Intermittency of rainfall at sub-daily timescales: New quantitative indices based on the number, duration, and sequencing of interruptions to rainfall

Abstract Hourly and other aggregated rainfall amounts reflect unknown proportions of raining and rainless time, arising from the intermittency of rainfall. This causes estimates of intensity derived from aggregated data to be too low, but the extent of this bias is difficult to estimate and remains in need of study. Rainfall data having high temporal resolution were analysed for sub-daily intermittency. The approach relied on the use of a ‘no-rain’ intensity threshold of 0.2 mm h−1, below which the intensity was regarded as effectively zero. Various aggregation durations (ADs) for rainfall amounts, including day, hour, and 15 min, as well as discrete rainfall events, were analysed for intermittency. Results show that the use of 1 h AD data results in lower levels of apparent intermittency, typically 42–47%, than does the use of unaggregated inter-tip times (ITTs, which permit higher resolution of intensity estimates) which for both days and events reveal intermittency of 62–67%. Evidence for both diurnal and annual (seasonal) patterns of intermittency is found. Measures of rainfall intensity extremes, such as extreme hourly rainfall, are affected by this intermittency variation. For exploring possible ecohydrological consequences of intermittency at sub-daily timescales, new metrics are proposed, based on the number of rainfall interruptions per rain day, the temporal pattern of their occurrence between first and last rainfall, and the distribution of the durations through the sequence of interruptions.