Rapid-Scan, Polarimetric Radar Observations and Ground Validation of a Hail-Producing Supercell in Colorado

Ground truth observations of hail are often limited, and most often only the maximum diameter of a hailstone is provided. The relationship between hail sizing and polarimetric radar data is quite complicated and sometimes ambiguous, partly due to the complexity in hail shape and resonance effects. In addition, longer update times provided by most operational radars can fail to resolve rapidly evolving hail growth processes and hail descent that can provide information about the resulting surface hail size. Rapid-scan radar systems have shown utility in resolving rapidly evolving hail processes and have successfully captured processes such as hail melting and size sorting. Most rapid-scan cases to date, however, have still been coupled with limited hail reports, so any linkage between the spatial distribution of hail and polarimetric evolution have been limited.

This study utilizes polarimetric radar data collected with the mobile Rapid X-Band Polarimetric Radar (RaXPol) on 13 June 2016 near Kiowa, CO. RaXPol collected volumetric observations from 2152 – 2219 UTC approximately every 60 seconds, scanning in the plane-position indicator (PPI) mode up to 30 degrees in elevation in steps of 2 degrees, providing high spatial resolution in both the horizontal and vertical dimensions. Hail disdrometers and physical measurements from the Insurance Institute for Business and Home Safety (IBHS) of hailstones that fell out of the storm were available, located about 30 km to the northwest of RaXPol. The rapid-scan and dense vertical sampling from RaXPol resolved a new updraft pulse that occurred to the west of the parent supercell that rapidly grew upscale before merging in with the parent supercell. This cell merger contributed hailstone embryos to the next updraft pulse and resulted in the appearance of the hail growth zone, marked by anomalously low rhohv and noisy ZDR values across the mid-levels and extended into the appearance of the rhohv column. The rapid-scan observations provided some insight into the evolution of the hail growth zone, including changes to the structure and appearance of the hail growth zone. Ground truth observations provided by IBHS revealed the spatial variability of the hailfall, including a transition from larger, wet growth hailstones to smaller, layered hailstones from west to east. Low-level RaXPol data revealed differences in the timing and polarimetric data of these measured hailstones. These coupled rapid-scan and ground truth observations can provide better insight into the evolution of hail growth through the variation of polarimetric data, which will be discussed.