<p>The project’s work ensures traceability for precipitation gauges which cannot be calibrated using established methods</p>
The project’s work ensures traceability for precipitation gauges which cannot be calibrated using established methods
Atmospheric precipitation, most commonly rain or snow, has profound effects on everyday life, from transport and productivity to recreation and personal safety. It is also defined by the World Meteorological Organisation (WMO) as an Essential Climate Variable and so its measurement and characterisation is key for studying the effects of climate change.
The most common instruments used to measure precipitation are catching gauges which physically collect precipitation to determine either the volume (e.g., tipping bucket gauges) or mass (e.g., weighing gauges). However, due to their direct exposure to the elements, these instruments are affected by harsh environmental conditions - such as very low temperatures or high winds - and require frequent maintenance. They can also underreport precipitation levels due to raindrops sticking to the instrument’s collecting funnel.
Non-catching gauges such as optical disdrometers, which instead ‘sense’ precipitation, are less affected by these issues and can also provide additional information beyond precipitation intensity alone, such as drop size, distribution and visibility. As they do not need to come into direct contact with the environment, they are also ideal for use in remote or automatic weather stations.
However, because non-catching gauges do not collect precipitation, they cannot be calibrated using a reference flow rate or traditional mass and time traceable standards. Instead, the characteristics of a rain event need to be reproduced with known drop size, frequency and fall velocity. This has meant that traceable calibration techniques for non-catching gauges have not been available.
Completed EMPIR project Calibration and accuracy of non-catching instrument to measure liquid/solid atmospheric precipitation (18NRM03, INCIPIT) has developed traceable calibration methods for non-catching precipitation gauges, specifically implemented in a form that can be incorporated into CEN and ISO standards. This has included contributions to CEN/TC 318/WG 12 (Rainfall Intensity) and ISO/TC 113 (Hydrometry).
Project achievements
The project has developed three raindrop generators to be used in the calibration of non-catching precipitation gauges.
The first, developed by Danish Designated Institute, DTI, is able to generate drops with diameters ranging from 0.2 mm (using flat-tipped needles) to 7 mm (using specially made nozzles). The second, developed by the University of Genoa (UNIGE), generates drops with diameters between 4.5 mm and 20 mm using high-precision syringe pumps, which are then measured using a high-resolution camera. The third, designed by Belgian National Metrology Institute, SMD, uses peristaltic pumps and different nozzle geometries to generate drops with diameters between 2.2 mm and 4.9 mm.
The generator from DTI has been employed successfully for test calibration measurements in both the lab and field tests, while the UNIGE generator is now operational at the UNIGE rain gauge laboratory, used mainly for research and to increase the awareness of the importance of traceability in meteorological measurements among users. The generator from SMD is used for in-lab tests.
The project undertook measurement campaigns in the lab and in the field at two WMO-recognised field test sites in Payerne, Switzerland and Vigna di Valle, Italy. In Switzerland, the DTI and UNIGE raindrop generators were used to verify a laser disdrometer. This also used a dedicated structure developed by INRiM which allows drops to fall vertically from up to 10 m, crossing the sensing area of the instrument without being deviated by the wind. In Italy, the UNIGE generator was used to verify a light scatter disdrometer.
This was the first time that this kind of in-field and laboratory validations had been performed.
- Contribution to standards
The project has contributed to CEN Technical Report Report CEN/TR 17993:2023 “Calibration and accuracy of non-catching precipitation measurement instruments” and to the draft European Standard CEN/prEN 18097:2024 “Hydrometry - Measurement of precipitation intensity - Metrological requirements and test methods for non-catching type rain gauges.”
The project has published a number of papers, including “Calibration of non-catching precipitation measurement instruments: A review” in the journal Meteorological Applications and “Calibration uncertainty of non-catching precipitation gauges” in the journal Sensors.
Project coordinator Andrea Merlone (INRiM) has said of the project’s work:
“Non-catching and non-contact instruments are finding a growing application in environmental measurements and the metrology community demonstrates its prompt response and service in establishing traceability, also for these new and evolving technologies. INCIPIT is a normative project originated within the large “family” of MeteoMet projects where participating National Metrology Institutes and Designated Institutes generated a unique direct involvement in such a fundamental field of measurements for climate, agriculture and hydrological protection: precipitation. The results of the project, in terms of technically developed and tested best practice for the calibration of instruments immediately delivered direct outreach to the user’s community, in terms of publication of new standards and guidelines. It also attracted the interest of manufacturers, in their mission to provide improved systems, by better knowing their measurement capabilities".
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.
Want to hear more about EURAMET?
Sign up for EURAMET newsletters and other information
Follow us on LinkedIn and X/Twitter
