|Urban flooding is often caused by high intensity rainfall, typical|
during summer thunderstorms.
1. Parties recognize the importance of averting, minimizing and addressing loss and damage associated with the adverse effects of climate change, including extreme weather events and slow onset events, and the role of sustainable development in reducing the risk of loss and damage.
As we move ahead, its important to quantify trends in extreme rainfall events, particularly as these relate to urban flooding. Recently Northern Illinois University (NIU) explored the effects of local urban heat island on storms. We review this in the context of trends in Toronto-area extreme rainfall, as well as trends across Ontario and Canada.
NIU research suggests that local heat island effects increase the incidence of thunderstorms. They found that for any given day in the 17-year study period (1997-2013) there was about a 5 percent greater chance that Atlanta would experience a thunderstorm compared to a similar sized rural area. As convective summer thunderstorms are the typical cause of urban flood damages, this could be one factor resulting in increased flood damages over past decades.
Researches studying Atlanta indicated that the possible mechanisms causing more thunderstorms in urban areas included:
- The urban heat-island effect. A landscape of concrete, asphalt and densely packed buildings can enhance heat. Low pressure forms atop the urban area with higher pressure in surrounding rural areas. The scenario might cause low-level atmospheric convergence, which forces air up into thunderstorms.
- Localized areas of upward moving air resulting from increased surface roughness. A cityscape modifies wind direction and speed, which might lead to enhanced convergence and thunderstorm formation.
- Urban pollution. The science isn’t settled on this issue, but some researchers believe enhanced aerosol production caused by urban pollution can enrich thunderstorms. “We discovered that there may be an aerosol effect with initiation as well, because we had greater storm initiation events during weekdays compared to weekend days.”
|Bloor Street gauge shows no increase in severity of rainfall events|
since the 1940's in downtown Toronto (Environment Canada).
|Toronto Island Airport annual maxima for rainfall over|
5 minute to 24 hour durations. Station 6158665.
|'Toronto' Buttonville Airport annual maxima for rainfall|
over 5 minute to 24 hour durations. Station 615HMAK.
|Toronto International Airport (Pearson) annual maxima for|
rainfall over 5 minute to 24 hour durations. Station 6158731.
|'Toronto North York annual maxima for rainfall over 5 minute to|
24 hour durations. Station 615S001 (composite gauge).
What has certainly changed in the Toronto area to explain increased flooding? Urbanization has increased runoff and that land use change doubles or triples peak flows relative to rural conditions. The Don River watershed urbanization has increased from 15% in the 1950's to nearly 90% today. Also, overland flow paths have been enclosed or obstructed due to intensification and infill. Perhaps Ontario needs a "Places to Flow Act" to address deficiencies in the overland flood conveyance capacity in developing cities.
|Days with rain include days with a fraction of a millimetre|
of rainfall accumulation (anything above a trace amount).
Numbers are up but do not affect flood risks.
Looking beyond Toronto and its urban heat island context, Environment Canada's assessment of rainfall trends can be found in the report at the following link:
This report is "Methodologies to Improve Rainfall Intensity-Duration-Frequency (IDF) Estimates: A Southern Ontario Pilot Study" by Environment Canada Adaptation and Impacts Research Climate Research Division, December 2011. See Section
6.1 entitled Trends in Precipitation and its Extremes in Southern Ontario, page 77 for the following:
“Significant increases, as well as decreases, were detected at some stations in a number of the extreme precipitation indicators. However, the majority of station trends were determined to be non-significant and no consistent geographical patterns for increases or decreases were observed across Canada. In most cases, the magnitude of the observed changes was also very small. These results are consistent with the daily Canadian extreme precipitation trend analysis of Zhang et al. (2001) and the Canadian component of global and North American trend analyses of daily precipitation extremes by Alexander et al. (2006) and Peterson et al. (2008), respectively.”
As further information we can refer to the work of Heather Auld, M.Sc. (Meteorology) who joined Risk Sciences International (RSI) as their Principal Climate Scientist in 2011 after spending 32 years with Environment Canada. Please see her presentation on extreme rainfall trends here:
|Ontario decreases and increases in short term rainfall intensities.|
"• Return Period Rainfalls DECREASE in many cases
• High profile extreme rainfall events in recent years lead to
expectations that return period rainfalls will all show INCREASES"
Raw rain intensity and statistics data is available from Environment Canada here under the IDF files link: