Extreme rainfall can cause rapid flooding, so-called “flash flooding”. How does such extreme rainfall change with temperature? This question has been studied for decades by using appropriate recordings of rainfall and temperature, measured at short intervals of one hour or less.
Rainfall and clouds form when the water vapor in the air saturates, thus forming small droplets that eventually lump together to form rain drops. According to the Clausius-Clapeyron relation, saturation requires roughly 7 percent more vapor when temperatures rise by one degree Celsius. This relation might, as an oversimplified image, be motivated by a sponge that can capture more water as temperatures increase. An extreme rainfall event in this image corresponds to squeezing the sponge to release most of its water.
This hypothesis had been challenged in 2008 by analysis of a long timeseries of rainfall data in the Netherlands. The authors of that study, Lenderink and van Meijgaard, concluded from their statistical approach that the Clausius-Clapeyron relation was insufficient to describe the increase in extreme rainfall, in particular that of thunderstorms which could increase at 14 percent per degree Celsius – thus twice the rate of Clausius-Clapeyron.
In the past 17 years the work by Lenderink and van Meijgaard, now cited more than 1000 times, has led to numerous investigations into the phenomenon, without being able to unambiguously confirm or reject the groundwork laid out by the Netherlands study. In particular, it was difficult to determine, in how far the blend of different rainfall types could give rise to statistical superpositions.
The current work takes a detailed look at two precipitation types: stratiform rainfall that is continuous and uniform in intensity compared to short rain showers typical for thunderstorms. ”We make use of a large and high-frequency dataset from Germany which is combined with a novel lightning detection dataset. Since lightning indicates thunderstorm activity, stratiform rainfall can be separated in this way,” explains Nicolas Da Silva from the University of Potsdam. “The result is rather striking: when carefully selecting only clear thunderstorm rainfall and studying the extremes at each temperature, the increase is almost perfectly along the Clausius-Clapeyron theory,” adds Jan O. Härterfrom the University of Potsdam who is also affiliated with the Leibniz Centre for Tropical Marine Research (ZMT). Equally, when selecting only for stratiform rainfall alone, the data fit the Clausius-Clapyron relation very well. Only when combining the statistics of both types of rainfall, much higher temperature increase rates emerge, as proposed in the study of Lenderink and van Meijgaard. The authors Da Silva and Härter state that this ’super-Clausius-Clapeyron’ increase is thus of purely statistical origin such that a long-standing controversy may now finally be settled.
However, the current study points out that the statistical ‘super-Clausius-Clapeyron’ increase in rainfall extremes does apply to clusters containing both thunderstorm and stratiform clouds. Such cloud clusters are responsible for most of the extreme flash flood inducing rainfall. “Assuming the temperature changes projected for the coming decades under climate warming, extreme rainfall may indeed reach unprecedented risk levels for humans and infrastructure, especially in urban areas,” the authors stress.
Link to Publication: Nicolas A. Da Silva and Jan O. Haerter, 2025, Super-Clausius-Clapeyron scaling of extreme precipitation explained by shift from stratiform to convective rain type, Nat. Geoscience, https://www.nature.com/articles/s41561-025-01686-4
Image 1: Installation of weather stations by the research group (left: Yahaya Bashiru, right: Maxime Colin), with a thunderstorm cloud cluster approaching in the background on 21 July 2023, Bremen (Germany). Image Credit: Irene Livia Kruse.
Image 2: A thunderstorm cloud cluster and its typical shelf cloud ahead of heavy rainfall near Bremen (Germany) on 21 July 2023. Image Credit: Maxime Colin.
Contact:
Prof. Dr. Jan Härter, Institute of Physics and Astronomy
Tel.: +49 331/977-5889 or -1628
E-Mail: jan.haerteruuni-potsdampde
Media Information 28-04-2025 / Nr. 042