February 25, 2013 | Rita, Katrina, Sandy—these are familiar as the names of three of the most destructive hurricanes in recent history. Early warnings from reliable forecasts can allow thousands of people to evacuate in the path of such frightening and potentially deadly weather events, and the sooner such a warning can be issued, the better.  But if a storm fizzles after a warning goes out, that also creates risks, as people may be less likely to take precautions the next time a storm threatens—a syndrome known as warning fatigue.

As a step toward meeting the goal of providing earlier warnings, NCAR scientists and their colleagues examined what enables poorly organized clusters of thunderstorms to develop into tropical storms and hurricanes. A study by NCAR scientists Chris Davis and David Ahijevych, published in the Journal of Atmospheric Sciences, details that the development of a tropical storm depends on certain critical features. These include a pre-existing tropical weather system that has a counter-clockwise swirl of air in the Northern Hemisphere (a clockwise one in the Southern Hemisphere) and is also aligned through a vertical column at least three miles high.

However, this swirling air may not align at different altitudes.  For instance, the center of the circling air at the ocean’s surface may be at a different location from the center of the circling air several miles above. When that happens, the misalignment seems to delay or even break down the formation of a tropical storm.

Davis and his team observed eight storm systems during one hurricane season to explore what conditions allowed storms to congeal and what made them fall apart. A field experiment funded by the National Science Foundation and NCAR known as PREDICT—the Pre-Depression Investigation of Cloud Systems in the Tropics—provided the means to collect data.

The researchers relied on the NSF/NCAR Gulfstream V aircraft that flew multiple missions over the Atlantic basin during the heart of hurricane season, from August 15 to September 30, 2010. Dropsondes—parachute-outfitted devices that gather data as they fall to Earth—were released on each occasion at different altitudes. Sensors on the dropsondes measured air pressure, temperature, humidity, location, height, dewpoint, windspeed, and wind direction.

Digging into the data, Davis and team confirmed that a high moisture content through at least the lowest three miles is important for tropical storm formation because it greatly increases the efficiency of rainfall. But an offset of circulating air contributes to delaying storm formation or makes things fizzle out altogether.

“One likely reason that misalignment is important,” Davis says, “is that it allows the intrusion of air from outside the circulation, and this air tends to be drier than the air inside the circulation.”

The structure of conditions that may or may not develop into a tropical storm are difficult to see in satellite data, Davis explains, unless the misalignment of the circulating air is extreme. However, these conditions can sometimes be depicted by computer models used in weather prediction.

The new findings bring the science a bit closer to distinguishing the real threats, vital to protecting populations and saving them from warning fatigue.

Christopher A. Davis and David A. Ahijevych, Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances, Journal of the Atmospheric Sciences (January 1, 2013); DOI: 10.1175/JAS-D-12-0278.1

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