NWC REU 2015
May 26 - July 31



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Dual-Polarization Hail Signatures Above the Melting Layer in Thunderstorms

Alexis Hunzinger, Kiel Ortega, and Jeff Snyder


What is already known:

  • The differential reflectivity column implies lofting of supercooled liquid water, which may contribute to accelerated hail growth and larger hail.
  • The evolution of a differential reflectivity column compared to surface hail fall has been modeled, but no observational studies have been completed.
  • Correlation coefficient is useful for hail detection and the connection between correlation coefficient within the differential reflectivity column and surface hail fall has not been explored.

What this study adds:

  • Simple measures of the differential reflectivity column, such as height above 0°C isotherm, may be too simple for robust hail identification.
  • The onset of large and giant hail at the surface was correlated with changes of the minimum correlation coefficient within the differential reflectivity column.
  • Highlights the importance of radar calibration for accurate differential reflectivity measurements, as two-thirds of the available cases were excluded due to large differential reflectivity bias.


Identification and sizing of hail is important for warning operations and post-storm activities, such as identifying where the largest hail may have fallen. Recently the National Weather Service in the United States upgraded its operational radar fleet to polarimetric capabilities. Dual-polarization variables, such as differential reflectivity (ZDR) and correlation coefficient (CC), can be useful in not only identifying areas with hail, but also the size of that hail. The Severe Hazards Analysis and Verification Experiment (SHAVE), run by CIMMS and the National Severe Storms Laboratory in Norman, OK, is tasked with collecting reports of hail, including maximum and average sizes, hail fall times, and ground coverage, in the wake of thunderstorms across the contiguous United States. The reports are collected at a high spatial resolution, with median report spacing near 2 km.


Eight cases with SHAVE reports were analyzed. These cases came from storms that were within 125 km of the nearest radar and produced at least 1 report of giant hail (diameter equal to or exceeding 51 mm). The primary signature investigated was the ZDR column and attributes of those columns. The ZDR column can be used as a proxy for updraft strength since it implies the lofting of supercooled liquid water droplets above the melting layer. These supercooled droplets may contribute to large hail growth. The ZDR column height relative to the melting layer and the CC values within the ZDR column were recorded. The location of the ZDR column was compared to SHAVE reports. The goal for the analyses were to spatially and temporally relate the ZDR column characteristics to the maximal surface hail size (e.g., does the appearance of a ZDR column mean surface hail fall of a certain hail size within 20 minutes?). The results of the analyses and discussions on the feasibility of a ZDR column algorithm and application of the results to the hail size discrimination algorithm will be presented.

Full Paper [PDF]