DPWX/Near-surface hail development: 6 - 7 June 2016


Author: Patrick C. Kennedy

CSU-CHILL reflectivity data from a 1.9 deg elevation angle PPI sweep through an isolated hailstorm. A three body scattering signature extends radially down range from the storm. Time lapse loops of the reflectivity and differential reflectivity data at this elevation angle have been assembled.


Near 1800 MDT on 6 June 2016 (the UTC boundary between 6 and 7 June), the CSU-CHILL radar was conducting continuous surveillance scans at an elevation angle of 1.9 degrees. In this unattended precipitation mapping operating mode, the scans repeated at ~90 second intervals. This scan sequence captured the development of dual polarization hail signatures in a thunderstorm that was passing approximately 30 km to the southwest of the radar.

Reflectivity and differential reflectivity (Zdr) loop

The frames in the following CSU-CHILL data image loops were generated using the SOLO software developed at NCAR. Reflectivity is on the left and Zdr is on the right. The maximum reflectivity levels in the echo core at the 1.9 degree elevation angle reach ~70 dBZ at 0005 UTC. As this intensification took place, a three body scattering signature appeared in the down range direction. The backscattered return received by the radar is only one component of the three dimensional electromagnetic field that the hydrometeors emit when they are illuminated by the transmitted radar pulse. The re-radiation in the downward direction illuminates the underlying Earth surface. In turn, components of scattering from the ground surface can re-illuminate the hydrometeors in the pulse volume. Some of the radiation produced by this second illumination of the hydrometeors is in the original backscattering direction towards the radar. Due to the path length added by the round trip to and from the underlying surface (i.e., a sequence of three scattering path nodes: (i) hydrometeors, (ii) underlying surface, (iii) hydrometeors), the the radar plots the resultant "three body" echo at ranges beyond that of the generating storm (Zrnic, Radio Science, 1987, 76-86.)

The Zdr values in the echo core decrease from their initially positive values towards ~ 0 dB and even slightly negative levels during this general intensification period. The Zdr reduction is due to the increasing signal return contribution made by quasi-spherical, tumbling hailstones. These shape and orientation factors tend to equalize the magnitudes of the horizontally and vertically polarized received signal levels (Zh and Zv respectively). Since Zdr is 10 Log10 (Zh / Zv), Zdr becomes 0 dB when Zh and Zv are equal. In the precipitation areas outside of the hail region, the scatterers are primarily raindrops. At diameters larger than ~1 mm, the equilibrum shape of raindrops becomes oblate. This flattened cross section acts to increase Zh relative to Zv, yielding positive Zdr values.

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Hail signatures at 005:45 UTC

The following plot provides an expanded view of several CSU-CHILL data fields associated with the presence of hail. In all of the plot panels, an irregular boundary has been subjectively added to define the area of clearly-defined hail signatures. As noted earlier, reflectivity (upper left) in the hail region was in the 60 - 70 dBZ range; this very strong echo region was also associated with a localized Zdr reduction (upper right). The correlation between the H and V co-polar received signals (rhoHV), is shown in the lower left panel. The combination of gyrating hailstones and oblate raindrops produced a wide variety of hydrometeor shapes and orientations with the radar pulse volume. This diversity reduces rhoHV from the near 1.0 values in pure rain areas to levels approaching 0.90 inside portions of the hail boundary. Mie scattering effects from suitably large diameter hailstones can also act to reduce rhoHV (Balakrishnan and Zrnic, J. Atmospheric Science, 1990, 1525-1540.) Linear Depolarization ratio (LDR) is shown in the lower right panel. LDR is based on the component of the received signal that appears in the "off" polarization channel. (For example, the V received signal level return following the transmission of an H pulse.) This cross-polarized return is increased when the major axis of non-spherical scatterers is oriented at large angles to the incident polarization plane. The hailstone's tumbling motions promote the occurrence of these significantly inclined orientation angles. As a result, relatively large LDR values (> ~-18 dB) are found within the hail contour. (The development of a water component in the hail's ice structure and on the stone's outer surface due to melting effects can significantly increase LDR. Also, the scattering interactions with the ground surface produce anomalously large (> -10 dB) LDR values in the three body scattering echo.)

CSU-CHILL and KFTG three body scattering patterns at 0006 UTC

The final plot shows the low elevation angle reflectivity fields observed by the CSU-CHILL and NWS KFTG radars at ~0006 UTC. The beams from these two radars passed through the hail core within 30 seconds of each other. Both radars show well-defined indications of three body scattering in their respective down-range directions.