HS3 is a five-year mission specifically targeted to enhance our understanding of the processes that underlie hurricane intensity change in the Atlantic Ocean basin. The HS3 science questions can be distilled down to the extent to which either the environment or processes internal to the storm are key to intensity change. Is storm formation and intensification mainly a result of characteristics of the large-scale environment or internal processes? Are these internal processes driven by large-scale forcing or do they act independently of this forcing?

The HS3 objectives are: • To obtain critical measurements in the hurricane environment in order to identify the role of key factors such as large-scale wind systems (troughs, jet streams), Saharan air masses, African Easterly Waves and their embedded critical layers (that help to isolate tropical disturbances from hostile environments). • To observe and understand the three-dimensional mesoscale and convective-scale internal structures of tropical disturbances and cyclones and their role in intensity change.

The mission objectives will be achieved using two Global Hawk (GH) Unmanned Airborne Systems (UASs) with separate comprehensive environmental and over-storm payloads. The GH flight altitudes (>16.8 km) allow overflights of most convection and sampling of upper-tropospheric winds. Deployments from Goddard's Wallops Flight Facility and ~26-hour flight durations will provide coverage of the entire Atlantic Ocean basin, and on-station times up to 5-22 h depending on storm location. Deployments will be in September of 2012 and from late-August to late-September 2013-2014, with eleven 26-h flights per deployment.

HS3's suite of advanced instruments will measure key characteristics of the storm environment and its internal structures. The measurements include:

Measurements from the Environmental GH Payload • Continuous sampling of temperature and relative humidity in the clear-air environment from the scanning High-resolution Interferometer Sounder (S-HIS). • Continuous wind profiles in clear air from the TWiLiTE instrument. TWiLiTE is a direct detection Doppler lidar capable of measuring the motion of air molecules in clear air environments. • Full tropospheric wind, temperature, and humidity profiles from the Advanced Vertical Atmospheric Profiling System (AVAPS) dropsonde system, which is capable of releasing up to 100 dropsondes in a single flight. • Aerosol and cloud layer vertical structure from the Cloud Physics Lidar (CPL).

Measurements from the Over-Storm GH Payload • Three-dimensional wind and precipitation fields from the High-altitude Wind and Rain Airborne Profiler (HIWRAP) conically scanning Doppler radar. • Surface winds and rainfall from the Hurricane Imaging Radiometer (HIRAD) multi-frequency interferometric radiometer. • Measurements of temperature, water vapor, and liquid water profiles, total precipitable water, sea-surface temperature, rain rates, and vertical precipitation profiles from the High-Altitude MMIC Sounding Radiometer (HAMSR).