NEXRAD Supplemental Lower Elevation Angle Impacts on MRMS Product Mosaics

Multi-Radar Multi-Sensor (MRMS) radar quantitative precipitation estimation (QPE) mosaics generally utilize 0.5° tilts from Weather Surveillance Radar 1988 Doppler (WSR-88D) radars outside of areas of significant blockage. In recent years, 18 WSR-88D radars started to provide supplementary lower elevation angles (SLEA) ranging from -0.22⁰ to 0.40⁰. Over the past year, MRMS researchers began to experiment with integrating SLEA tilts into the radar-based mosaics. The goal was to determine how the SLEA data influenced the mosaics as well as to document any unexpected impacts on the various MRMS algorithms used to generate them. These experiments were executed using available SLEA tilts near low level radar coverage gaps where SLEA tilts are more likely to impact radar-based mosaics and improve the overall radar coverage.

The experiments showed that the SLEA tilts exhibited significantly more detail for radar echoes, particularly those comprised of vertically shallow precipitation, at farther distances from the radar site. This was expected as the SLEA tilt remains closer to the terrain surface hence the sample beam volume does not overshoot as much of the precipitation bearing clouds than the 0.5° tilt. It was also noted that a dual pol vertical profile of reflectivity correction (dpVPR) was sometimes not attempted for SLEA tilts, hence bright band contaminated reflectivity data would negatively impact QPE. Subsequent investigation revealed that the melting layer top pattern in the RhoHV field extended to near the maximum distance of dual pol data in these instances. Hence, the dpVPR algorithm could not determine a full melting layer structure, including the top, and therefore did not make a bright band correction to reflectivity within the melting layer. Further, the melting layer bottom signature in the RhoHV field appears less significant in the SLEA tilt than in the 0.5° tilt due to the associated larger sample beam volumes at farther distances. This impact sometimes caused the melting layer bottom to be diagnosed further from the radar than it should be, resulting in a wet bias in specific attenuation derived QPE due to contamination from frozen hydrometeors.