## 3 Results
### Influence of Measurement Strategies and Instrument Metrologies
When a RR threshold of 0.2 mmhr\({}^{-1}\) is applied, the probability of precipitation from the OTT Parsivel\({}^{2}\) exceeds that from the other two sensors. Over three-quarters of all 1 min periods when the optical, impact and video disdrometers were operating without error \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **RR \textgreater{} 0.1 mmhr\({}^{-1}\)** & \multicolumn{2}{c}{**RR \textgreater{} 1 mmhr\({}^{-1}\)**} & \multicolumn{2}{c}{**RR \textgreater{} 10 mmhr\({}^{-1}\)**} \\ \hline  & **HR** & **FAR** & **HR** & **FAR** & **HR** & **FAR** \\ \hline Optical v video & 0.781 & 0.0020 & 0.788 & 0.0018 & 0.705 & 0.0003 \\ \hline Optical v impact & 0.600 & 0.0017 & 0.570 & 0.0013 & 0.525 & 0.0003 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Hit Rates (HR) and False Alarm Rates (FAR) for the co-occurrence of RR above the specified thresholds from the optical, impact and video disdrometers operating at the DoE ARM site in the Southern Great Plains (SGP). See also Figure 16.  
Figure 16: Scatterplots of 1 min rainfall rates (RR, in mmhr\({}^{-1}\)) from the three disdrometers operating at US SGP. The optical disdrometer (OTT Parsivel\({}^{2}\)) is shown in the \(x\)-axis and the red and blue dots show all 1 min periods when the video (red) or impact (blue) disdrometers also show RR above the specified threshold; (**a**) 0.1 mmhr\({}^{-1}\), (**b**) 1 mmhr\({}^{-1}\) and (**c**) 10 mmhr\({}^{-1}\). Panels on the right show the mean droplet size distributions for those same 1 min periods. Two values are shown for each diameter for the optical disdrometer, they denote conditional sampling for co-occurrence of the specific RR in the video and impact disdrometers. The values shown in the legends of the left-hand panels denote the Spearman (rank) correlation coefficients (r\({}_{\text{s}}\)) and the sample size (n). Hit Rates (HR) and False Alarm Rates (FAR) for the co-occurrence of RR above the specified thresholds are given in Table 4.  
flags and the optical displacement indicated a RR above 0.1 mmhr\({}^{-1}\) are also identified as having RR > 0.1 mmhr\({}^{-1}\) (and RR > 1 mmhr\({}^{-1}\)) in the video disdrometer (Table 3). It is notable that this HR is lower than that reported from an earlier study in Switzerland [43], but the FAR is also lower. This may reflect use of a screen to deselect periods with very light RR in the current analysis. For a RR threshold of 10 mmhr\({}^{-1}\), which equates to approximately the 96 th percentile of all non-zero RR, and the 99.85 th percentile of all 1 min periods, the HR drops to 0.705 (Table 4). Thus, a quarter of all 1 min periods when the optical disdrometer indicated very high RR are not identified as such in the video disdrometer time series. This finding is consistent with the existing literature summarized above and also suggests that discrepancies between disdrometers are amplified at high RR that have greatest relevance to LEE. Comparisons between the impact disdrometer and the optical disdrometer indicate lower HR, but similarly low FAR. When the data time series are conditionally sampled to select only periods when either the optical disdrometer and the video disdrometer or the optical disdrometer and the impact disdrometer exhibit RR exceeding either 0.1 mmhr\({}^{-1}\), or 1 mmhr\({}^{-1}\) or 10 mmhr\({}^{-1}\), the Spearman correlation coefficients (\(\rho_{\rm s}\)) all exceed 0.78, indicating relatively good but incomplete agreement between RR from the different instruments (Figure 16). The mean droplet size distributions for RR with different thresholds from the optical, impact and video disdrometers at US SGP also exhibit important differences. For example, for both the 1 mmhr\({}^{-1}\) and 10 mmhr\({}^{-1}\) RR the video disdrometer indicates a much greater abundance of droplets of all diameters (Figure 16).