Examining Severe Storm Characteristics with the Airborne Phased Array Radar (APAR) Observing Simulator

Airborne radars provide a unique opportunity to understand high-impact weather events, such as hurricanes and mesoscale convective systems, due to their ability to sample large areas of a storm and to follow a storm as it propagates. While some airborne cloud radars are available to the research community, the NSF and the National Center for Atmospheric Research (NCAR) are developing a new and transformational airborne radar to investigate a multitude of atmospheric phenomena and severe storms. Known as the Airborne Phased Array Radar (APAR), it is designed to fly on the NSF/NCAR C-130 aircraft and provide both dual-Doppler and dual-polarization capabilities using agile scanning methods associated with PAR technology. While APAR is still in development, a tool was created at NCAR that allows scientists to test various uses of APAR in a controlled setting. The APAR Observing Simulator (AOS) currently uses model output and user-defined radar scan and flight parameters to provide realistic radar data that would be expected with APAR’s given specifications.

The AOS uses high spatial and temporal resolution Cloud Model 1 (CM-1) output of a supercell, squall line, and tropical cyclone to provide a base set of weather information by which to generate the simulated APAR data. Previous experiments with the AOS output investigated some of the radar's technical limitations related to flight and scan strategies, some initial estimates of the expected beam broadening away from the radar boresight, and attenuation comparisons with X-band radars (currently in use on the NOAA WP-3D aircraft). Initial analyses also quantified the effects of altering proximity to a storm and the ability to observe certain storm characteristics from various flight altitudes. For the current presented work, two sets of experiments are studied within the AOS. The first set of experiments evaluates the Doppler winds and their comparison with known storm characteristics with confirmation that APAR produces this information with a high level of accuracy. These velocity experiments show that at a minimum, APAR can support objectives that were designed for mechanically scanning tail Doppler radars. Further discussion of the velocity performance for different storm types is also provided from the context of resolution and reliability.

APAR is also equipped with dual-polarimetric capabilities, providing a new opportunity to study the microphysical and precipitation characteristics of these severe storms in ways that have not previously occurred for airborne platforms. A second set of experiments showcases the results associated with investigating particle identification (PID) distributions and Quantitative Precipitation Estimates (QPE). The initial PID determination and analysis results suggest that the hydrometeor classifications match well between the simulated cases and known distributions. For instance, the rain and snow distributions are well-represented, but there is a noticeable difference in the amount of ice crystals depicted in the simulated storms compared to observations. The results from the QPE analysis indicate that specific methods are preferable for APAR, where “hybrid” rain rate determination schemes tend to provide more accurate results. These “hybrid” schemes use APAR’s hydrometeor classifications and dual-polarization parameters. Preliminary results of a more tailored scheme specific to APAR for the QPE techniques and PID classification is also presented along with some initial techniques using the PID classification to improve the estimation of vertical velocity.