Forecasts by meteorologists: more heavy rain and hail in sight?

Flooded streets, cars sinking in hail – the storm images of the past few days beg the question: Are heavy rain and heavy thunderstorms the consequences of climate change?

Heavy thunderstorms with heavy rain and hail are not unusual in summer. If, however, images of flooded streets and cars sinking in hailstones repeat themselves within a few days, it can be astonishing. And some will wonder whether this is still normal or whether these events are already consequences of climate change.

As is well known, weather and climate are two different things, as processes are viewed on different time scales. Weather phenomena sometimes take place within seconds, such as in the case of a dust devil.

Long-lived high pressure areas can hold out for two weeks. However, everything that happens in the atmosphere within months or millions of years is viewed as a climate phenomenon. Slow changes, such as global warming of the past decades, but also short-term processes such as summer thunderstorms, have an impact.

Low pressure sump XERO: powerful and slow

But let’s stay with the weather this week for now. How did the extreme rains come about? The culprit has a name: XERO is the name of a low pressure area that has moved from France to Germany in the past few days.

The special thing about it: This is not a strong storm depression – weak and only shifting slowly. In meteorology, the term “low pressure sump” has also been used for this.

Due to the weak contrasts in air pressure, there is only little current, so that thunderstorm cells that have once formed only move slowly across the country and thus cause long-lasting rainfalls in one place.

Water is abundant in the humid and warm air mass that originally set out from the Mediterranean region. Since the low is not only pronounced on the ground, but also in the form of cold air in higher atmospheric layers, thunderstorms triggered by the air that is warmed up in the afternoon force further ascent. Additional condensation from the mighty thunderstorms releases a lot of energy and additionally intensifies the process.

Clusters grow from moisture

On Monday and Tuesday, large thunderstorm complexes, so-called clusters, were able to incorporate the moisture from the environment and thus continue to grow and live longer than normal summer storms. The southwest and the Ruhr area were particularly hard hit.

In Essen, for example, 63 liters per square meter fell within a few hours on Tuesday. Today the low XERO and the associated thunderstorms have shifted to the northeast. The emergency services are likely to have their hands full, especially in northern Brandenburg and southern Mecklenburg. Periods of drought alternate with heavy rain.

Consequences of the rise in temperature

But now to the climate: Do you have to reckon with heavy rain more frequently in this country in the course of climate change? An increase in the world mean temperature due to the increasing concentration of greenhouse gases in the atmosphere is very likely and also easy to understand. However, this warming leads to further changes, such as the spatial and temporal redistribution of precipitation.

In Germany, for example, winters will be more humid, but summer precipitation will be more extreme overall, ie both longer periods of drought and more productive heavy precipitation will be expected in the future.

However, these observed and expected systematic changes are mostly covered by natural fluctuations and therefore do not always come to light. For this reason, it is advantageous to consider data from a wide variety of regions that go back as long as possible. Individual weather capers, however extreme, tell us nothing about climatic developments.

Increased heat waves over the past 50 years

A worldwide increase in humidity has been observed since around 1980. This is obvious because the air in the lower layers of the atmosphere, warmed up by the greenhouse effect, can store more water due to physical principles.

In addition, summer heat waves have systematically increased in Central Europe over the past 50 years or so. The hot summer of 2003 and the drought summer of 2018 are two extreme examples of this.

It is therefore entirely plausible that in the course of global warming there may be more violent thunderstorms and heavy rain.

Extreme-value statistical studies of radar data by the German Weather Service actually indicate an increase in small-scale thunderstorms since 2001. Analyzes going back further would be desirable, but are unfortunately not yet possible due to a lack of high-resolution measurements.