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Improving understanding of circulation changes during hurricane intensification


A research team funded in part by the Climate Program Office’s Climate Observations and Monitoring Program, in collaboration with the Atlantic Oceanographic and Meteorological Laboratory’s Hurricane Research Division, has published a new paper describing the circulation changes that occur when hurricanes strengthen. The paper, published online July 17, 2019 in Monthly Weather Review, is entitled “Observed Kinematic and Thermodynamic Structure in the Hurricane Boundary Layer during Intensity Change.”

Hurricane research is an integral part of climate science, and the life of hurricanes receives keen interest from the general public. As hurricanes form near the United States, coastal and inland communities monitor the probability of landfall and prepare for potentially damaging winds, storm surge, and precipitation. The financial cost of a hurricane can be staggering.  According to NOAA, from 1986 to 2015, hurricanes cost the United States over $515 billion. In 2017, four of the five costliest U.S. hurricanes on record hit (Harvey, Maria, Sandy, Irma). Hurricane Harvey alone cost $125 billion.

Within the large field of scientific inquiry into hurricanes, hurricane intensity forecasting holds great significance. Several recent hurricanes intensified at an astonishing rate, highlighting the importance of improving hurricane intensity predictions. Hurricane Michael, which struck the Florida Panhandle in 2018, strengthened from a Category 2 storm to almost a Category 5 in 24 hours. Hurricane Maria, which devastated Puerto Rico in 2017, strengthened from Category 1 to Category 5 in 24 hours. Currently, forecasters can provide more information about a hurricane’s future path than its future strength.

Kyle Ahern, graduate student and former NOAA Hollings Scholar, advisor Dr. Mark Bourassa, and their co-authors developed a comprehensive dataset to study processes occurring within the bottom layer of the atmosphere when hurricanes intensify. 

The team processed over 12,000 atmosphere profiles collected during hurricanes from 1998 to 2015 to produce the dataset. These long-term NOAA observations include measurements of hurricane pressure, temperature, humidity, wind speed, and direction.   

The new dataset allowed the researchers to analyze hurricane circulation features related to air-ocean heat and moisture fluxes as well as frictional forces, and highlight their different kinematic and thermodynamic properties. They found that strengthening hurricanes have strong vertical turbulence and varying regions of stability inside versus outside the eyewall, which may have implications for where convection occurs during periods of changing intensity. The authors noted that these results improve scientists’ understanding of how air flows within a hurricane as a function of its dynamic and evolving physical properties, and could be used as guidance for hurricane intensity forecasting.

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