HIWRAP observations from the Hurricane and Severe Sentinel (HS3) campaign: Comparing retrieval techniques

The High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) is a dual-beam, dual-frequency, Doppler radar system designed for operation on board the Global Hawk aircraft unmanned aircraft system. The HIWRAP radar system recently obtained observations of Hurricane Ingrid during the 2013 Hurricane and Severe Storm Sentinel (HS3) field campaign. The antennas of HIWRAP point downward and scan conically at two different tilt angles. This scanning geometry, which is unlike previous airborne radar systems, presents unique challenges to retrieving the full three-dimensional wind field. We present observations from the HS3 field campaign using two dual-Doppler retrieval techniques that were specifically modified for the HIWRAP geometry.

The first technique is a coplanar retrieval method that solves for the wind field in a cylindrical coordinate system natural to the radar's scanning geometry. Utilizing scans pointing fore and aft along coplanar beams, two wind components are calculated while the third wind component is retrieved using mass continuity and boundary conditions in the nadir plane and at the surface. The second technique is a global optimization approach that minimizes the difference between radar-measured and retrieved velocity components while approximately adhering to mass continuity and boundary conditions at nadir and at the surface.

Each technique was first tested using a radar-scanning simulator on model output in a hurricane environment. A comparison of the retrieved and true velocities revealed low errors throughout most of the field. This application also highlighted relative strengths and weaknesses for each method. The coplanar method performed well with all wind components near the nadir plane, while the optimization method was more capable near the surface and near the domain edges. Results from the HS3 observations show that both techniques retrieved a realistic wind field that documents the outer regions of Hurricane Ingrid. The observed features contain kinematic structures that are consistent with previous modeling and observational studies. The coplanar method performs well at capturing convective-scale structures, while the optimization method appears to be less sensitive to noise and other measurement errors in the radar data.