An Intercomparison Methodology for Non-Catching Precipitation Measurement Instruments Applied in Norway

Precipitation measurements from two disdrometers (Thies LPM and OTT Parsivel²) and an optical sensor (Vaisala PWD12) are intercompared and evaluated against a Geonor T-200B precipitation gauge under low-wind conditions in Oslo. Proper use of those instruments in practical applications requires understanding their accuracy in measuring precipitation amounts and their reliability in reporting present weather. We establish a detailed, reproducible methodology to harmonize data from multiple precipitation instruments and meaningfully compare measurements of precipitation occurrence, amount, and type. The methodology is applied to one year of hourly and sub-hourly data from a test site in Oslo. Relative to the Geonor, liquid precipitation amounts are underestimated by the three devices by 3-18%; solid precipitation amounts are largely overestimated by the Parsivel2 (up to 100%), by 14-21% by the LPM, but are underestimated by 11-18% by the PWD12. The disdrometers agree nearly perfectly on liquid versus solid precipitation types, while the PWD12 detects more liquid and less solid precipitation. We identify two weather conditions that challenge the instruments: i) trace precipitation occurrence is inconsistently characterized and highly dependent on device detection thresholds, with the LPM reporting far more wet observations than any other instrument; ii) mixed-type precipitation is agreed upon only 53% of the time by the Parsivel2 and the LPM, and less than 20% by the PWD12. Our methodology can be reproduced to study other instrument combinations in other locations, and our results can help users of those instruments critically consider their measurements.