Updated Observations Provide Stronger Evidence for Increases in Sub-hourly to Hourly Extreme Rainfall in Canada

Abstract Global warming is expected to lead to increases in atmospheric moisture and intensify sub-hourly to hourly rainfall extremes. However, signal-to-noise ratios are low, especially at the local scale, making detection of changes in the observational record difficult. For Canada, previous studies based on short data records from 1965-2005 did not show conclusive evidence of increases in short-duration extreme rainfall. This study updates single-site and regional trend analyses of 5 minute to 24 hour annual maximum rainfall in Canada using data from 1950-2021. Estimates of temporal trends are extended to also consider the association between rainfall intensity and dew point temperature, a measure of moisture availability. With longer records, evidence for increases in extreme rainfall at individual sites is stronger. Field significant increasing trends are found for the majority of durations, whereas before results were mixed and typically not statistically significant. Intensification is even more pronounced in single-site scaling of rainfall intensity with summer mean dew point temperature. Field significant positive scaling rates are detected for all durations. When data are pooled in space – irrespective of choice of regionalization – the results are even more clear. Notably, the strongest and most spatially homogeneous intensification of short-duration extreme rainfall is detected in sub-hourly to 2 hour durations. When data are pooled across Canadian climate regions, field significant positive scaling is found in 72.7% to 81.8% of regions for 5 minute to 2 hour durations, with median scaling rates ranging from 5.3 to 9.4% °C−1. For durations ≥ 6 hours, this falls to 27.3% to 53% of regions, with scaling rates less than 4% °C−1.