Cities boosted rain, sent storms to the suburbs during Europe’s deadly summer floods

Peer-Reviewed Publication


Flooding in Altenburg, Germany

When it comes to extreme weather, climate change usually gets all the attention. But according to a study from The University of Texas at Austin and two universities in China, the unique effects of cities – which can intensify storms and influence where rain falls – need to be accounted for as well.

“In addition to cutting emissions we need to recognize that understanding the effects of urbanization is part of the solution,” said Dev Niyogi, a professor in the UT Jackson School of Geosciences and Cockrell School of Engineering.

Niyogi is a co-author on a study published in Geophysical Research Letters that used computer models to investigate how cities and climate change influenced the rainstorm that struck the Rotterdam-Brussels-Cologne metropolitan region on July 14, 2021.

The model found that the interplay of large-scale climate and local-scale urbanization intensified the storm, causing more rainfall than either climate or urbanization on its own.

The severe weather system was part of a storm complex that dropped heavy rain across Western Europe in the summer of 2021, causing destructive and deadly flooding that killed at least 242 people, making it one of the deadliest floods in European history.

The research team also included scientists from Nanjing University and Tsinghua University.

Urbanization is known to impact local climate and invigorate storms. City buildings are taller and closer together, which can stall storm systems while directing them away from the city center. And cities’ warmer relative temperatures and higher levels of pollution can often increase moisture in clouds.

In the new study, researchers found that interplay between a warmer climate and city environment focused the rainstorm on suburban areas and boosted rainfall by 50% when compared to the influence of the city alone. That means that of the near 6 inches of rainfall that fell on the metropolitan region on July 14, about three inches of it can be attributed to these interactions.

To conduct the study, the scientists created a storm computer model that included the effects of the environment, city and climate. The model of the storm proved to be representative of the actual event. The storm centered over the same regions in eastern Belgium and western Germany and unfolded over the same timeline with the simulation running from July 13 –17 with the most rain falling on July 14. The model slightly overestimated the total amount of rain that fell, simulating 7.2 inches instead of the near 6 inches measured by rain gauges.

In addition to simulating the actual storm environment, the researchers also created simulation models that replaced cities with undeveloped land and lowered the temperatures to pre-industrial levels. Doing this helped the researchers determine how cities and climate each influenced the storm, as well as their combined impact.

Although the influence of the climate and the city together had the greatest impact, lead author Long Yang, an associate professor at Nanjing University, said that, when viewed individually, the influence of the city equals or outweighs that of climate change.

“We are the first group to reveal that the regional impacts through land-atmosphere interactions on extreme rainfall is comparable or more critical than that induced by climate-scale processes,” Yang said.

The models apply to one extreme storm. Nevertheless, Niyogi said that the results conform with different urban rainfall studies and show the importance of incorporating the influence of urbanization and regional landscapes into climate models in general.

He also added that adapting to climate change provides an opportunity to plan more resilient communities that can help shape desired climate outcomes, such as cities that can send storms away from flood-prone regions.

“At more local scales, there are immediate ways to develop climate resiliency where you don’t have to wait for 100-plus nations signing on to declarations,” Niyogi said. “It’s something you can do at the city scale, the regional scale.”

The study’s additional co-authors are Professor Guangheng Ni and Fuqiang Tian from Tsinghua University in China.


Geophysical Research Letters


Computational simulation/modeling


Not applicable


Urbanization Exacerbated Rainfall Over European Suburbs Under a Warming Climate



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January 8, 2022 10:07 am

Climate models?

A complete waste of time

Reply to  fretslider
January 8, 2022 10:44 am

Depends. Salary and benefits for an average modeler amounts to around $200,000 per year. The organization where the modeler works gets about a third more on top of this.

Reply to  Scissor
January 8, 2022 10:56 am

And a complete waste of money, too.

