Tropical Atlantic, Gulf of Mexico
While NOAA has made steady progress in forecasting the track of a hurricane, forecasting hurricane rapid intensification—wind speeds increasing 30 knots in 24 hours or less—remains a significant 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 2021, NOAA and Saildrone launched a multiyear mission to deploy Explorer-class uncrewed surface vehicles (USVs) into tropical storms and hurricanes. The goal of the mission is 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 are equipped with ruggedized “hurricane wings” designed especially for operating in winds over 78 knots and waves over 15.2 meters. They are strategically positioned in operational areas in the Tropical Atlantic and Gulf of Mexico that have a high likelihood of encountering a hurricane, based on historical track data. NOAA and Saildrone Mission Control work together to task the vehicles into as many storms as possible.
The USVs transmit 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 is used to improve ocean-atmosphere initial conditions in forecast models.
During the missions, NOAA orchestrates coordinated sampling with underwater gliders and aerial assets to create a complete picture of the atmospheric and water column, from 9,000 meters above the surface to 1,000 meters below.
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 Category 4 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.
In 2022, four of seven vehicles engaged with Hurricane Fiona: It was still a tropical storm when it passed over SD 1083, stationed 400 nm east of Montserrat; the vehicle measured wind speeds gusting over 34 knots. The storm continued on a trajectory due west and had strengthened to a Category 1 as it passed over SD 1031, stationed just south of Puerto Rico, where Fiona first made landfall. The vehicle recorded waves up to 14 meters high and wind speeds over 61 knots, which dropped abruptly to as low as 8.5 knots when SD 1031 was in the eye of the storm. While inside the eye, SD 1031 recorded a minimum central pressure of 986 mb. Stationed north of Puerto Rico, SD 1040 recorded wind speeds over 52 knots and 12-meter waves on the edge of the storm. Fiona had strengthened to a Category 4 storm when it reached SD 1078. Inside the storm, the vehicle sailed at sustained speeds over 7.8 knots, reaching a momentary peak speed of 34.5 knots before surfing down a massive 17-meter wave.