HS3

HS3 is a NASA mission that deploys manned and unmanned aircraft to study hurricane formation and intensification. The program, which began operations in 2012, flies instruments into and around tropical cyclones to collect data on atmospheric conditions, ocean surface temperature, and wind shear that influence storm development. The acronym stands for Hurricane and Severe Storm Sentinel, though the mission focuses primarily on hurricanes rather than general severe weather.

The HS3 fleet typically includes a piloted WB-57F high-altitude aircraft carrying remote sensing instruments, and multiple unmanned Global Hawk aircraft equipped with dropsondes, radiometers, and microwave sensors. The WB-57F can reach altitudes above 60,000 feet to observe the upper-level outflow from hurricanes, while the Global Hawks operate at lower altitudes closer to the storm core. This combination allows researchers to sample atmospheric profiles and sea surface conditions simultaneously across different layers and spatial scales.

Dropsondes are central to HS3 operations. These small instrument packages are released from aircraft and transmit pressure, temperature, humidity, and wind data as they fall through the atmosphere. A typical mission might deploy 20 to 30 dropsondes around a hurricane's circulation, providing detailed cross-sections of the storm environment. The microwave instruments aboard the aircraft penetrate cloud cover to measure sea surface temperature and wind speed directly beneath the storm, data that visible and infrared sensors cannot obtain.

Why this matters to hurricane forecasters

Operational hurricane forecasters rely on satellite estimates of storm intensity and track, but these estimates contain systematic errors. HS3 research has addressed gaps in understanding rapid intensification, the process where a hurricane's maximum sustained winds increase by 35 knots or more in 24 hours. By documenting the atmospheric and oceanic conditions preceding rapid intensification events, HS3 data helps improve the physical basis for intensity guidance in numerical models.

HS3 operations are seasonal, running during the Atlantic hurricane season from August through October in most years. The mission is not a forecast or warning system; its primary product is research data shared with the scientific community months after collection. Maintenance personnel supporting HS3 must be familiar with both the Global Hawk unmanned aerial vehicle systems and the specialized instrumentation packages, which differ significantly from routine aircraft maintenance.