Joseph Zorzin
Reply to  Scissor
January 8, 2022 12:35 pm

hmmm…. when wars occur, there are profiteers- so now with the “climate wars”, we see a new kind of profiteer

Reply to  Scissor
January 8, 2022 7:53 pm

Exactly. They can get traction on the global scale, so they go regional. Doesn’t really matter who pays only that someone pays.

Thomas Gasloli
Reply to  fretslider
January 8, 2022 10:56 am

Agreed. I stopped reading at “climate models”. When will they stop publishing computer generated results as if they are real research?

Reply to  Thomas Gasloli
January 8, 2022 7:57 pm

Actually these are weather models, so they can be accurate forecasts for a few days out. Cities cause heating, which means uplift, which suctions in humid air from surrounding areas, which creates thunderstorms. You can do that model on the back of an envelope. Somewhere there is an old photograph of a huge thunderstorm of Paris, that illustrates this effect.

Curious George
Reply to  fretslider
January 8, 2022 11:59 am

Do they consume less electricity than Bitcoin mining?

Reply to  Curious George
January 8, 2022 2:19 pm

BTC is worth having…

J Mac
January 8, 2022 10:08 am

RE: “The models apply to one extreme storm.”
And sweeping generalizations are made from there! Ugh….

Old Retired Guy
January 8, 2022 10:17 am

In the new study, researchers found that interplay between a warmer climate and city environment focused the rainstorm on suburban areas and boosted rainfall by 50% when compared to the influence of the city alone. That means that of the near 6 inches of rainfall that fell on the metropolitan region on July 14, about three inches of it can be attributed to these interactions.

If it boosted rainfall from 3″ to 6″ isn’t that 100% increase?

Secondly, the effect of UHIs would indicate CO2 is a lesser reason than this report would suggest.

Reply to  Old Retired Guy
January 8, 2022 10:52 am

I read it differently. If 6 inches fell and the “interplay” between climate and city boosted rainfall by 50% when compared to the influence of the city alone, that means the rainfall would have been 4 inches originally, by the city alone, boosted by 50% equals the 6 inches that fell. Only 2 inches can be attributed to the “interactions”, which included climate, a 33% smaller amount than claimed.

That makes UHI responsible for twice the effect of the warmer climate.

January 8, 2022 10:17 am

A meaningfull work, I would say.
Debunks a lot of Griff meaning about CC floods in general and specific 😀

January 8, 2022 10:21 am

Back in the 1970’s I read a study that found that the uplift in air caused by UHI increased rainfall in areas downwind of cities. The larger the city, the bigger then impact.

No global warming needed.

Bruce Ranta
Reply to  MarkW
January 8, 2022 11:12 am

Then there is the cumulative impact of big cities in close proximity – common in much of Europe, the eastern seaboard of the USA, and many other areas.

Tom Halla
January 8, 2022 10:24 am

Perhaps the only use for this model would be to plan adequate storm drains, the lack of which is the usual cause of urban flooding.

Thomas Gasloli
Reply to  Tom Halla
January 8, 2022 10:58 am

Exactly. Floods are not caused by rainfall they are caused by build wrong in the wrong place.

Reply to  Thomas Gasloli
January 8, 2022 12:38 pm

Floods ARE caused by rainfall, and can cause massive destruction of the natural envoronment. Human death and property damage are caused by “build wrong in the wrong place”.

Clyde Spencer
Reply to  Tom Halla
January 8, 2022 11:06 am

Hydrologists use models to estimate urban runoff. The most important parameter is the percentage of impervious surface of each of the hydrologic basins in the urban area.

Reply to  Clyde Spencer
January 8, 2022 11:43 am

Slope is also important.
Flat places like Houston, it doesn’t matter how many storm drains are in place, when you get a couple of feet of rain in a few days, the water is going to back up.

Reply to  Clyde Spencer
January 8, 2022 11:47 am

Exactly Clyde, when you start doing the civil engineering calcs, you realize that climate change adds about 1” (which you can ignore) to the height of the several feet you are allowing for the 100 year flood berms in your new housing development, or the 2-3 feet you are adding to existing berms due to someone building a city worth of parking lots upstream over the last 50 years….plus you add another half a foot because your confidence in the 100 year flood data is not 100%….

