High-Resolution Hurricane Boundary Layer Properties Using the Airborne Phased Array Radar (APAR) Agile Scanning Techniques

The current landscape of airborne weather radars in the United States consists of the operational Tail Doppler Radars (TDRs) on the National Oceanic and Atmospheric Administration (NOAA) WP-3D hurricane hunter aircraft and various cloud radars within the university community. With the retirement of the National Science Foundation (NSF) ELectra DOppler RAdar (ELDORA) more than ten years ago, there has been a gap in ability to perform innovative research related to high-impact weather events, especially in remote locations. Because of this lack of research quality airborne radar data, the NSF, the National Center for Atmospheric Research (NCAR), and NOAA are in the process of developing a new and transformational airborne radar: the Airborne Phased Array Radar (APAR). This new radar is designed to fly on the NSF/NCAR C-130 aircraft and provide dual-Doppler and dual-polarization capabilities using agile scanning methods associated with PAR technology. Operating at C-band as opposed to X-band (or W-band) for the existing and previous airborne radars, APAR will probe deeper into convection and severe weather events due to less rain-attenuation and will serve as a unique airborne radar for retrieving combined dynamics and storm microphysical processes. While APAR is still in the development phase, a tool was created at NCAR that allows scientists to test various uses of APAR in a controlled setting. The APAR Observing Simulator (AOS), which incorporates functions developed for the Cloud Resolving Model Radar Simulator (CR-SIM) software package, uses model output and user-defined radar scan and flight parameters to provide realistic radar moment output that would be expected with APAR’s given technical specifications.

The purpose of this paper is to discuss the results of several experiments performed with the AOS for tropical weather events, specifically applications for tropical cyclones (TCs). High spatial and temporal resolution Cloud Model 1 (CM-1) output of a TC is used as input into the AOS, where specific flight patterns and scan strategies are employed to allow evaluation of various characteristics of the Doppler and polarimetric radar parameters within the TC boundary layer. One of the advantages of using APAR compared to TDRs, such as ELDORA, are highlighted by comparing the differences of storm structure representation. For example, APAR’s sensitivity of -11 dBZ at 10 km with 0 dB Signal-to-Noise Ratio (SNR) is very similar to the sensitivity provided by ELDORA (-12 dBZ at 10 km). However, a significant benefit of APAR is the use of a 5.5 cm wavelength signal that is less attenuated than the 3.2 cm wavelength signal employed by ELDORA and TDR. For example, a 20 mm hr-1 rain rate produces 1.3 dB two-way attenuation at C-band while it produces a 7.2 dB loss for X-band over a 10 km path. Therefore, more storm information is retained at farther ranges from the radar.

Another major benefit of APAR compared to the existing or retired TDRs is the agile scanning strategies that allow for higher spatial and temporal resolution data collection for specific regions of a storm. The current mechanically rotating TDRs have a constant rotation pattern with a rotation rate of between 15-20 rotations per minute. This rate produces a full rotation every 3-4 seconds. At typical aircraft speeds of ~120 m/s (~240 kt), a full scan contains data along a flight track of 400-500 m. For APAR, four phased array radar panels collect data simultaneously, and the typical design of a range height indicator (RHI) scan from one of the side panels is completed in 2.2 seconds (fore and aft scanning). Compared to the TDR, this cuts the sampling time nearly in half, which also reduces the distance between scans to ~260 m. Changes in the precipitation or velocity fields that occur quickly during storm progression are more likely to be observed with APAR in this example. The agile scanning of APAR also enables alternate scanning patterns to focus on specific regions of a storm, such as the TC boundary layer. Rather than produce a full vertical scan every time, it would be possible to scan only within the boundary layer and would allow even higher along-track resolution (< 150 m), introducing a means by which small scale storm characteristics are observable by APAR. Proximity of the aircraft to features of interest is important in this consideration due to beam spreading at increasing ranges. Results of this modified type of scanning are examined in this study in terms of velocity and precipitation with direct comparisons between the TDR style and APAR style scanning. These scanning patterns are the first of a set of iterative testing that is being performed during the APAR development and will provide a basis for establishing protocols for innovative observing techniques for future operations.