Progress towards a calibration facility for non-catching disdrometers

Precipitation is among the most important meteorological variables for meteorological, hydrological and climate studies. From an operational point of view, non-catching precipitation gauges are attractive, because they require less maintenance compared to the more traditional catching types (e.g. gravimetric or tipping-bucket). For that reason, in recent years non-catching precipitation gauges are increasingly adopted in meteorological networks. Despite the growing number of such gauges, calibration procedures and dedicated setups are missing. The result is that sensors are not calibrated, and this has a negative influence on the accuracy and reliability of the measured data. Therefore, Danish Technological Institute (DTI) is currently developing at setup for test and calibration of non-catching rain gauges. The setup is intended to simulate real rain, i.e. it will produce water drops that are similar in both size and velocity to those found in real rain. The setup can produce drops with diameters from about 0.2 mm to several mm. Traceability to the SI-unit system is obtained by flow and area measurements. In this context a volumetric pump is employed and calibrated gravimetrically in the DTI micro-flow laboratory. The drop velocity is determined using a photographic system. A key advantage of the setup is that the non-catching gauges can be calibrated under well-controlled, stable laboratory conditions. The presentation describes the design principle and summarizes recent progress of the setup at DTI.