Extreme rainfall
Quebec has been experiencing episodes of extreme rainfall in which the amount of precipitation is well above normal. It often happens in the summer with short rainstorms around one hour long, or in the fall or spring with rainfalls of a longer duration.
Due to climate change, the frequency and intensity of extreme rainfall is increasing and will continue to rise. This change will have major impacts on infrastructure and communities that are not equipped to cope with this type of hazard.
Definition | Extreme rainfall
Extreme rainfall is defined in two ways.
The first, which is the most common, consists of defining it as the maximum rainfall of a given duration recorded each year.
For example, if we look at rainfall events lasting for one hour, we identify the maximum amount of rainfall recorded each year for this duration. That gives us the maximum annual rainfall with a duration of one hour.
The second approach consists of looking only at rainfall events with an intensity greater than a given threshold. This threshold must be high enough so that only a few rainfall events exceed it each year.
The characteristics of precipitation vary across the world depending on region and time of year. That makes it difficult to determine fixed precipitation thresholds to define extreme rainfall globally.
Environment and Climate Change Canada uses region-dependent thresholds to define its criteria for public weather alerts. For example, the threshold for a heavy downpour warning for Quebec, Ontario and the Prairies is 50 mm or more of rain per hour.
Factors contributing to extreme rainfall
Several factors affect the generation of extreme rainfall, such as humidity and atmospheric stability.
Humidity is influenced by temperature: the proportion of water vapour in the atmosphere increases at a rate of 7% per degree Celsius. Extra humidity leads to greater amounts of rain. Extreme rainfall is the sudden release of atmospheric moisture, decreasing the amount of water in the atmosphere. This type of process is particularly associated with extreme rainfall from thunderstorms in summer.
Atmospheric stability, on the other hand, is influenced by the difference (gradient) in temperature between the air at the surface and at higher altitudes, and its water saturation. The warmer and more humid the air at the surface is relative to the air higher up, the faster it rises and the more it tends to produce severe thunderstorms and rain.
Other important factors influencing extreme rainfall include changes in atmospheric circulation patterns and the number and trajectories of extratropical hurricanes.
