November 21, 2010
The recent floods in Canada, Europ, China and elsewhere in recent months have again brought to the fore the ability of extreme rainfall to take lives, destroy homes and displace communities. Evidence suggests climate change is causing increases in extreme precipitation, leading to a greater risk of flooding in urban areas. As a result, agencies around the world are responding to this threat by incorporating climate change into their decision-making. But often the focus around rainfall – in the data being collected and the projections for the future – is on daily totals. Yet the increasing severity of “sub-daily” rainfall – such as hourly – accumulations can be overlooked.
This article looks at why sub-daily rainfall is crucial for flood risk, how it is becoming more severe as the climate warms, and the implications for planning our cities. In urban settings, drainage systems can typically cope with rain from a long-duration storm with a relatively low intensity. But a short storm of high intensity can bring rain that falls faster than the system can drain it away, resulting in a flash flood. Flash flooding is the result of short, intense bursts of rainfall that is followed within minutes or hours by an increase in surface water flow. Due to their rapid onset and difficulty in providing early emergency warning, flash floods can be particularly devastating.
This type of flooding is generally caused by convective storms. These occur when warm air at the Earth’s surface rises quickly on a hot day. This air cools as it ascends and the moisture it contains condenses to form clouds. In the right conditions, huge cumulonimbus clouds can form, which are commonly associated with thunder, lightning, strong winds and sudden changes in temperature. A warming climate means that the risk of these short-duration rainfall extremes is increasing. There are three lines of evidence for understanding how sub-daily rainfall extremes are changing
- Physics: as the Earth warms the atmosphere can store more moisture.
- Historical changes: extreme rainfall has been increasing – and the more extreme the event, the greater the increase. .......
- Model projections: convection-permitting models, detailed enough to resolve convective rainfall processes, consistently show increases in the intensity of the most extreme precipitation events of 10-14% per degree of warming.