Reply to  Clyde Spencer
January 8, 2022 1:19 pm

in urban stormwater design it is call run-off coefficient C.

Typical C,
For original forest likely less than 0.2
For farmland about 0.2
For post war USA or Australian subdivisions (1500sqf house on 7500sqf lot) C about 0.5
For current subdivisions in outer suburbs (2500sqf house on 4500sqf lot) C about 0.7

the densifying of inner city suburbs leads to C of upto 0.9.

Whether the total or peak RAINFALL has increased a 5-10% is irrelevant when the peak RUNOFF has increased by more than 40%( 0.7/0.5) in just about every growing city.

Reply to  Waza
January 8, 2022 8:40 pm

… but,

when the urban area is less than 1% of the total basin, the peak runoff for the basin increases by ? as a result of the 0.5% increase in impervious area for the entire basin.

And if you want to discuss urban only … see Tom Halla above.

Regional flooding is regional flooding. The fact that an urban area is inundated usually means that the urban area is in the area of flooding and/or the local infrastructure is inadequate.

Reply to  Waza
January 8, 2022 11:58 pm

Agreed urban drainage DESIGN is different to both regional and rural.
Please note I used terms rainfall and runoff but not flooding.
What the hell does flooding actually mean?
A. In an urban setting you get rainfall which leads to runoff.
Flooding occurs when runoff is in excess of what the drainage system was design for. This drainage system needs to be constantly upgraded as the area intensifies.
B. In a rural setting you get runoff and in large storm events inundation of the catchment. I argue that this is not flooding as no drainage system was actually overloaded. Inundation of the catchment is natural.
C. The issue you an Tom are correctly highlighting is when humans has settled along major watercourses with large upstream catchments. Upgrading the urban drainage system will not help.

The German towns which were flooding suffered from both A and B above.
These medieval towns built on rivers which large snow covered catchments continue to grow with more impervious area.

January 8, 2022 11:04 am

OK, once they drop their preconceived notions, then they start to accomplish real science. However, there are considerable weaknesses in this “study”. First off, it appears to be more of taking the GCM models, producing output and calling that “data” to input into another model, this one their own. So I think the conclusions have <50% chance of accuracy, and therefore almost no good for forecasting. But still, they may be on to something. How would we devise a test/experiment, or even collect other data from the real world, to verify/validate their model?

…lowered the temperatures to pre-industrial levels…

This is where I would do the most digging. Did they use the raw temperatures, or did they fall for the “adjusted” and “homogenized” temperatures used by the Warmunist Doom Casters? I would think that “…replac[ing] cities with undeveloped land…” would include lowering the temperatures, since we already know UHI affects local temperatures and in fact may be responsible for 100% of the “Global Warming™” observed in the surface temperature records. So lowering temperatures to pre-industrial levels, was that based on weather stations uncontaminated by UHI, or was that another product of a model?

…such as cities that can send storms away from flood-prone regions…

How would they do that? What exact shape(s) and material(s) would it take to redirect a storm in a particular direction? They don’t say, and until they can, they certainly haven’t proven anything.

I support their work, if only because they finally admitted that local land use changes have more of an impact than “Climate Change™” (and by that I’m assuming they mean warming caused by increased atmospheric CO2 concentrations? It’s not clear in this write-up, I may have to read the paper.)

Climate believer
January 8, 2022 11:05 am

 “…making it one of the deadliest floods in European history.”

80% of the deaths happened in one country, lets keep things in perspective, Europe’s a big place.

There was however a “deadly European flood” back in 1910, that killed more than 1200 people from four different countries.

CO² was 300ppm and there was a lot less concrete about.

