Forecasting the floods: New clues about extreme rainfall

Heavy rain has always been part of Earth’s water cycle, but lately something is different. Cities are flooding more often, storm drains are overwhelmed, and weather records keep getting broken.

Now, scientists have solved a puzzling mystery about extreme rainfall that’s been bugging researchers for years.

Led by Dr. Sarosh Alam Ghausi at the Max Planck Institute for Biogeochemistry in Jena, Germany, researchers have determined why some data seemed to contradict what we know about rainfall in a warming world.

What makes rainfall “extreme”?

Extreme rainfall refers to an unusually high amount of rain over a short period, often surpassing local thresholds set by meteorologists.

To put this in perspective, scientists consider “extreme rainfall” to be the heaviest 5% of rain events in any given area.

Its definition varies by region due to differences in climate and historical weather patterns. For example, what is considered extreme in arid regions may be normal in tropical areas.

Extreme rainfall can result from slow-moving weather systems, tropical cyclones, or atmospheric rivers – long, narrow bands of moisture in the atmosphere.

These events can disrupt ecosystems, overwhelm urban drainage, and cause significant damage to agriculture and infrastructure.

The cloud confusion

For a long time, scientists noticed something odd in their data. While we know that warmer air holds more moisture and should lead to heavier rain, the numbers weren’t adding up in tropical and mid-latitude regions.

When daily temperatures went above 23-25°C (73-77°F), extreme rainfall actually seemed to decrease.

This didn’t make sense. The science is clear: warmer air can hold more water vapor, so why weren’t we seeing more intense rainfall in these warmer conditions?

The missing piece of the puzzle

Interestingly, clouds were found to interfere with temperature measurements during rainfall. When it rains, clouds block sunlight from reaching the Earth’s surface, causing a cooling effect.

This cooling was distorting the temperature data, particularly in tropical regions where clouds reflect a significant amount of sunlight back into space.

This made it appear as though extreme rainfall was less linked to warming than it actually is. By accounting for this cooling effect, scientists uncovered the true relationship between rising temperatures and increasing extreme rainfall rates.

New approach to track extreme rainfall

The research team found a clever way around this problem. Using satellite data that measures radiation, they developed a method to remove the cooling effect of clouds from their temperature measurements.

When they looked at the corrected data, everything fell into place. The relationship between temperature and extreme rainfall matched what science predicts and what climate models have been showing us.

What this means for the future

“This confirms what is widely expected: Extreme rainfall intensifies in a globally warmer climate,” said Dr. Ghausi. His team found that the heaviest increases in extreme rainfall are happening in tropical humid areas like India, Northern Australia, and the Amazon.

The implications are serious. Without proper preparation, these more intense rainfall events could lead to increased flooding.

“These results support the physical expectations that the whole hydrological cycle becomes more intense and extreme with warmer temperatures,” said Dr. Axel Kleidon, senior author of the study from Max Planck Institute for Biogeochemistry.

“We will not only see more extreme rainfall rates, but also more intense and longer dry spells in the future.”

Complexity of climate science

This research gives us a clearer picture of what’s happening with extreme rainfall and why. It confirms what many communities are already experiencing: rain patterns are changing, and they’re changing fast.

The study also highlights the complexity of climate science. Sometimes what looks like a contradiction in the data is actually telling us something important about how our atmosphere works.

By figuring out the role clouds play in temperature measurements, scientists have given us a better understanding of what to expect as our planet continues to warm.

For cities and communities around the world, this knowledge is crucial. It can help planners design better drainage systems, improve flood defenses, and prepare for the more extreme weather that lies ahead.

This isn’t just about numbers and measurements. It’s about helping communities prepare for a future where the phrase “when it rains, it pours” takes on a whole new meaning.

The study is published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–