## 3 Results
### Influence of Measurement Strategies and Instrument Metrologies
Data from the MRR operated at the Norway site fall below the Best DSD approximation for large \(D\) but recall instrument intercomparisons suggest the MRR underestimates the presence of large rain droplets (Table 3). Conversely, data from the OTT Parsivel2 at that site indicate both the Best and Marshall-Palmer approximations underestimate the concentration of droplets at \(D\) 0.5 to 1.5 mm, and the Marshall-Palmer estimate more closely approximates the observations at larger \(D\) (Figure 7). Even data from the same type and generation of disdrometer are not always comparable. Data from an experiment wherein 16 identical Parsivel1 disdometers were deployed within a 6 m\({}^{2}\) area found substantial deviations of two of the 16 instruments. Although no details were provided, the authors reported 'two instruments yielded different measurements from the other 14 in terms of cross correlation for integral parameters' [99]. For this reason, a short instrument closure experiment from four brand-new OTT Parsivel2 didrometers that have consecutive serial numbers was performed on the Cornell University campus in upstate New York during 21 October to 7 December 2021. The disdometers were placed on a grid such that the detection heads are all within 2 m of each other. RR from the four instruments when subject to linear fits exhibit intercept values close to 0 (Figure 17b), and three of the four instruments exhibit RR that agree within the instrument specification of \(\pm\)10%. The slope of the linear fit is in the range 0.9-1.1 and the mean ratio of RR from disdometers 2-4 to that from disdrometer 1 are close to 1 (Figure 17a,b). Data from the third disdrometer (D3) exhibits a positive bias relative to the other three in terms of the RR. The bias in RR appears to be due to excess detected abundance of droplets in the 1-4 mm diameter range as is evidenced in the mean DSD from all four disdometers (Figure 17c). Footnote 1: The \(\sim\)10% of the data is not available for the \(\sim\)10% of the data. Disdrometer D3 was subsequently returned to the manufacturer for repair/recalibration. Upon return of that instrument a second closure experiment was performed to collect an approximately equal number of 1 min periods with precipitation. The results indicate that after the repair/recalibration the RR from this disdrometer have a mean ratio to RR from disdrometer D1 close to 1 (Figure 18a) and a slope of a linear fit of \(\sim\)0.9 with RR from the other disdometers. However, the repaired D3 now exhibits a negative bias in terms of RR (Figure 18b), and on average exhibits a smaller number density for \(D>1.5\) mm (Figure 18c). A key recommendation based on this, and other related research, is that prior to deployment of a disdrometer network the instruments should be collocated and subject to intensive closure experiments. Figure 17: (**a**) Probability distribution and (**b**) Scatterplots of 1 min rainfall rates (RR, in mmhr\({}^{-1}\)) from the four OTT Parsivel\({}^{2}\) disformeters (D1, D2, D3, D4) operating at Cornell University during the first instrument closure experiment. The probability distribution of RR shown in panel (**a**) is discretized into 0.5 mmhr\({}^{-1}\) intervals and the legend reports the mean ratio of RR from the disformeters. The legend in (**b**) denotes best-fit lines to the datasets where c is the intercept and m is the slope of a linear fit. (**c**) Mean DSD from the four instruments for all 8526 1 min periods when RR \(>\) 0 mmhr\({}^{-1}\). Figure 18: (**a**) Probability distribution and (**b**) Scatterplots of 1 min rainfall rates (RR, in mmhr\({}^{-1}\)) from the four OTT Parsivel\({}^{2}\) disformeters (D1, D2, D3, D4) operating at Cornell University during the second closure experiment, after disdrometer D3 had been repaired/recalibrated. The probability distribution of RR shown in panel (**a**) is discretized into 0.5 mmhr\({}^{-1}\) intervals and the legend reports the mean ratio of RR from the disformeters.