DPWX/Hail signatures at X-band: 24 May 2016


Author: Patrick C. Kennedy

NWS KFTG reflectivity data from a 1.3 deg elevation angle PPI sweep through a strong thunderstorm located just northeast of the radar. The CSU-CHILL radar collected dual wavelength (S and X-band) data from this same storm. Data plots identifying X-band Mie scattering-related signatures been assembled.


Thunderstorms developed in several portions of the CSU-CHILL radar's coverage area during the afternoon hours of 24 May 2016. Near 2200 UTC, the focus of the radar scanning shifted to a storm that was developing near the NWS KFTG radar (located ~75 km to the south-southeast of CSU-CHILL). According to Storm Prediction Center records, reports of one inch diameter hail were received at 2217 and 2231 UTC from the path of this storm. Since hailstone diameters larger than ~1 to 1.5 cm are large enough to develop Mie scattering resonance effects at a wavelength of 3 cm; selected samples of CSU-CHILL X-band data were examined for indications of Mie scattering.

163 degree azimuth RHI scan at 2155 UTC

The following two images show the vertical storm structure as depicted by the S-band reflectivity and radial velocity fields. The reflectivity data showed a deep, high reflectivity core with a forward overhang at ~70 km range.

The unfolded radial velocity field contained a well-defined low level convergence / storm top divergence pattern, implying the existence of a strong updraft in the vicinity of the forward overhang (indicated by the blue 55 dBZ contour line overlay.)

The next two plots show the corresponding X-band data. (An SNR threshold of +10 dB was applied to all of the presented X-band data.) For reference, the S-band 55 dBZ reflectivity contour is also included. The color fill shows the X-band differential propagation phase (phidp) data. The primary feature of interest is the vertical swath of tan / yellow (> -90 degrees) phidp values that extends across the 55 dBZ S-band reflectivity contour. At ranges on either side of this feature, phidp values are more negative (green shades). Mie scattering increases the magnitude of differential phase shift upon scattering. This is typically manifest as a local positive perturbation in the range profile of phidp. (See Figure 1d in Tromel et al, JAMC, November 2013). Thus, the observation of this "delta column" indicates that hail diameters large enough to enter the Mie scattering regime at X-band were probably present in this portion of the RHI plane. (It should be noted that presence of liquid water, for example due to wet hail growth conditions, can significantly enhance Mie scattering effects.)

The final RHI plot shows the X-band rhoHV multiplied by 10. An area of significantly lowered rhoHV occurred in the same general region as the phidp delta signature. Since Mie resonance effects also are effective at reducing rhoHV, this localized reduction in correlation is an additional indication of the existence of ice particles with diameters that are a relatively large fraction of the X-band (3 cm) wavelength.

The next plot shows the range profiles of phidp, rhoHV and beam height (in km MSL) along a single ray of X-band data in the RHI scan. The red color highlights the local excursions in the phidp and rhoHV traces that occurred in the ~65 - 69 km range interval. The bottom plot panel shows that the height of this data ray was just over 6 km MSL in this range segment.

KFTG and CSU-CHILL dual Doppler analysis at 2157 UTC

Immediately following the collection of the RHI scan shown above, both the NWS KFTG and CSU-CHILL radars started well-synchronized volume scans. The S-band radial velocities observed by these two radars were used to synthesize the three dimensional airflow using NCAR's CEDRIC software. Since the storm core was only at a range of ~20 km from KFTG, the NWS radar did not scan to a high enough elevation angle to observe the top of the storm. As a result, upward integration of the horizontal divergence fields was done to obtain the vertical air velocities. In all of the dual-Doppler plots, a blue line segment has been added that shows the 59 to 73 km range increment along the 163 degree RHI azimuth.

The first dual Doppler plot depicts the Earth-relative horizontal wind vectors and CSU-CHILL S-band reflectivity field at the 6 km MSL level. At this height level, strong, cyclonically-curved flow was found over a broad area to the east of the echo core.

The color fill in the next plot shows the vertical air velocities (mps) as calculated by upward integration. Due to the storm's relatively poor location with respect to dual-Doppler geometry (~20 km from KFTG and ~70 km from CSU-CHILL), the accuracy of the derived vertical air motions is degraded. The indicated updraft band located along the inflow (south) side of the higher reflectivity region seems qualitatively reasonable. (The updraft line to the northwest of the storm is probably due to the proximity to the radar baseline.)

The next plot combines the gridded X-band phidp data with the horizontal flow field at 6 km MSL. On this color scale, the region of orange shades that intersect with the blue RHI line segment are associated with the delta bump that was seen in the RHI data. This delta bump / Mie scattering region shows a fair degree of spatial correlation with the updraft.

The final plot shows the X-band rhoHV x 10 field combined with the 6 km horizontal wind field. The active hydrometeor growth promoted by the moisture-laden updraft probably supported the growth of a wide spectrum of hydrometeor sizes. The existence of such a broad range of particle sizes, especially if the largest diameters entered into the Mie scattering regime, would promote significant rhoHV reductions.