In recent years, uncrewed surface vehicles (USV) have become vital tools used to collect observations to improve hurricane forecasts. To fully understand how storms intensify, scientists need data at the air-sea interface where energy is transferred from the ocean to tropical cyclones. This data is most effectively collected by USVs because of their unique ability to operate in dangerous environments and extreme conditions. In a new study, scientists are able to directly quantify the air-sea momentum exchange (termed ‘wind stress’) and related frictional drag effect that acts to slow a hurricane’s rotating winds. These direct measurements of this process are the first to be collected over the ocean in hurricane-force winds.
A key part in the formation and intensification of tropical cyclones is the exchange of energy between the ocean and atmosphere in the forms of heat and momentum. The ocean supplies energy for the storm and slows its winds through surface friction, which exerts a force on the ocean, known as wind stress. During this process, wind momentum is transferred from the storm to the ocean, generating ocean currents, surface waves, storm surge, and mixing. This mixing cools the ocean’s surface which can act as a negative feedback and impact the tropical cyclone’s intensity. Therefore, it is critical for forecast models to simulate wind stress properly.
Hurricane forecast models cannot resolve the high-frequency wind variations that drive the wind stress and instead must approximate it using a drag coefficient (the resistance to the air moving over the ocean’s surface) together with average winds and air density. The drag coefficient is the key variable that converts wind speed to wind stress. Waves can also impact the wind stress and drag coefficient, so understanding how waves interact with tropical cyclones is important.
However, due to extremely limited direct observations, wind stress and the drag coefficient in tropical cyclones, and their relationships with wind speed and surface waves, are poorly known. To estimate the drag coefficient, measurements need to be taken near the surface of the ocean during storms, which is very difficult to do. Historically, scientists used moored buoys or aircraft, or conducted lab experiments to estimate the value. However, previous measurements over the ocean were limited to lower wind speeds and didn’t include simultaneous data on waves.
In this study led by scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and Pacific Marine Environmental Laboratory (PMEL), researchers used direct measurements from saildrones, a USV, collected in 11 Atlantic hurricanes to quantify the drag coefficient in winds up to 98 miles per hour and waves up to 46 feet.
During the 2021-2024 Atlantic hurricane seasons, saildrones were used to collect high-quality observations near the air-sea interface in hurricanes. These autonomous platforms are equipped with sensors to measure essential ocean variables, are remotely piloted, and can collect observations in extreme conditions including hurricane force winds and large waves. Saildrones allow scientists to monitor conditions continuously throughout the core of a storm, where most of the energy exchange occurs.
The study analyzing saildrone data revealed that the drag coefficient levels off as wind speed surpasses 67 miles per hour, but it does not decrease significantly after that as many previous studies using indirect methods have indicated. It was also found that the drag coefficient is larger than what is currently used in hurricane forecasts models. This finding offers vital information that could be used to improve the accuracy of future forecast models.
“Our results provide the first direct quantification of the air-sea momentum flux and drag coefficient in hurricane-force winds using field observations and have important implications for understanding and modeling air-sea interactions in tropical cyclones. Our finding that the drag coefficient varies significantly from one storm quadrant to another emphasizes the need for drag coefficients in forecast models that depend not only on wind speed but also wind-wave misalignment,” said Greg Foltz, a NOAA AOML oceanographer and lead author of the study.
To evaluate the changes in the ocean’s surface with changing wind speed, researchers analyzed 97 images taken by saildrones in 7 different hurricanes. The pictures show that as wind speed increases from 56 to 71 mph, the ocean surface changes significantly, showing much more foam at higher wind speeds. However, the surface doesn’t appear to change as dramatically at extremely high wind speeds, which is consistent with the leveling off of the drag coefficient.
The study shows that as wind speeds increase, the interaction between waves and wind direction causes the ocean’s drag coefficient to vary more than previously expected. Accounting for these variations, especially on the left side of the storm, could lead to improvement in hurricane forecast models. Ignoring variations in wind stress in forecast models can lead to errors in predicting upwelling, ocean cooling, and overall storm intensity.
This study is the first to quantify a key tropical cyclone air-sea interaction variable using direct observations. The results also highlight how this interaction changes depending on the specific part of the storm and wind-wave interactions, which is important for understanding how tropical cyclones behave. Continuing to collect direct measurements of key ocean variables is needed to further advance our understanding of air-sea processes in tropical cyclones.
This study was made possible by NOAA/OMAO Uncrewed Systems Operations Center, NOAA Weather Program Office, NOAA Global Ocean Monitoring and Observing, NOAA Atlantic Oceanographic and Meteorological Laboratory, NOAA Pacific Marine Environmental Laboratory, the Cooperative Institute for Climate, Ocean, and Ecosystem Studies (CICOES), the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), and Saildrone Inc.