Clyde Spencer
January 8, 2022 11:08 am

I have read that as poor as the temperature predictions of Global Circulation Models are, precipitation is even worse. Sometimes the regional predictions are even contradictory (+ and -).

Rud Istvan
Reply to  Clyde Spencer
January 8, 2022 12:15 pm

Some data. Based on ARGO salinity measurements, the climate models are underestimating precipitation over the oceans by about half. That means they get the WVF too high, and thus overheat temperatures.

January 8, 2022 11:13 am

One of the deadliest floods in European history?
According to Wikipedia, between 50,000 and 80,000 people lost their lives from flooding in the Netherlands, Germany and England in 1287. Between 8,000 and 15,000 lost their lives due to flooding in Britain, the Netherlands, Belgium and Germany in 1634. 10,000 people lost their lives in the St Petersburg floods of 1824. Over 2,000 people in the UK and the Netherlands lost their lives from flooding in 1953. And, according to the same source, 230 people lost their lives from flooding in 2021. 

Reply to  Alba
January 8, 2022 12:24 pm

Fortunately for them, today’s scientific and social nitwits will not read your very good post. That way they will continue to be comfortable in their ignorance.

Devils Tower
January 8, 2022 11:14 am

Live in country(midwest) and have had my share of heavy rain fall(>6in) over the years. So they are telling me large cities get twice the amount in storms. What absolute bs….

Joseph Zorzin
January 8, 2022 12:32 pm

urban flooding occurs because people build in flood plains- duh!

Reply to  Joseph Zorzin
January 9, 2022 6:26 am

Or inside storm water detention basins! In the floods of Hurricane Harvey in Houston, a large number of the flooded homes had been built inside and below the emergency spillway pool elevations of large, regional detention basins. The basins are so large that they are crossed by many roadways and filled with parks and athletic fields. With no local zoning ordinances, developers had constructed whole subdivisions in these basins and lower than the emergency spillways. Apparently nobody noticed until the basins filled as designed during the severe rain event.

Not sensing their danger, many people in these neighborhoods were caught too late to prepare. They got out with their lives, but left their flooded cars in their driveways and all of their belongings in place on the ground floors to be inundated.

If one lives in a city or plans to buy real estate there, it is wise to know your elevation and to consult FEMA and updated local flood hazard products, including forecasted changes due to development. The same is true for rural locations, but the countryside is less prone to the impacts of surrounding development and infrastructure.

Joseph Zorzin
Reply to  Pflashgordon
January 9, 2022 6:31 am

“With no local zoning ordinances”

we all know Texans love their freedom, but are they really against all zoning?

No doubt some zoning is poorly done or excessive, but I should think basic zoning to protect people and their stuff from flooding isn’t taking away their liberties.

Joao Martins
January 8, 2022 1:30 pm

Emergency! Save the climate!

Study says that, according to models, you must go live in caves!

Insufficiently Sensitive
January 8, 2022 6:16 pm

in the new study, researchers found that interplay between a warmer climate and city environment focused the rainstorm on suburban areas and boosted rainfall by 50% when compared to the influence of the city alone. 

Well then, when will the public-radio greenies start a series of debates wherein the urban vertical-sprawl Grand Poobahs of densification are accused by suburban residents of floodification of their once-liveable suburbs by that ‘interplay’? I’d pay to hear those ones!

Geoff Sherrington
January 8, 2022 6:41 pm

“… the influence of the city equals or outweighs that of climate change.”
Showing that it is not hard to to be more powerful than climate change, without stating how powerful climate change is.
What a mess of a paper!!! Geoff S

January 8, 2022 11:30 pm

Refreshing to read that the lead author admits that the urbanization is responsible for more than half the effect. Of course, 100% is more than half…

How bad was the weather in the early 1300’s, the years with no summer in Europe, just rain. Why did they not compare their model results with historical data, especially for areas that are recently paved over and developed, in that area of flooding?

One would think that it’s required to compare and contrast all possible data available with the model results in the discussion and results sections of the paper in order to be able to properly access the model.

