Non-parametric analysis of deviations of one-minute rain intensity measurements from the WMO field intercomparison

The analysis of counting and catching errors of both catching and non-catching types of rain intensity (RI) gauges was possible for the first time over a wide variety of measuring principles and instrument design solutions based on the recent Field Intercomparison of Rainfall Intensity Gauges promoted by WMO, the World Meteorological Organisation. This paper investigates the frequency distribution of the observed deviations of one-minute RI measurements from the assumed reference value (a weighted average of four selected pit gauges) obtained in real world conditions during the measurement campaign in the field. The presented non-parametric analysis of these deviations confirms that the accuracy of the investigated RI gauges is generally high and contained within the limits established by WMO. Exceptions are the majority of non-catching gauges, especially the optical and acoustic disdrometers, which show significant biases. The intermediate measurement precision was also investigated revealing that the frequency distribution of deviations around their mean value is not indicative of an underlying Gaussian population, being much more peaked in most cases than what can be expected from samples extracted from a Gaussian distribution and indicative of a better precision. Non-catching gauges showed a markedly different behaviour. The analysis of variance (one-way ANOVA), assuming the instrument model as the only potentially affecting factor, does not confirm the hypothesis of a single common underlying distribution for all instruments. Pair-wise multiple comparisons revealed that this hypothesis is generally acceptable for two paired instruments. The cases where significant differences are observed could be easily identified.