Marshall Shepherd’s new research with NASA focuses on the “brown ocean” effect.
The 2017 Atlantic hurricane season may well go down in history as one of the most devastating.
Many experts agree that the remnants of Hurricane Harvey, which dumped more than a year's worth of rain on parts of Texas and Louisiana in just a few short days, will be remembered as one of the worst flood disasters on record.
The current season, which officially runs from June 1 to November 30, is also one of only six in history to feature at least two Category 5 hurricanes-Hurricane Irma and Hurricane Maria-and more could be on the way.
"When a hurricane is particularly devastating, its name is retired. This year, I suspect we will see three names receive that dubious distinction," said Marshall Shepherd, Georgia Athletic Association Distinguished Professor of Geography and Atmospheric Sciences.
Now, researchers at the University of Georgia and NASA's Goddard Space Flight Center are working to understand how wet soils could strengthen these devastating storms as they move over land, and Shepherd said Hurricane Harvey could provide a good case study for their investigation.
The scientists are building on an idea first advanced in two UGA-based studies suggesting that wet soils could sustain or intensify hurricanes, typhoons and other tropical cyclones, a phenomenon they call the "brown ocean" effect.
"The more common behavior is that hurricanes weaken when they move over land so this research has implications for assessing storm hazards and flood potential as storms move over land," said Shepherd, the study's principal investigator. "Our previous work found that smaller or weakening storms can be the biggest rain producers, not the powerhouse Category 5 storms."
Harvey was a tropical storm when it produced over 4 feet of rainfall in parts of Texas.
The new research, funded by NASA's Modeling and Analysis Program, will augment Earth system models by using satellite-based soil moisture estimates to better predict when storms will strengthen over land. Shepherd will explore a variety of case studies, including the possibility that Tropical Storm Harvey fed from the "brown ocean" in parts of Louisiana later in its life span.
"The brown ocean effect may also go beyond hurricanes, as we possibly saw with the storm system that caused the devastating Louisiana floods in 2016," Shepherd said. "The combination of advanced NASA datasets and models will allow us to better understand how the land and atmosphere talk to each other during storms."
Shepherd says the team will explore how wet soil, swampy regions like the Everglades and even irrigated areas affect storms as they move inland.
Recent research from one of Shepherd's former doctoral students now working at the U.S. Forest Service suggests that irrigated landscapes in southwest Georgia may provide a moisture source for storms in that region.
"The concept of the brown ocean effect is very intriguing scientifically and has practical implications for tropical cyclone forecasting," said Joseph Santanello of the NASA Goddard Center Hydrological Sciences Laboratory. "This proposal will allow us to bring together the expertise and tools required-including NASA weather and land surface models, and satellite data-to properly address the processes and predictability of this phenomenon."