Tropical Storms Gain Strength Over Land Due To 'Brown Ocean' Phenomenon
July 17, 2013

Tropical Storms Gain Strength Over Land Due To ‘Brown Ocean’ Phenomenon

April Flowers for - Your Universe Online

Meteorologists were stumped by Tropical Storm Erin in 2007. Unlike most tropical cyclones, which dissipate after making landfall and being weakened by everything from friction and wind shear to loss of the ocean as a source of heat energy, Erin intensified as it tracked through Texas.

Erin's eye formed over Oklahoma, and grew stronger than it had been over the ocean as it spun over the southern plains. According to NASA-funded research by Theresa Andersen and J. Marshall Shepherd of the University of Georgia in Athens, Erin is representative of a newly defined type of inland tropical cyclone that maintains or increases strength after landfall.

Tropical storms gather power from the warm waters of the ocean before they make landfall. Those in the newly defined category pull their energy from the evaporation of abundant soil moisture -- a phenomenon Andersen and Shepherd call the "brown ocean."

"The land essentially mimics the moisture-rich environment of the ocean, where the storm originated," Andersen said.

The study, published in the International Journal of Climatology, is the first global assessment of the post-landfall strength and structure of inland tropical cyclones, and the weather and environmental conditions in which they occur.

"A better understanding of inland storm subtypes, and the differences in the physical processes that drive them, could ultimately improve forecasts," Andersen said. "Prediction and earlier warnings can help minimize damage and loss of life from severe flooding, high winds and other tropical cyclone hazards."

The researchers accessed data archived by the National Oceanic and Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC) for tropical cyclones from 1979 to 2008 to gain a better understanding of tropical cyclones that survive beyond landfall. To be included in the study, storms had to meet the criteria of retaining a measureable central pressure by the time they tracked at least 220 miles inland, away from the maritime influence of the nearest coast. Then, the research team analyzed atmospheric and environmental data for before and after the storms from NASA's Modern Era Retrospective-Analysis for Research and Applications (MERRA).

Of the 227 inland tropical cyclones identified, the research team found 45 that maintained or increased strength, as determined by their wind speed and central pressure. Not all such storms are fueled equally, the study found.

Hurricane Sandy, in October 2012, demonstrated the destructive power of extratropical cyclones -- a well-studied storm type that undergoes a known physical and thermal transition. Systems such as these begin as warm-core tropical cyclones that gain energy from the ocean. Once on land, however, the storms transition to cold-core extratropical cyclones that derive energy from clashes between different air masses. Of the 45 inland storms that maintained or increased strength, 17 belong to this category of extratropical cyclones.

Another 16 of the 45 storms are the newly described storm category, including Tropical Storm Erin. These storms maintain their tropical warm-core characteristics instead of transitioning to a cold-core system once on land.

Anderson and Shepherd call this type of storm tropical cyclone maintenance and intensification events, or TCMIs. TCMIs have the potential to deliver much more rainfall than their extratropical counterparts.

"Until events like Erin in 2007, there was not much focus on post-landfall tropical cyclones unless they transitioned," Andersen said. "Erin really brought attention to the inland intensification of tropical cyclones."

"This is particularly critical since a study by former National Hurricane Center Deputy Director Ed Rappaport found 59 percent of fatalities in landfalling tropical cyclones are from inland freshwater flooding," Shepherd said.

Many inland tropical cyclones happen in the US or China. The majority of TCMIs during the 30-year study period, however, occurred in Australia. The uneven geographic distribution led the researchers to examine the environment and conditions surrounding the brown ocean phenomenon that gives rise to the storms.

The study revealed that a brown ocean environment consists of three observable conditions: the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature; soils in the vicinity of the storms need to contain ample moisture; and evaporation of the soil moisture releases latent heat, which the team found must measure at least 70 watts averaged per square meter. In contrast, the latent heat flux from the ocean averages about 200 watts per square meter.

When Erin tracked across the US Gulf Coast and Midwest, all three conditions were present and yet, questions remain about the factors -- such as variations in climate, soil and vegetation -- that make Australia the region where brown ocean conditions most often turn up.

The study findings also point to possible implications for storms' response to climate change. "As dry areas get drier and wet areas get wetter, are you priming the soil to get more frequent inland tropical cyclone intensification?" asked Shepherd.