Identifying changes in the timing of heavy rainfall occurrence in Ontario, Canada

Changes in heavy rainfall events have been characterized with a focus on quantifying and qualifying their magnitude and frequency. Knowing the magnitude and frequency of heavy rainfalls have changed and will continue to change, due to the impact of climate change, insufficient attention is being paid to recognizing changes in the timing of heavy rainfall event occurrence. In this work, a systematic methodology is presented to assess changes and trends in the timing of heavy rainfall events from the past to the present using the Annual Maximum Series (AMS) for 44 climate stations across Ontario from 1960 – 2017. Three statistical parameters (mean – x̅, variance – S 2 , and coefficient of variation – CV) were used to identify changes in AMS occurrence times. This work used the rolling time window technique with standard t-test, F-test, Mann-Kandal test, and Sen’s Slope, where the time window length is chosen to be 10 years. Results show that, on average, considering all measuring durations (5min to 24hr), the mean AMS occurrence time has advanced by 44 days, where the maximum and minimum changes are 86 and 21 days, respectively. Meanwhile, results from F-test indicate that heavy rainfall events occur at altered time windows of the year. There are 24 stations identified to have significant changes in their AMS occurrence time variance, where 10 stations have increased variance, and 14 stations have decreased variance. Trend analyses were used to verify the change identified in the previous step between the past and the present. It was found that all stations at all durations are showing increasing trends of CV, indicating increased variabilities of heavy rainfall occurrence time. Results from this work open up a new era of assessing climate change impacts on rainfall from the timing perspective.

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