To improve future tropical cyclone forecasts, researchers sent a remotely operated saildrone into the extreme winds and towering waves around the eye of a category 4 hurricane.

By Gregory R. Foltz, Chidong Zhang, Christian Meinig, Jun A. Zhang and Dongxiao Zhang

Saildrone Inc.

Hurricanes are among the most severe natural hazards on Earth, often inflicting extensive damage and loss of life when they strike coastal communities. Seven of the 10 costliest natural disasters in U.S. history have been hurricanes, and the combination of continued coastal development and climate change will likely only increase the risks they pose and the damage they cause. Storms that intensify rapidly before landfall—such as Hurricanes Harvey, Michael, Laura, and Ida in just the past 5 years—pose particularly serious threats because they often reach major hurricane strength and leave limited time for preparations and evacuations.

Last year, NOAA scientists deployed five uncrewed saildrones in regions of the western Atlantic Ocean and the Caribbean Sea where hurricanes have been historically prevalent. In August, the scientists and pilots in charge of remotely operating the saildrones got a chance to practice positioning them ahead of Tropical Storms Fred and Grace. But the real test came in September as Hurricane Sam headed westward across the Atlantic.The accuracy of hurricane track forecasts has steadily increased over the past several decades. However, intensity forecasts have seen uneven progress and significantly less improvement overall since 1990 [Cangialosi et al., 2020]. Predicting rapid intensification, normally defined as an increase in maximum wind speed of at least 15.5 meters per second (30 knots) in 24 hours or less, remains extremely challenging and is a top research and forecast priority [Gall et al., 2013; Cangialosi et al., 2020]. If we are to continue improving hurricane intensity forecasts, we will need further advancements in our knowledge and understanding of the processes that affect hurricane intensification. We will also need expanded measurements of the ocean and the atmosphere ahead of and within hurricanes, as well as improvements to hurricane forecast models.

Interactions of Air and Sea

The ocean and its interactions with the atmosphere play critical roles in hurricane intensification, supplying heat energy that is converted into the mechanical energy of a hurricane’s winds. Turbulent heat exchange, called enthalpy flux, between the ocean and the atmosphere depends on sea surface temperature, air temperature, wind speed relative to the ocean surface, and humidity. The rate at which the enthalpy flux increases with increasing wind speed is not well known for hurricane force winds of 35 meters per second and higher [Bell et al., 2012]. The uncertainty is due in large part to a scarcity of direct flux measurements at the ocean-atmosphere interface inside strong hurricanes.