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This study was to assess the raindrop fall speed measurement capabilities of OTT Parsivel²disdrometer through comparisons with measurements of a collocated High-speed Optical Disdrometer (HOD). Raindrop fall speed is often assumed to be terminal in relevant hydrological and meteorological applications, and generally predicted using terminal speed–raindrop size relationships obtained from laboratory observations. Nevertheless, recent field studies have revealed that other factors (e.g., wind, turbulence, raindrop oscillations, and collisions) significantly influence raindrop fall speed, necessitating accurate fall speed measurements for many applications instead of reliance on laboratory-based terminal speed predictions. Field observations in this study covered rainfall events with a variety of environmental conditions, including light, moderate, and heavy rainfall events. This study also involved rigorous laboratory experiments to faithfully identify the internal filtering and calculation algorithm of OTT Parsivel². Our assessments revealed that, for the smaller diameter bins, Parsivel²filters out many of the observed raindrops that fall faster than predicted terminal speeds, bringing down the mean fall speed for those size bins without observational evidence. Furthermore, Parsivel²fall speed measurements exhibited notable artificial bell-shaped deviations from the predicted terminal speeds toward subterminal fall starting at around 1 mm diameter raindrops with peak deviations around 1.625 mm diameter bin. Such bell-shaped fall speed deviation patterns were not present in collocated HOD measurements. Assessment results along with the faithfully identified Parsivel²algorithm are presented with discussions on implications on reported raindrop size distributions (DSD) and rainfall kinetic energy.

 

Abstract This study investigates the OTT Pluvio 2 weighing precipitation gauge’s random and systematic error components as well as stabilization of the measurements on time-varying rainfall intensities (RI) under laboratory conditions. A highly precise programmable peristaltic pump that provided both constant and time-varying RI was utilized in the experiments. Abrupt, gradual step, and cyclic step changes in the RI values were evaluated. RI readings were taken in real time (RT) at different time resolutions (6–60 s) for the RI range of 6–70 mm h −1 . Our analysis indicates that the lower threshold for the OTT Pluvio 2 ’s real-time RI measurements should be redefined as 7 mm h −1 at a 1-min resolution. Tolerance intervals containing 95% of the repeated measurements with a probability of 0.95 are given. It is shown that the measurement variances are unequal over the range of RI and the measurement repeatability is not uniform. A statistically significant negative bias was observed for the RI values of 7 and 8 mm h −1 , while there was not a statistically significant linearity problem. Through the use of statistical control limits, it is shown that means of the RI measurements stabilized on the actual RI value. A detailed investigation on RT bucket weight measurements revealed a time delay in bucket weight measurements, which causes notable errors in reported RI measurements under dynamic rainfall conditions. To demonstrate the potentiality of large errors in Pluvio 2 ’s real-time RI measurements, a set of equations was developed that faithfully reproduced the Pluvio 2 ’s internal (hidden) algorithm, and results from dynamic laboratory and in situ rainfall scenarios were simulated. The results of this investigation show the necessity of modifying the present Pluvio 2 RI algorithm to enhance its performance and show the possibility of postprocessing the existing Pluvio 2 RI datasets for improved measurement accuracies.