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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 Wind and turbulence effects on raindrop fall speeds were elucidated using field observations over a 2-yr time period. Motivations for this study include the recent observations of raindrop fall speed deviations from the terminal fall speed predictions ( V t ) based upon laboratory studies and the utilizations of these predictions in various important meteorological and hydrological applications. Fall speed ( V f ) and other characteristics of raindrops were observed using a high-speed optical disdrometer (HOD), and various rainfall and wind characteristics were observed using a 3D ultrasonic anemometer, a laser-type disdrometer, and rain gauges. A total of 26 951 raindrops were observed during 17 different rainfall events, and of these observed raindrops, 18.5% had a subterminal fall speed (i.e., 0.85 V t ≥ V f ) and 9.5% had a superterminal fall speed (i.e., 1.15 V t ≤ V f ). Our observations showed that distributions of sub- and superterminal raindrops in the raindrop size spectrum are distinct, and different physical processes are responsible for the occurrence of each. Vertical wind speed, wind shear, and turbulence were identified as the important factors, the latter two being the dominant ones, for the observed fall speed deviations. Turbulence and wind shear had competing effects on raindrop fall. Raindrops of different sizes showed different responses to turbulence, indicating multiscale interactions between raindrop fall and turbulence. With increasing turbulence levels, while the raindrops in the smaller end of the size spectrum showed fall speed enhancements, those in the larger end of the size spectrum showed fall speed reductions. The effect of wind shear was to enhance the raindrop fall speed toward a superterminal fall. 

Abstract Rainfall microphysical characteristics including raindrop fall speed, axis ratio, and canting angle were measured through field observations by using a high-speed optical disdrometer (HOD) during and after tornadic severe storm passage. High and low wind and turbulence characteristics were observed during and after passage, respectively, which provided an opportunity to compare the effects of the different wind and turbulence characteristics on raindrop characteristics. During passage, 9.4% of the raindrops larger than 1.0 mm in volume equivalent diameter ( D ) were identified as subterminal, whereas only 0.5% of the raindrops of the same size were detected as subterminal after passage. Contrary to findings in literature, we could not find any distinct superterminal fall speed behavior for raindrops with D < 1.0 mm during or after passage. For raindrops with D > 2.0 mm, deviations of the axis ratio distribution from the predicted distribution for the equilibrium raindrops were observed, and the deviations during passage were larger than those after passage. The deviations of the axis ratio distributions from the predicted distributions for the equilibrium raindrops were also observed for midsized (1.0 < D < 2.0 mm) raindrops; however, these deviations during and after passage were of similar magnitude. The canting angle distribution for raindrops with D > 2.0 mm was found to have the mean value of approximately 0° both during and after passage and the standard deviation values of 24.7° during passage and 13.6° after passage. This study shows the clear influence of wind on various rainfall microphysical characteristics and documents the observed value ranges of these characteristics under strong wind that are of importance for a number of rainfall applications, including radar rainfall retrievals and rainfall modeling. 

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. 

Abstract This study investigates the shapes and fall speeds of freezing and frozen raindrops through field observations using an instrument called the high-speed optical disdrometer (HOD) that we developed recently. Our field observations showed that while the shapes of all of the observed freezing raindrops and a portion of the frozen raindrops (39% of the frozen raindrops that are larger than 1.0 mm in volume equivalent diameter D ) resemble the shapes of warm raindrops, majority of frozen raindrops (61% of the frozen raindrops with D > 1.0 mm) exhibited a distinct feature such as a spicule, bulge, cavity, or aggregation. Field observations of axis ratios (i.e., ratio of the vertical to horizontal chord) and fall speeds were compared with the predictions of available models. Separate empirical axis ratio parameterizations were developed for the freezing and frozen raindrops using the HOD field observations and extensions to an available shape model were also incorporated. For the fall speeds of freezing and frozen raindrops, field observations demonstrated a good agreement with the predictions of the available parameterizations. Frozen raindrops showed a larger scatter of fall speeds around the mean fall speed of a given drop size than those of the freezing raindrops due to the shape variety among the frozen raindrops with the aforementioned distinct features. The drag coefficients for the observed hydrometeors were compared with the predictions of the available drag coefficient models. Separate “drag coefficient–Reynolds number” relationships for freezing and frozen raindrops were developed.