## 4 Discussion
Causes of these differences are only partly resolved and the discrepancies in data from different instruments may vary with the prevailing hydroclimate. Instrument closure experiments to resolve these matters and those arising from difference in instruments from the same manufacturer are urgently needed. There would also be tremendous benefit from greater transparency on the part of instrument manufacturers regarding the data inversion used to derive DSD, RR and the weather codes (hydrometeor phase), in terms of reconciling data from the different sensors. Two key recommendations are advanced regarding closure experiments. First, a controlled experiment should be designed andperformed for the in situ disrometers where known DSD at varying RR are applied in a laboratory setting to characterize the different instrument response more comprehensively. Second, a comprehensive and extended field experiment should be performed to better characterize the relative performance and comparability of data from all the different instrument technologies. Such an experiment should ideally be conducted in a location with a high range of RR, and a range of hydrometeor types (including hail) and unlike most past research would be fully inclusive of all metrologies. The data from such experiments should be made public to allow both for advancements in instrument metrology, improvements in signal processing and improved uncertainty quantification. Data and analyses presented herein also indicate that reliance on the Best approximation to project droplet size distributions for use in whirling-arm experiments is not appropriate and may lead to biases if extrapolated to accumulated material fatigue. The Marshall-Palmer distribution also fails to fully capture the DSD at any of the sites from which data are reported herein. This implies that further method development is required to describe DSD for LEE studies, although better characterization of hydroclimatic conditions is also likely to be beneficial for other applications. Inclusion of solid hydrometeors in whirling-arm experiments for material stress assessment is also critical to correctly represent the hydroclimate in many regions of the world where wind turbines are being deployed. Data summarized herein indicate the droplet size distributions of hydrometeors, precipitation frequency and intensity and hail frequency vary markedly across locations where wind turbines are currently deployed or may be deployed in the near future, particularly offshore. The implication is that atmospheric conditions associated with leading edge erosion (e.g., frequency of hail and heavy precipitation co-occurring with wind speeds above cut in) will also exhibit large site-to-site variability. The state of the art (including data summarized above) suggests there are likely to be pronounced regional differences in terms of the relative importance to total accumulated kinetic energy transfer to the blades of; (i) hail versus rain, (ii) low intensity (low rainfall rates) but sustained precipitation periods versus high intensity but relatively short duration precipitation periods. Accordingly, there is likely to be substantial geographic variability in the need for LEE preventative measures and the relative utility of erosion safe operation mode versus use of protective tapes will also be location specific. However, in the absence of reliable and high-fidelity measurements of hydroclimatic conditions, identification of optimal solutions will remain elusive.