View from window obscured by raindrops
Intense storms are deadly not because of wind but because of the precipitation they deliver. Credit: krzys16/Pixabay

Every year, several tens of tropical cyclones, hurricanes, and typhoons occur on Earth. These tempests—brethren defined by the geographic location in which they occur—are capable of delivering extreme levels of rainfall: When Storm Daniel swept across part of the Mediterranean last month, it triggered catastrophic flooding and dam failures that resulted in thousands of deaths. However, researchers have now found evidence that the rate of precipitation delivered by tropical cyclones has actually decreased over time relative to background rainfall not associated with storms. That’s a surprise, the team suggests, and something that should be incorporated into modeling to improve future climate projections.

With Wind Comes Churning

When a large storm passes over a body of water, powerful winds churn up the water. That has the effect of mixing the upper water column, said Zhanhong Ma, a tropical cyclone researcher at the National University of Defense Technology in Nanjing, China. “It can stir the surface.” And because the ocean is stratified by temperature—with warmer water residing near the surface and colder water lingering at depth—the mixing results in overall cooling of the surface water near the location of a storm. The so-called cold wake that occurs below and behind a storm at sea is a well-documented phenomenon that’s been shown to persist for several weeks.

Those cold wakes can have pronounced climatic effects, Ma and his colleagues recently showed. The group studied more than 1,300 tropical cyclones and found that both cloud coverage and rainfall tended to be suppressed behind the storms. The researchers concluded that the storms’ cold wakes were the culprit—as the temperature of the ocean’s surface decreases, it’s less apt to evaporate, and therefore, less moisture is transferred into the atmosphere. That effect strongly contributes to suppressing both precipitation and cloud formation above a colder-than-usual patch of ocean water, the team suggested in a study published in 2020.

A Global Census of Storms

Now, Ma and another team have investigated how cold wakes affect the rate of rainfall within a storm itself. Focusing on 2,100 tropical storms that occurred worldwide from 1998 to 2019, the team mined precipitation data based on microwave wavelength measurements collected by the Global Precipitation Measurement Mission network of satellites and the Tropical Rainfall Measuring Mission satellite. The researchers compiled, on average, more than 50 data points for each storm, and they tracked the tempests over both water and land. When they homed in on rainfall occurring within roughly 500 kilometers of each storm, the researchers found that the rainfall rate tended to increase over time: Each year, storms delivered, on average, about 0.8% more rain per hour.

But background rainfall rates also increased. When the researchers compared their storm measurements with rainfall at the same location as the storm recorded 1 year earlier, they found that less rain per hour was falling within 500 kilometers of each storm over time. In particular, storms’ inner cores (the region within 200 kilometers of a storm’s center) were delivering, on average, about 1.5% less rain per hour per year.

Precipitation on the Downswing?

These findings highlight that storms’ contributions to global precipitation rates are on the downswing overall. That’s likely occurring because cold wakes are getting colder over time, the researchers proposed. The team’s data backed up that assertion.

“Most climate models don’t consider cold wakes.”

That trend also makes sense, Ma said, because water near the sea surface heats up more rapidly than deeper water as the atmosphere warms. That, in turn, increases the temperature gradient between surface and subsurface waters, which means that a storm of a given size is more likely to produce a colder wake, Ma said. “A larger sea surface temperature anomaly will be caused by the same storm forcing.”

Such patterns should be accounted for in models, Ma said. “Most climate models don’t consider cold wakes.” These results were published in Climate and Atmospheric Science.

Cold wakes undoubtedly play a role in storm precipitation, but the team’s conclusions about rainfall rates in the storms’ inner cores might be underestimated, said Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology not involved in the research. That’s because the relatively coarse spatial resolution of the team’s satellite observations—roughly 25 kilometers—might not be sufficient to resolve the rainfall that’s occurring in the middle of a storm, he explained.

“Rain does much more damage and kills many more people than wind.”

“I just don’t think the data is up for this kind of analysis,” he said. A theoretical study by Emanuel and his colleagues has suggested that rainfall rates in storms’ inner cores could, in fact, increase by more than 30% by the end of the century. But understanding precipitation patterns in storms remains critically important, Emanuel said. “Rain does much more damage and kills many more people than wind.”

In the future, Ma and his colleagues plan to model the spatial structure of cold wakes in greater detail to better understand precipitation patterns. Getting a better handle on any possible asymmetry could help researchers more accurately predict ensuing changes in rainfall, the team suggests.

—Katherine Kornei (@KatherineKornei), Contributing Writer

Citation: Kornei, K. (2023), Rainfall from tropical storms might be on the downswing, Eos, 104, Published on 20 October 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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