Evolution of a three body scattering signature: 26 May 2010

Three body scattering patterns were observed in association with a severe hailstorm on 26 May 2010. Plots of the time evolution of the three body scattering patterns are shown.

Introduction

Echoes due to three body scattering are typically seen down-range of thunderstorms that contain high reflectivity regions located at heights of several kilometers above the ground. When these high reflectivity areas are illuminated by the radar pulse, they re-radiate a scattered component of the incident energy in all directions. Some of this scattered energy is reflected off of the underlying ground surface, producing a re-illumination of particles in the pulse volume. A portion of this re-scattered energy is directed back towards the radar where it may result in a detectable echo. The lengthened path travelled by these signals (radar -> hydrometeors -> ground -> back to hydrometeors -> back to the radar) causes them to appear as down-range extensions of the generating thunderstorm echo.

The presence of hailstones aloft with significant diameters and concentrations can cause the appearance of the three body signature. During CSU-CHILL operations on 26 May 2010, an intensifying hailstorm was observed to develop an increasingly well defined three body signature.

RHI Data

The following sequence of RHI scans were all collected on an azimuth of 191 degrees. In each image frame the upper panel is reflectivity and the lower panel is differential reflectivity. The development and descent of the intense reflectivity area is apparent. In the differential reflectivity data, a region of anomalously positive $Z_{dr}$ values develops in the three body scattering region down range of the storm. This is one of the differential reflectivity characteristics of three body echoes that were reported in Hubbert and Bringi (J. Atmos. Oceanic Tech., January, 2000, 51-61).

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The next plot includes the radial velocity field observed in the final RHI of the above time lapse sequence. The spatial variability of the radial velocities is relatively smooth in the thunderstorm echo; the velocity variability is visibly greater in the three body region. Due to the large fall velocity of the hailstones, the radial velocities in the three body echo are also predominately negative (Wilson and Reum, J. Atmos. Oceanic Tech., April 1988, 197-205).

PPI Data

A short loop of 0.5 degree elevation PPI scans that bracket the RHI image loop period is shown below. In these images, the upper panel is reflectivity and the lower panel is linear depolarization ratio (LDR). As hail becomes an increasingly large component of the precipitation, the reflectivity levels approach 70 dBZ and the LDR values increase to ~ -15 dB. The final PPI image time is 1932:37 UTC. The SPC severe weather report records contain an observation of damage-producing 2 inch diameter hail from this storm in Commerce City at 1933 UTC.

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References

• J. C. Hubbert, and V. N. Bringi, 2000: The Effects of Three-Body Scattering on Differential Reflectivity Signatures. Journal of Atmospheric and Oceanic Technology., 17, 1 (January 2000) pp 51-61.
• James W. Wilson, Darelyn Reum, 1988: The Flare Echo: Reflectivity and Velocity Signature. Journal of Atmospheric and Oceanic Technology., 5, 2 (April 1988) pp 197-205.
• Three Body Scattering on Wikipedia