Using NASA Airborne HIWRAP Observations to Study the Impact of Precipitation Variability on Path Integrated Attenuation and Near-Surface Reflectivity

Abstract:

Typically, precipitation radar retrieval algorithms assume that the precipitation is uniform throughout the radar resolution volume. As the radar Field-of-View (FOV) increases, the precipitation variability increases and retrieval algorithms need to account for this non-uniform beam filling (NUBF). While ground-based scanning radars can be used to quantify precipitation variability over different FOV scales over land, NASA HIWRAP (High-altitude Imaging Wind & Rain Airborne Profiler) radar observations from the Global Hawk during the NOAA SHOUT (Sensing Hazards with Operational Unmanned Technology) HRR (Hurricane Rapid Response) field campaign provide observations needed to examine spatial precipitation variability over open-ocean conditions.

Previous studies have shown that without accounting for precipitation variability across the radar FOV, the satellite radar derived areal-averaged rain rates are under estimated. This is primarily due to over estimating the PIA which causes the near-surface reflectivity to be underestimated, and thus, leads to under estimating rain rate. This study uses the native HIWRAP PIA and reflectivity measurements to study the impact of areal-averaging on PIA and near-surface reflectivities.

Global Hawk HIWRAP observations are conical scanning and provide a forward and backward view of the same precipitation area. With Ku- and Ka-band frequencies (13 and 35 GHz), HIWRAP provides dual-wavelength measurements and path integrated attenuation (PIA) estimated from ocean surface returns using the surface reference technique. The native HIWRAP resolution is 75 m in range and approximately 1 and 0.4 km diameter surface footprints at Ku- and Ka-band, respectively (with nominal aircraft altitude of 18 km). As the Global Hawk moves forward, there are approximately 75 HIWRAP conical scans within a 5-km surface spatial footprint.

Due to the detailed statistics needed to describe the impact of areal-averaging on PIA and near-surface reflectivity, this presentation is well suited for a poster presentation.