Calibration of non-catching type rain gauges: preliminary tests on an optical disdrometer

The last WMO Field Intercomparison on Precipitation Intensity highlights that non-catching type rain gauges show limited performance, in terms of precision and accuracy of the measurements, when compared to the traditional catching type rain gauges. Moreover, the performance of these instruments differ significantly from each other, this fact suggests that the lack of a standardized calibration procedure leads to different behaviour in field measurements. Building on these results and in order to understand the causes of this behaviour, the CIMO Lead Centre on Precipitation Intensity has developed a rain drop generator, able to reproduce drops with various diameters. The Thies LPM disdrometer has been tested using the rain drop generator and the results are presented in terms of total number of droplets detected by the instrument, droplets per diameter class, total accumulation volume and measured rainfall intensity. Preliminary results show that the disdrometer detects a larger number of droplets than those actually generated. Most of them are attributed by the instrument to the lower diameter class, and have a low impact in the total volume account or rainfall intensity estimation, but could affect the shape of the Drop Size Distribution (DSD) provided by the instrument. Moreover, in many cases it has been found that the disdrometer assigns most of the real droplets to the upper diameter class, therefore resulting in a significant overestimation of the rainfall amount and intensity. These results, although still preliminary, reveal that a standardized and rigorous calibration procedure is needed for the non-catching type rain gauges to foster more reliable and comparable measurements.