What are the trends in regions of Canada that have experienced significant flooding in the past? And do the trends projected by models for long durations (1 day precipitation) match observed data trends? No - some 24-hour trends are decreasing despite models estimating they will go up (or have gone up because of increasing temperatures).
Also, what is happening with observed short duration intensities, the ones responsible for flooding in urban areas, compared to the observed 1-day trends?
The data show short duration and long duration trends diverge. Therefore relying on models of 1 day precipitation to estimate what is happening with short duration, sudden, extreme rainfall should be done with caution.
A couple charts help illustrate these observed data trends and show what is wrong with relying directly on models to project local extreme rainfall.
This is the trend in observed rainfall for southern Ontario climate stations, using median changes in IDF statistics:
|Southern Ontario Extreme Rainfall Trends|
These are the trends for Alberta observed rainfall when new data are added and are reflected in the most current v3.10 datasets:
|Alberta Extreme Rainfall Trends|
In northern Ontario, trends are different than in southern Ontario as shown below:
|Northern Ontario Extreme Rainfall Trends|
In northern Ontario the long duration intensities have increased but short duration intensities have decreased on average. So we see short and long duration rainfall trends are diverging when we consider new data.
Climate modellers may suggest that simulated 1 day precipitation can guide what happens during short durations too. Observed data suggest otherwise. Trends actually diverge.
In brief, for this sample of regions shown above, we see these trends:
Location Short Duration Trend Long Duration Trend
Southern Ontario Larger Decrease Decrease
Northern Ontario Decrease Increase
Alberta Increase Decrease
Remember "All models are wrong, some are useful". Climate models do not accurately project changes in extreme rainfall in Canada based on observed data. Furthermore, simulated 1 day precipitation trends from models cannot be used to assume short duration trends related to flooding in urban areas - short and long duration rainfall trends are observed to change in opposite directions in sample regions across Canada.