Why are big storms bringing so much more rain? Warming, yes, but also winds
For three hurricane seasons in a row, storms with record-breaking rainfall have caused catastrophic flooding in the southern United States: Harvey in 2017, Florence in 2018 and Imelda in 2019.
A new analysis by Princeton researchers explains why this trend is likely to continue with global warming. Both the higher moisture content of warmer air and storms’ increasing wind speeds conspire to produce wetter storms, the researchers reported in a study published in the Nature Partner Journal Climate and Atmospheric Science and supported by the Carbon Mitigation Initiative based in the Princeton Environmental Institute (PEI).
The results help resolve a puzzle that’s emerged from recent climate-modeling studies. Models have projected that hurricane rainfall rates will increase up to twice as fast as would be expected by the end of the century due to increasing moisture from rising sea surface temperatures. The Princeton team wanted to understand what other forces might contribute.
The researchers suspected that wind might play a role. Climate models also project that tropical storm winds will strengthen as temperatures rise. Observational studies have shown that storms with stronger winds tend to produce higher rainfall rates. The researchers reasoned that a combination of higher sea surface temperatures and stronger storms might explain the predicted increases in rainfall rates.
“This study makes a statement about the future,” said co-author Gabriel Vecchi, professor of geosciences and the Princeton Environmental Institute (PEI). “We’re having this convergence where our observations are starting to show the increased rainfall that our models have been predicting for quite a while, and now we also have a clear theoretical understanding as to why it should be happening.”
In recent years, heavy rainfall from tropical cyclones such as Hurricane Harvey has unleashed devastating flooding on areas in the storm’s path, particularly low-lying heavily developed coastal cities such as Houston.
“Potential changes in the frequency of occurrence and rainfall rates from tropical cyclones are major concerns for flood hazards in the United States, especially for urban regions along the Gulf and Atlantic coasts,” said co-author James Smith, Princeton’s William and Edna Macaleer Professor of Engineering and Applied Science and PEI associated faculty. “This study provides an important step in understanding the rainfall-rate piece of the problem. The picture is one in which changes in rainfall extremes should be carefully examined in assessing flood hazards.”
To test this hypothesis, lead study author Maofeng Liu, a postdoctoral research associate in civil and environmental engineering, devised an approach to isolate the effect of increasing wind speeds: He considered the rainfall rates of projected storms in separate groups according to their wind intensity.
For each of six ocean basins where tropical cyclones form, Liu grouped storms according to their maximum sustained wind speeds: tropical storms, with winds between 39 and 73 miles per hour; and hurricanes in Saffir-Simpson categories 1 through 5, with categories 4 and 5 grouped together due to the smaller number of storms at this intensity level.
Liu used this re-sampling method to analyze more than 4,000 simulated storms under current and future climate conditions. The approach revealed that within each storm intensity category, increases in rainfall rates with rising temperatures aligned well with the increase per degree Celsius of warming expected according to classical thermodynamics (about 7%). Only when storms of all intensities were grouped together did rainfall rates appear to outpace what would be expected to happen as a result of temperature increases alone.
“We found that not only did a storm’s holding capacity for water vapor increase because of global warming,” Liu said, “but also that the storms were getting stronger and contributing to higher rainfall rates.”
A fourth co-author of the study was Thomas Knutson, a research meteorologist at the U.S. National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory at Princeton’s Forrestal Campus. The work was supported in part by the National Oceanic and Atmospheric Administration (U.S. Department of Commerce), the National Science Foundation, the National Aeronautics and Space Administration and PEI’s Carbon Mitigation Initiative.