Quantifying the Hydrometeorological Impacts of Lowering Operational Weather Radar Scan Elevation Angle

Abstract

The operational Weather Surveillance Radar-1988 Doppler (WSR-88D) network forms the cornerstones of the national weather and climate observation infrastructure, to support forecast and warning operations, as well as aviation safety and flood mitigation systems. The current regulations limit the lowest elevation angle used by WSR-88D to 0.5°, i.e., one-half the antenna beamwidth. However, due to the Earth’s curvature and terrain blockage, more than 70% of the atmosphere below 1 km above ground level cannot be observed, especially when it is far from the radar stations. Unfortunately, that is where most severe weather hazards occur such as floods and tornadoes. Radar hydrometeorology observations and subsequent precipitation estimates will be closer to the ground if they are obtained at lower heights. Recently, the National Weather Service (NWS) has initiated a prototype demonstration of lowering the WSR-88D scan elevation angle to 0° or even lower. The KMUX WSR-88D radar in California is one of the first radar stations that have executed the lower elevation angle scan. Using the KMUX radar as an example, this study quantifies the hydrometeorological impacts of lowering WSR-88D scan elevation angle. In particular, the distribution of polarimetric radar measurements at 0° and 0.5° elevation angles are investigated to highlight the hydrometeor characteristics, especially when the 0.5° beam is shooting or overshooting the melting layer while the 0° beam is observing liquid precipitation regions close to the ground. The enhanced rainfall microphysical structures brought by the 0° scans are investigated to reveal the changes of precipitation during the falling processes. In addition, enhanced radar rainfall algorithms for local application in the San Francisco Bay Area are derived based on raindrop size distribution data, and the enhanced algorithms are compared with operational radar rainfall relations adopted by the WSR-88D systems.