Climate Models Predict Decreasing Extreme Rainfall Intensities and No Change for Moderate Storms In Southern Ontario

Abstract
A study by University of McMaster researchers entitled "Assessment of Future Changes in Intensity-Duration-Frequency Curves for Southern Ontario using North American (NA)-CORDEX Models with Nonstationary Methods" predicts that extreme 50-year rainfall amounts will decrease in Southern Ontario by 2050, that moderate 25-year rainfall amounts will remain flat, and frequent 10-year rainfall amounts will increase overall for long durations but are mixed for short durations.

The paper is available at this link.

Ganguli and Coulibali state in the abstract "Our results showed that extreme precipitation intensity driven by future climate forcing shows a significant increase in intensity for 10-year events in 2050s (2030-2070) relative to 1970-2010 baseline period across most of the locations. However, for longer return periods, an opposite trend is noted."

The following tables illustrate how rainfall intensities are predicted to change over periods of 1 hour to 24 hour at Southern Ontario Locations including Toronto, Hamilton, Ottawa, Windsor, London Trenton, Stratford and Shand (Fergus Shand Dam).

Southern Ontario Extreme Rainfall Predicted to Decrease With Climate Change 

Southern Ontario Moderate Rainfall Predicted to Not Change Overall With Climate Change
Southern Ontario Frequent Rainfall Predicted to Increase With Climate Change For Long Durations
Specifically, the Toronto-Hamilton 50-year rainfall amounts over 1-2 hours are predicted to drop by up to 5% or increase by 1% assuming non-stationary distributions - such changes are considered insignificant in the realm of infrastructure design given uncertainties with other factors and analysis methods. Across Ontario, the largest predicted decrease is 44% at Shand and the largest increase of 14% is in Windsor. These short duration amounts are most relevant to peak flow affecting urban flooding.

Meanwhile, the 25-year rainfall amounts are predicted to increase or decrease by 7% and 4% respectively, again an insignificant amount in design. Across Ontario, the largest predicted decrease is 35% at Shand and the largest increase of 10 % is in Windsor.

In contrast, the 10-year rainfall is predicted to increase overall, especially for long durations. For short durations of 1-2 hours the maximum increase is 22% in Hamilton and the maximum decrease is 32% in London.  For the 1-hour duration 5 stations show a decrease in 10-year rainfall (-5% to -22%), while only 3 stations show increases (+5% to + 22%).

The authors conclude that "The findings, which are specific to regional precipitation extremes, suggest no immediate reason for alarm, but the need for progressive updating of the design standards in light of global warming."

It is interesting that authors refer to 'global warming' as opposed to climate change, perhaps since extreme rainfall if not predicted to change with future temperatures.

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Previously, we analyzed the trends in the Engineering Climate Datasets for long term Southern Ontario gauges:

http://www.cityfloodmap.com/2018/01/short-duration-frequent-rainfall-show.html

The review showed for a 2-hour duration the 10-year intensities decreased on average 0.8% from 1990 to the most current Version 2.3 dataset. The McMaster research predicts an average increase of 4.3% for 2-hour 10-year rainfall (non-stationary vs non-stationary). A greater increase is predicted for stationary vs stationary. Question: when will the real data observations start to show an increase like the model suggests? Maybe it won't. Reminds us of this quote:

"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
Richard P. Feynman

Given rainfall design intensities are decreasing in many Southern Ontario cities based on past observations it is refreshing to see climate modelling that predicts trends that are consistent with real data - yeah !


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