Wind Retrieval Algorithms for the IWRAP and HIWRAP Airborne Doppler Radars with Applications to Hurricanes

Algorithms for the retrieval of atmospheric winds in precipitating systems from downward pointing, conically scanning airborne Doppler radars are presented. The focus in the paper is on two radars: the Imaging Wind and Rain Airborne Profiler (IWRAP) and the High-altitude IWRAP (HIWRAP). The IWRAP is a dual-frequency (C and Ku band), multi-beam (incidence angles of 30° – 50°) system that flies on the NOAA P3 aircraft at altitudes of ~ 2 – 4 km. The HIWRAP is a dual-frequency (Ku and Ka band), dual-beam (incidence angles of 30° and 40°) system that flies on the NASA Global Hawk aircraft at altitudes of ~ 18 – 20 km.

Two basic retrieval algorithms are described: (1) retrieval of vertical and along-track winds in a vertical plane along the aircraft track (“plane” method) and (2) retrieval of the three Cartesian wind components over the entire radar swath (“swath” method), which can be determined using either a direct or variational solution procedure. The random errors in the retrievals are evaluated using both a theoretical approach and a numerical simulation of a hurricane. These error analyses show that the vertical and along-track wind errors have strong across-track dependence with values of ~ 0.25 m s-1 and ~ 0.20 m s-1 at nadir to ~ 2.0 m s-1 and ~ 1.00 m s-1 at the swath edges, respectively. The cross-track wind errors have a more complicated distribution and are on average ~ 3.50 m s-1 or ~ 10% of the local wind speed. For typical rotated figure four flight patterns through hurricanes, the zonal and meridional wind speed errors are ~ 2 – 3 m s-1.

Examples of real data retrievals from IWRAP during an eyewall replacement cycle in Hurricane Isabel (2003) and from HIWRAP during the development of Tropical Storm Matthew (2010) are shown.