January 9, 2022 1:04 am

<i>The model slightly overestimated the total amount of rain that fell</i>

A “slight” 20% overestimate

January 9, 2022 1:04 am

Regardless of this, the issue with both Germany and china is the sheer amount of rain which fell over a wide area in a short timescale…

For example the heavy rainfall in the south of North Rhine-Westphalia and north of Rhineland-Palatinate in Germany produced accumulations which averaged 100 to 150 mm (3.9 to 5.9 in) in 24 hours, equivalent to more than a month’s worth of rain. In Reifferscheid, 207 mm (8.1 in) fell within a nine-hour period while Cologne observed 154 mm (6.1 in) in 24 hours. Some of the affected regions may not have seen rainfall of this magnitude in the last 1,000 years.

Reply to  griff
January 9, 2022 5:46 am

“May not”.
If “ifs” and “ands” were pots and pans there’ld be plenty of work for the tinkers.
How about some data, griff?

Ben Vorlich
Reply to  Oldseadog
January 9, 2022 8:59 am

I learnt that as

If “ifs” and “ands” were pots and pans there’d be no need for tinkers.

But how do we know what the rainfall was like more than 100years ago. We don’t so I could make a claim that before the LIA rainfall like this was commonplace.

Reply to  griff
January 10, 2022 10:21 am

What do you call it when you believe in things that you don’t understand?

Reply to  griff
January 10, 2022 10:22 am

When you believe in things
That you don’t understand
Then you suffer
Superstition ain’t the way

Very superstitious
Nothing more to say
Very superstitious
The devil’s on his way

… Stevie Wonder

Tim Crome
January 9, 2022 2:23 am

This paper from 2014 concludes that historical flood levels, in exactly the same river valley as impacted by the 2021 floods, were higher.

Before major urbanisation or climate change!

Tom Abbott
Reply to  Tim Crome
January 9, 2022 11:07 am

Griff is going to be *so* disappointed. He thought he had an unprecedented climate change event to point to, and it turns out he doesn’t. Poor Griff.

January 9, 2022 2:25 am

“deadly flooding that killed at least 242 people, making it one of the deadliest floods in European history.”

utter bollox!
There have been many many floods in the last 2000yrs far worse.
In my limited memory since the 1960s quite a lot that have been comparable or even worse.
The difference being greed.

Build stuff surrounded by concrete in known historic flood plains with little or no drainage and nature comes back to bite, sooner or later.

Rainer Bensch
January 9, 2022 3:48 am

Well, the Ahr took their old bed back.
Bad Bath & Beyond.

Peta of Newark
January 9, 2022 8:22 am

The severe, extreme, unusual and unexpected rainstorm was because these people are living in a desert. The rainstorm is exactly the sort of weather that happens in such places.

Next and 2 points:
1) Flatly and basically, cities are deserts. They are very dry places with no stored water and get very hot when the sun is up. Thus, how many cities and large conurbations are down there on the ground.
Are they now so big, numerous and close together so as to be acting as one great big desert?

2) Again and on the subject of desertification, what state are the soils in around that place where the rain landed? Are they badly eroded from farming and tillage – have they lost their Soil Organic Matter?
Because a 1 foot depth of high organic soil can ‘disappear’ 2 inches of rain and nobody would even notice. Under an ancient grassland would be at least 2 feet of high organic soil (make the 2 metres under an ancient forest)
Now then, with non eroded soils at least 4 inches of that rain would have ‘disappeared’ into the ground and stayed there, leaving 2 inches for ‘the city’ to handle.

Frankly, if the city planning, building and infrastructure can not handle 2 inches of rain, some number of people need their heads banging together and their <expletive> backsides given a good kicking.

January 9, 2022 12:48 pm

The 1981 book “The Urban Climate” by Landsberg has maps and diagrams examples of increased rainfall over urban areas. 

Teddy Lee
January 10, 2022 3:28 am

Slightly overestimated by 20%!

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