboratories involved in the intercomparison. The 19 pairs of participating instruments from 18 manufacturers were divided into three groups, with each group being tested for a period of about three to six months in each of the laboratories, in order to obtain a high degree of confidence in the results. The first phase of tests had successfully concluded by 15 February, the second by 15 May 2005 and the third by September 2005. All the cost related to laboratory intercomparisons was born by the laboratories and the manufacturers involved.
The majority of the participating instruments were tipping-bucket gauges which are the most widely used in operational networks. Another group of instruments are weighing gauges, while two of the participating instruments use a different measuring principle, namely, a water level sensor based on conductivity measure.

A general methodology was adopted for the tests, based on the generation of a constant water flow from a suitable hydraulic device within the range of operational use declared by the instrument’s manufacturer. The water is conveyed to the funnel of the instrument under test in order to simulate a constant rain water intensity. The flow is measured by weighing the water over a given period of time. The relative difference between the measured and generated “rain intensity” is assumed as the relative error of the instrument for the given reference flow rate. In addition to measurements based on constant flow rates, the step response of each instrument was checked based on the devices developed by each laboratory.
Each laboratory developed its own testing device, with some differences in the principle and technology used to generate a constant water flow, as well as in the way the water is weighed in the device. These provided a basis for the development of a standardized procedure for generating consistent and repeatable precipitation flow rates for possible adoption as a laboratory standard for calibration of catchment type rainfall intensity gauges.



The results of the Intercomparison showed that the tipping-bucket rain gauges that were equipped with proper correction software provided good quality rainfall intensity measurements. The gauges where no correction was applied had larger errors. In some cases problems of water storage in the funnel occurred that could limit the usable range for rain intensity measurement.
The uncertainty of the rainfall intensity is generally less for weighing gauges than for the tipping-bucket rain gauges under constant flow rate condition, provided the instrument is properly stabilized. The measurement of rainfall intensity is affected by the response time of the acquisition system. Significant delays were observed in “sensing” the variation in time of the rain intensity. The delay is the result of the internal software which is intended to filter the noise. Only one instrument had a delay that met the WMO 1-minute rainfall intensity requirement.
The two gauges using a conductivity measurement for determining water level showed good performances in terms of uncertainty under these controlled laboratory conditions. Siphoning problems for one gauge limits its ability to measure a wide range of rainfall intensity. For the other one, a limitation is related to the emptying mechanism, in which case 2-minute delay was observed. These gauges are potentially sensitive to the water conductivity, but with no demonstrated problems during the laboratory tests.
The laboratory tests were performed under controlled conditions and constant flow rates (rain intensities) so as to determine the intrinsic counting errors. It must be considered that rainfall intensity is highly variable in time, thus catching errors may have a strong influence on the overall uncertainty of the measurement. The need to combine the assessment of both counting and catching errors for the instrument analyzed in the laboratory is paramount. Provided the instrument is properly installed in the field, according to the WMO specifications, the question to be answered is what kind of instrument (measuring principle, manufacturer, model) is the most suited to the specific requirements of the user. This question cannot be answered based on the Laboratory Intercomparison alone, although the results obtained can provide preliminary information to manufacturers and the first-step selection criterion for the user.
It was therefore deemed necessary to proceed with the quality assessment procedure initiated in the laboratory by organizing a follow-up Intercomparison in the field where the instruments tested in the laboratory should have priority. This would allow continuity in the performance assessment procedure and result in the estimation of the overall operational error to be expected in the measurement of rainfall intensity in the field. Other instruments could be included in the field intercomparison, even if not tested in the previous laboratory phase, with priority given in this case to the non-catching type of instruments.
For the Field Intercomparison a working reference rain gauge(s) should be inserted in a pit according to the EN-13798 Reference Raingauge Pit, adopted by ISO, in order to minimize the effect of weather related errors on the measured rain intensities. According to the results of the laboratory intercomparison, it was recommended to select the best performing dynamically corrected TBRGs and the weighing gauges showing the shortest step response and the lowest uncertainty as reference gauges. The combined analysis of the reference gauges allows the best possible estimation of the rainfall intensity in the field, given their demonstrated performance in the laboratory. The use of one reference instrument alone is not recommended.
Finally, the improvement of the uncertainty of rainfall intensity gauges brings the risk of affecting the homogeneity of rainfall time series. Older data may be adjusted computed from the most recent subset of recorded data. The improvement of the measurement of rainfall intensity may produce a discontinuity of the historical rain intensities records, which especially apply to the handling of high rainfall intensities, as is the case for the study of extreme events. The bias introduced by non-corrected records propagates through any rainfall-runoff model down to the statistics of flow rates in water courses, with non negligible effects on the study of floods and flash floods.

LC-PrIN - WMO-CIMO Lead Centre "B.Castelli" on Precipitation Intensity