WORLD METEOROLOGICAL ORGANIZATION INSTRUMENTS AND OBSERVING METHODS REPORT No. 84 WMO LABORATORY INTERCOMPARISON OF RAINFALL INTENSITY GAUGES Trappes (France) - Genoa (Italy) - De Bilt (Netherlands) September 2004 – September 2005 L. Lanza (Italy) M. Leroy (France) C. Alexandropoulos (France) L. Stagi (Italy) W. Wauben (Netherlands) WMO/TD-No. 1304 2006 NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or its authorities, or concerning the limitation of the frontiers or boundaries. This report has been produced without editorial revision by the Secretariat. It is not an official WMO publication and its distribution in this form does not imply endorsement by the Organization of the ideas expressed. TABLE OF CONTENTS Page FOREWORD PART I RAINFALL INTENSITY MEASUREMENT INSTRUMENTS AND UNCERTAINTY 1. Rainfall Intensity 1 2. Rainfall Intensity Gauges 3 3. Uncertainty Sources and Measurement Errors 7 4. Previous related intercomparisons 12 PART II LABORATORY INTERCOMPARISON RESULTS AND CONCLUSIONS 1. Rationale 13 2. Applied Methods 16 3. The testing devices 18 4. Uncertainty of reference intensity 22 5. Description of the database 23 6. Quality control 23 7. Data policy 23 8. Participating instruments 24 9. Results 27 10. Conclusions 41 11. Recommendations 41 ACKNOWLEDGEMENTS 43 REFERENCES 44 ANNEXES Annex I: Overview of the Laboratories 46 Annex II: Questionnaire I on potential participants 52 Annex III: Questionnaire II to selected participants 54 Annex IV: Uncertainty assessment for the QM-RIM developed at the DIAM 58 laboratory, Italy Annex V: Uncertainty assessment for the bench calibration at the Trappes 62 laboratory, France Annex VI: Uncertainty assessment for the bench calibration at the KNMI laboratory, 64 The Netherlands Annex VII: Summary table: synthesis of the results 66 Annex VIII: Correction of historical rain series: a sample exercise 68 APPENDIX Instrument Data Sheets 73 FOREWORD The Thirteenth Session of the Commission for Instruments and Methods of Observation (CIMO) recognized the need to conduct an in depth assessment of rainfall intensity instruments. During this Intersessional period CIMO conducted a laboratory Intercomparison of Rainfall Intensity (RI) gauges from September 2004 through September 2005 in the laboratories of the Royal Netherlands Meteorological Institute, Meteo-France and the Italian Meteorological Service (Department of Environmental Engineering, University of Genoa). The Project Team analyzed the results of the Intercomparisons and the Final Report was published and posted to the CIMO/IMOP website in January 2006. Based on the analysis, proposals were made for standardizing procedures for laboratory calibration of RI gauges and on reference equipment for field Intercomparisons. Follow-on actions have included contacting manufacturers of the Intercomparison results assisting manufacturers in synchronizing current practices and future planning. This document constitutes the Final Report of the WMO Laboratory Intercomparison of Rainfall Intensity (RI) Gauges launched, simultaneously, in September 2004, at the laboratories of the Royal Netherlands Meteorological Institute (The Netherlands), Météo-France (France) and the Department of Environmental Engineering of University of Genoa (Italy) in collaboration with the Italian Meteorological Service. Prior to this Intercomparison there had been no international intercomparison of instruments for the measurement of RI. Following the recommendations of the Expert Meeting on Rainfall Intensity Measurements, Bratislava (Slovakia) April 2001, it was proposed that the first necessary step to be taken was to organize and conduct the first intercomparison of RI instruments under laboratory conditions. Some laboratory tests of RI gauges had been conducted and their results documented in literature, however no intercomparison of RI instruments in multiple laboratories, had ever been conducted. The main objective of the Intercomparison was to test the performance of catchment type rainfall intensity gauges utilizing different measuring principles under controlled conditions. Other objectives of the Intercomparison 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. The CIMO Project Team consisted of the Chair of the Expert Team on Surface-based Instrument Intercomparisons and Calibration Methods, the International Organizing Committee on Surface-Based Instrument Intercomparisons, the Project Leader and three Site Managers, all of whom coordinated the work of the three laboratories involved. The 19 pairs of participating instruments proposed by 18 manufacturers were divided into three groups, with each group being tested for a period of three to six months in each of the laboratories with the purpose of obtaining a high degree of confidence in the results. All costs related to laboratory intercomparisons were born by the laboratories and the manufacturers. The gauges fell into three categories; the majority fell into the first category, tipping-bucket gauges, which are the most widely used in operational networks; the second group of instruments consisted of weighing gauges; and the third group consisted of two participating instruments using a non-common measuring principle, namely, a water level based on conductivity measurement. A general methodology was adopted for testing based on the generation of a constant water flow from a suitable hydraulic device within the range of operational use as declared by the instrument manufacturer. Water was conveyed to the funnel of each instrument under test in order to simulate constant rainwater intensity. Flows were measured by weighing the water over a given period of time. The relative difference between the measured and generated rainfall intensity was assumed as the relative error of the instrument for a given reference flow rate. In addition to measurements based on regular flow rates the step response of each instrument was checked, based on suitable devices developed by each laboratory. The results of the Intercomparison showed that tipping-bucket rain gauges equipped with proper correction software provided 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 limitting the usable range for RI measurement. The uncertainty of the rainfall intensity is generally less for weighing gauges than for the tipping-bucket rain gauges under constant flow rate conditions 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 time variation of the RI by weighing gauges. The delay is the result of the internal software 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 controlled laboratory conditions. Siphoning problems with one gauge limited its ability to measure a wide range of rainfall intensity; the other was limited by the emptying mechanism, in which case a 2-minute delay was observed. These gauges are potentially sensitive to water conductivity but demonstrated no problems during the laboratory tests. Laboratory tests were performed under controlled conditions and constant flow rates so as to determine the intrinsic counting errors. It must be assumed that RI is highly variable in time, thus catching errors may have a strong influence on the overall uncertainty of the measurement. The need to combine counting and catching errors for the instruments analyzed in the laboratory is paramount. Provided the instrument is properly installed, according to 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 Intercomparisons alone, although the results do provide preliminary information to manufacturers and are the first-step in the selection criteria for the user. Therefore, it is necessary to proceed with the quality assessment procedure initiated in the laboratory by organizing a follow-up intercomparison in the field where instruments tested in the laboratory have a priority. This allows continuity in the performance assessment procedure and results in the estimation of the overall operational error to be expected in the measurement of RI in the field. Other instruments would be included in the field intercomparison, if not tested in the laboratory phase, with a priority given to the non-catching type instruments. Finally, the improvement in uncertainty of RI gauges introduces the risk of affecting the homogeneity of rainfall time series. The improvement of RI measurement may produce a discontinuity in historical rainfall intensities records, which could influence studies of extreme events. The bias introduced by non-corrected records propagates through any rainfall-runoff model