Trends in Canadian Short‐Duration Extreme Rainfall: Including an Intensity–Duration–Frequency Perspective

Abstract

Short-duration (5 minutes to 24 hours) rainfall extremes are important for a number of purposes, including engineering infrastructure design, because they represent the different meteorological scales of extreme rainfall events. Both single location and regional analyses of the changes in short-duration extreme rainfall amounts across Canada, as observed by tipping bucket rain gauges from 1965 to 2005, are presented. The single station analysis shows a general lack of a detectable trend signal, at the 5% significance level, because of the large variability and the relatively short period of record of the extreme short-duration rainfall amounts. The single station 30-minute to 24-hour durations show that, on average, 4% of the total number of stations have statistically significant increasing amounts of rainfall, whereas 1.6% of the cases have significantly decreasing amounts. However, regional spatial patterns are apparent in the single station trend results. Thus, for the same durations regional trends are presented by grouping the single station trend statistics across Canada. This regional trend analysis shows that at least two-thirds of the regions across Canada have increasing trends in extreme rainfall amounts, with up to 33% being significant (depending on location and duration). Both the southwest and the east (Newfoundland) coastal regions generally show significant increasing regional trends for 1-and 2-hour extreme rainfall durations. For the shortest durations of 5-15 minutes, the general overall regional trends in the extreme amounts are more variable, with increasing and decreasing trends occurring with similar frequency; however, there is no evidence of statistically significant decreasing regional trends in extreme rainfall amounts. The decreasing regional trends for the 5-to 15-minute duration amounts tend to be located in the St. Lawrence region of southern Quebec and in the Atlantic provinces. Additional analysis using criteria specified for traditional water management practice (e.g., Intensity-Duration-Frequency (IDF)) shows that fewer than 5.6% and 3.4% of the stations have significant increasing and decreasing trends, respectively, in extreme annual maximum single location observation amounts. This indicates that at most locations across Canada the traditional single station IDF assumption that historical extreme rainfall observations are stationary (in terms of the mean) over the period of record for an individual station is not violated. However, the trend information is still useful complementary information that can be considered for water management purposes, especially in terms of regional analysis.

Observed Trends in Severe Weather Conditions Based on Humidex, Wind Chill, and Heavy Rainfall Events in Canada for 1953–2012

“…Changes in wind chill are consistent with the results of Keimig and Bradley (2002), which show that apparent temperatures based on wind chill have become warmer in North America. The results for heavy rainfall events are in agreement with the lack of change in the number and intensity of heavy precipitation events Vincent and Mekis, 2006) and with the lack of significant trends in extreme short-duration rainfall amounts (Shephard et al, 2014). Globally, changes in precipitation are much more spatially heterogeneous than temperature changes (Donat et al, 2013); therefore, caution must be used when applying these findings.…”

“…Future work would require further analysis of changes in extreme rainfall events using a larger number of measurements (including shorter-duration precipitation estimations). The analysis of trends in severe weather conditions would greatly benefit from new additions to the autostation rainfall rate observation network (newer TB3 TBRG replacing the older MSC TBRG gauges) and weighing gauge observations (Shephard et al, 2014).…”

“…No consistent trends were found in either the number or intensity of heavy precipitation events . No evidence of significant trends was found in single-station extreme shortduration rainfall amounts (Shephard et al, 2014). Freezing rain has become more frequent in northern regions (Wang, 2006).…”

“…Data were merged at 28% of the 126 temperature and wind stations and at 17% of the 285 rain stations. The potential effect of merging station observations was investigated by visually examining the station time series for temperature and wind speed and by using a detailed procedure for TBRG rainfall (Shephard et al, 2014). The locations and number of stations reporting data per year are given in Figs 1 and 2, respectively.…”

“…Hourly rainfall amounts measured by the tipping bucket rainfall gauge (TBRG) observation network were used to produce the heavy rainfall indices (details on the TBRG instrument characteristics can be found in Devine and Mekis (2008) and Shephard et al (2014)). All data were retrieved from the National Climate Archive of the Meteorological Service of Canada (MSC) (Government of Canada, 2015a).…”