## 3 Results
### Influence of Measurement Strategies and Instrument Metrologies
Most have focused on comparisons of different optically based instruments (see summary given in Table 3 and following discussion). For example, one study at a site in Austria compared data from two PWS100, a Thies LPM and an OTT Parsivel1 against a weighing rain gauge. All of the disdrometers underestimated accumulated precipitation by 2 to 29% relative to the rain gauge [145]. Mean droplet size spectra from Thies LPM showed lower number density for \(D>1\) mm. The PWS100 sensors had highest modal droplet \(D\) but exhibited important differences from each other. PWS100 showed 31 and 22% of droplets were in the diameter range 1-2 mm. Equivalent data from the Thies LPM and OTT Parsivel1 indicated 6% and 16% [145]. Conversely a long-term study of multiple Thies LPM relative to tipping-bucket rain gauges found the Theis LPM tended to overestimated long-term accumulated precipitation, and that the Theis LPM exhibited differences in long-term accumulated precipitation of up to 18% [146]. A further study of the Thies LPM and an OTT Parsivel1 conducted over 3 years in southern Australia found only small discrepancies between the two Thies LPM sensors, but greater differences between the two OTT Parsivel1 instruments. The authors report \"LPM recorded 1 to 2 orders of magnitude more smaller droplets for drop diameters below 0.6 mm compared to the OTT Parsivel1, with differences increasing at higher rainfall rates\" [147]. Analyses of accumulated precipitation and DSD from impact, 2 DVD and OTT Parsivel1 disdrometers indicated \"reasonable agreement\" in terms of total event accumulated precipitation with a reference rain gauge, with mean biases of \(-3.5\%\), \(+0.4\%\) and \(-7.5\%\), respectively [148]. Relative to the impact and 2 DVD, the OTT Parsivel1 was negatively biased in terms of the number concentrations for \(D<0.76\) mm, and exhibited a positive bias for \(D>2.44\) mm. The divergence between the instruments was most marked for RR \(>2.5\) mmhr\({}^{-1}\)[148]. A further study lasting 2 years and employing two Thies LPM and two OTT Parsivel2 disdrometers during \(>\)200 rain events, also found the Thies sensors exhibited a higher total number of droplets but considerably lower median droplet diameter, even after correcting for the differences in measured diameter range. The discrepancies were amplified at high RR [149]. An evaluation of the Thies LPM in terms of precipitation detection, intensity and phase relative to an OTT weighing precipitation gauge and a 2 DVD found the HR for precipitation detection is 99% (FAR = 9.9%) during periods with liquid precipitation [43]. However, the Thies disdrometer underestimated rainfall intensities by 16.5% which the authors linked to under-sampling of the number concentration of hydrometeors with \(D\) of 0.5 to 3.5 mm relative to the 2 DVD [43]. The slope of a linear fit of RR averaged to 30 min from the Thies disdrometer and the OTT  
Figure 15: Mean droplet size distributions from the OTT Parsivel2 disdrometer at US SGP conditionally sampled for the presence of WC indicating hail and/or graupel and an equal number of liquid (rain only events) centered on the same mean RR as prevails during the hail/graupel events. pluviometer was equal to 0.80. The authors proposed adjusting the number concentrations of rain droplets in the first three diameter classes (\(D\): 0.125-0.5 mm) down by factors of 0.83 to 0.92, and those in classes 4 to 12 (\(D\): 0.5-3.5 mm) up by factors of 1.08 to 1.43, and those in classes 13 to 22 (\(D\): 3.5 to \(>8\) mm) down by factors of 0.92 for class 13 to 0.19 for the 22 nd (and largest) class [43]. A further study comparing optical and 2 DVD discharges with an MRR found best consistency between number concentrations for \(D\) = 1.1 to 3.8 mm and for longer averaging periods [102]. That study also reported much higher number concentrations (higher by a factor of 10) for \(D\) < 0.5 mm from the MRR relative to the ground-based gradiometers.