Dynamic calibration of two catching type drop-counting rain gauges

This study reports the results of laboratory tests performed to assess the performance of two drop counting rain gauges of the catching type, and to propose suitable correction to make them compliant with the specifications of the World Meteorological Organization (WMO) at one-minute time resolution for Rainfall Intensity (RI) measurements. The tests were limited to the steady state conditions, with known and constant flow rates provided to the instrument at various reference intensities for a sufficient period of time, in order to compare the measures provided by the gauge with the reference figures (which is known as dynamic calibration). The instruments investigated are manufactured by Ogawa Seiki Co. Ltd (Japan) and the Chilbolton RAL (UK). They are designed as high-sensitivity drop counter type rain gauges. The dynamic calibration adopting a constant volume of the droplets declared by manufacturers reveals that these instruments do not fulfil the WMO requirements. A calibration curve of the drop volume with respect to the measured drop frequency has been derived for each instrument. Using a suitable correction algorithm, based on calibration curves as obtained from the tests performed in the laboratory, the precision of the instruments is improved and the results are fully compatible with the WMO required maximum admissible error (WMO, Pub. No 8). Laboratory tests also reveal the operational limit of this kind of instrument that is given by the rainfall intensity at which the water flux from the nozzle start to be continuous, therefore a standalone installation is discouraged.