The HIWRAP is the High-Altitude Imaging Wind and Rain Airborne Profiler, a "conically scanning" Doppler radar, meaning it scans in a cone-shaped manner. Wind measurements are crucial for understanding and forecasting tropical storms since they are closely tied to the overall dynamics of the storm. The HIWRAP instrument is able to measure line-of-sight (along the radar beam) and because it scans in a cone beneath the aircraft, it gets two looks at most parts of the storm, allowing calculations of the 3-dimensional wind and rain fields. In the absence of rain, it can also measure ocean surface winds.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 188.8.131.52.0