Flooding in Europe, China and the United States in recent months has once again highlighted the ability of extreme rainfall to kill people, destroy homes and displace communities.
Evidence suggests that climate change is causing an increase in extreme precipitation, leading to an increased risk of flooding in urban areas. As a result, agencies around the world are responding to this threat by integrating climate change into their decision making.
But often the focus on precipitation – in the data collected and projections for the future – is on daily totals. Yet the increasing severity of “sub-daily” precipitation – such as hourly accumulations – can be overlooked.
In this guest post, we take a look at why intraday precipitation is critical to flood risk, how it gets worse as the climate warms, and the implications for planning our cities.
Extreme intraday precipitation
In urban settings, drainage systems can usually cope with rain from a long-lasting storm with relatively low intensity. But a short, high-intensity storm can bring rains that fall faster than the system can drain them, resulting in a flash flood.
Flash floods are the result of short, intense gusts of rain followed within minutes or hours by increased surface water flow. Due to their rapid onset and the difficulty in providing an early emergency warning, flash floods can be particularly devastating.
This type of flooding is usually caused by convective thunderstorms. These occur when the warm air on the Earth’s surface rises rapidly on a hot day. This air cools as it rises and the moisture it contains condenses to form clouds. Under the right conditions, huge cumulonimbus clouds can form, which are usually associated with thunder, lightning, strong winds and sudden changes in temperature.
Global warming means that the risk of these short-lived extreme rains is increasing.
The figure below illustrates the processes that cause the short, intense storms responsible for the sub-daily extreme precipitation, and how climate change and other human actions are altering these processes – and the resulting flooding.
Increases in sub-daily extreme precipitation in a warming climate are due to three main mechanisms.
The main one is the increase in the moisture-holding capacity of the atmosphere at higher temperatures. This means that when precipitation does occur, it is possible that it will be more abundant.
This increase in the moisture holding capacity of the atmosphere leads to two secondary mechanisms.
First, if there is more humidity in the atmosphere and the temperature is higher, the cloud base is closer to the ground. This means that the risk of precipitation increases.
Second, if more moisture condenses as precipitation, the resulting increased buoyancy and updraft makes the storm even stronger.
According to the Clausius-Clapeyron (CC) relationship, for each degree of temperature rise, the atmosphere can contain 7% more humidity. However, evidence suggests that intraday precipitation may actually see larger increases with warming for the reasons mentioned above.
For example, the graph below shows the observed change in the magnitude of hourly precipitation across Australia. The dotted line labeled “CC” indicates an expected increase according to the Clausius-Clapeyron equation – with rarer and more intense precipitation towards the right side of the graph. Here, the observed change (red line) shows that the increasing magnitude of precipitation is two or three times greater than the rate of CC.
Thanks to the measurement of precipitation by radar, we have become accustomed to seeing thunderstorms evolve in real time. However, the way physical measurements of precipitation are collected has limited our understanding of the impact of climate change on intraday precipitation extremes.
Precipitation has traditionally been measured using daily reading gauges – a cylinder that fills with precipitation and is manually read and emptied once a day. Automatic measurement of precipitation is still relatively recent and has only become routine in recent decades.
To detect changes in intense precipitation due to convective storms, we need long records of intraday precipitation measurements and dense gauging networks. In the UK precipitation stations are usually 40 km away, but in other parts of the world they are rarer. This means that our gauges don’t always pick up the most intense bursts of convective storms, or may even miss them altogether.
Detecting changes in extreme weather events due to global warming is notoriously a difficult task due to the great natural variability or “noise” when studying events that are not observed very frequently. Compilation and analysis of new precipitation data sets indicate recent increases in precipitation intensity over large continental areas in several parts of the world. However, there are large areas for which data is not available and for which we are unable to identify changes.
Climate models can help fill the gaps in our understanding, but global models – the main tool for understanding future potentials under climate change – are too crude to solve the processes that govern intraday precipitation. Instead, we rely on fine-resolution models – similar to those used for weather forecasting – with increased temperatures used in these models to simulate the possible effects of global warming.
We therefore have three data sources to understand how intraday precipitation extremes change:
- Physical: As the Earth warms up, the atmosphere can store more moisture.
- Historical changes: Extreme precipitation has increased – and the more extreme the event, the greater the increase.
- Model projections: Convection-enabling models, detailed enough to resolve convective precipitation processes, consistently show increases in the intensity of the most extreme precipitation events of 10 to 14% per degree of warming.
Our three data sources all match: extreme sub-daily precipitation intensifies with higher temperatures that are due to man-made climate change.
Planning for a warmer future
The increase in intraday extreme precipitation poses a significant challenge to our existing urban areas.
The existing stormwater infrastructure in much of the world has typically been designed without these increases in mind. The latest report from the Intergovernmental Panel on Climate Change (IPCC) indicates a likely doubling and tripling in the frequency of heavy rainfall events over 10 years and 50 years, respectively – and as a society we must prepare for it.
For example, an infrastructure that was previously designed to withstand an event that we experience on average once every 50 years, can now expect to see it three times during that time.
A recent review of the global flood guidelines revealed that significant progress has been made in integrating the impacts of climate change into the design of cities. Based on the latest scientific advances, many guidelines recommend that extreme precipitation intensities be “taken into account” in their planning and design decisions.
For example, the UK now has a series of climate ‘lift factors’ that can be applied to sub-daily extreme precipitation data to ensure that the infrastructure we are building today is designed to cope. to tomorrow’s changes. The idea is to apply these uplift factors to the observed data in order to provide “future” precipitation data sets to run in models of sewage and flood defense systems.
The maps below illustrate the size of these factors for Scotland and northern England for the âcentralâ (left) and âhighâ (right) projections for future precipitation.
While there remain limits to our understanding of the physics of the current climate system and our ability to observe and simulate it, the scientific community has made tremendous progress in our understanding of the impact of warming on climate change. intraday precipitation.
What is now clear is that there is a substantial – and widening – gap between the severity of climate risks and adaptation actions. With planning decisions often reactive rather than proactive, it is increasingly crucial that the risks of increased extreme intraday precipitation be taken into account when designing and building our future societies.
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