Florida, US Virgin Islands
While NOAA has made steady progress in forecasting the track of a hurricane, forecasting hurricane rapid intensification (wind speeds increasing 35 mph in 24 hours or less) remains a big challenge. To improve understanding of hurricane rapid intensification, scientists need to understand the ocean processes that are occurring as intensity increases, which means collecting data immediately before and during a storm.
In July 2021, NOAA and Saildrone deployed five Explorer uncrewed surface vehicles (USVs) equipped with ruggedized “hurricane wings” designed especially for operating in winds over 90 mph and waves over 50 feet. The goal of the mission was to measure near-surface atmospheric and upper-ocean parameters to calculate energy and momentum fluxes between the atmosphere and ocean outside and within hurricanes. The observations collected would be used to understand how ocean-atmosphere interaction affects hurricane intensity and improve hurricane prediction models.
The vehicles were strategically positioned in five operational areas that have a high likelihood of encountering a hurricane, based on historical track data. NOAA and Saildrone Mission Control worked together to task the vehicles into as many storms as possible.
The USVs transmitted meteorological and oceanographic data including air temperature and relative humidity, barometric pressure, wind speed and direction, water temperature and salinity, sea surface temperature, and wave height and period from the eastern tropical Atlantic in near real time; the NRT data was used to improve ocean-atmosphere initial conditions in forecast models.
The mission also sought to pair the USVs with underwater gliders to measure the coupling between the surface atmosphere and upper ocean.
On September 30, 2021, SD 1045 encountered Category 4 Hurricane Sam approximately 450 nautical miles NE of Puerto Rico. The vehicle remained in the storm for several hours, experiencing winds gusting over 120 mph and 50+ foot waves and sent near-real-time data and video throughout the storm to Saildrone Mission Control.
The data collected during Hurricane Sam enabled NOAA collaborators to plot the surface fluxes from the edge through the eyewall. Preliminary findings show that salinity may have played a role in hurricane intensification.
Saildrones and ocean gliders were coordinated to take near-collocated, simultaneous measurement of the upper ocean and the surface oceanic-atmospheric conditions. Comparisons between Saildrone data and that of moored buoys was made to cross-validate measurements, and found to agree very well, and dropsondes and oceanic floats were launched by NOAA and NASA aircraft to provide additional observations.
Initial results suggest that fresh (low saline) water, likely emanating from the Amazon River, may have fueled Hurricane Sam, keeping the ocean surface warm and allowing for more evaporation of energy.
“The point of the whole scientific mission was to measure the surface flux within hurricanes, especially around the eyewall—and we got it! But before the mission began, my primary goal was to see if the new short-wing vehicle would work, because we just didn’t know. I told everyone, ‘If this vehicle can survive a hurricane, then this would be a big success story.’ The whole mission exceeded my expectations.”
Chidong Zhang, director of the Ocean Climate Research Division at NOAA PMEL
All five vehicles in the 2021 mission contributed important insight into hurricane rapid intensification by sampling near other tropical storms: SD 1031 sailed into Tropical Storm Henri, SD 1040 sailed into Tropical Depression Mindy, SD 1048 got into the strong winds of Tropical Storms Fred and Grace and SD 1060 was to the north of Tropical Storm Grace, and SD 1045 experienced the strong winds of Tropical Storm Peter before sailing into Hurricane Sam. SD 1031 and SD 1040 also sailed in the strong winds of Tropical Storm Mindy after the storm had weakened to a post-tropical low.