NASA-funded Researchers Classify New Type of Storm Fueled by 'Brown Ocean'

In 2007 Tropical Storm Erin puzzled meteorologists as it inexplicably intensified after making landfall in Texas, growing stronger over the southern plains than it was at sea.

Erin is now being considered a prime example of a newly define class of inland cyclone that maintains or increases strength after landfall.

Typical tropical storms gather power from the warm waters of the ocean. But this new class of cyclone generates its energy from the evaporation of abundant soil moisture - a phenomenon that researchers are calling the "brown ocean."

"The land essentially mimics the moisture-rich environment of the ocean, where the storm originated," explains Theresa Andersen of the University of Georgia in Athens.

Andersen and her colleague J. Marshall Shepherd's NASA-funded study is the first of its kind to measure the post-landfall strength and structure of inland tropical cyclones.

"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."

To gain a better understanding of how tropical cyclones survive after landfall, Andersen and Shepherd accessed a NOAA climate data archive for tropical cyclones from 1979 to 2008. To be considered for the data set, the storms has to meet the criteria of "retaining a measureable central pressure by the time they tracked at least 220 miles (350 kilometers) inland, away from the maritime influence of the nearest coast," NASA reported. The researchers also gathered atmospheric and environmental data before and after the storms.

Of the 227 inland tropical cyclones identified, 45 maintained or increased strength after landfall. Of the 45 highlighted storms, 16 could be classified under the new category of inland cyclone. The researcher call the new type of storm a Tropical Cyclone Maintenance and Intensification events, or TCMIs.

"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."

Most of the TCMIs accounted for in the 30-year study period occurred in the United States and China, but researchers said the "hotspot" turned out to be Australia, where the brown ocean effect was in play the most.

Andersen and Shepherd's work resulted in three observable conditions for brown ocean effect: that the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature, that soils in the vicinity of the storms need to contain ample moisture and, finally, that evaporation of the soil moisture releases latent heat, which must measure at least 70 watts averaged per square meter. (For comparison, the latent heat flux from the ocean averages about 200 watts per square meter.)

When Erin was tracked across the U.S. Gulf Coast and Midwest, all three conditions were present.

But about questions regarding variations in climate, soil and vegetation are still present and researcher are trying to understand what makes Australia the region where brown ocean conditions most often turn up.

The research, according to NASA, also points 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.