This document summarizes research on using dual-polarization radar to detect hail at X- and S-band frequencies. It presents two case studies of hail storms in Colorado with hail up to 1.5" in diameter where differences were seen between radar parameters at X-band compared to S-band, indicating Mie scattering detection at smaller sizes on X-band. A third case study of a large hail storm in Illinois with hail over 4" showed similar differences between parameters at S-band, demonstrating Mie scattering effects. The document concludes dual-polarization radar shows potential for improved hail detection compared to single-polarization methods.

1. Kyle Chudler
University of Michigan 2015
Patrick Kennedy
Colorado State University
CSU-CHILL
POLARIMETRIC RADAR DETECTION OF HAIL
USING X- AND S-BAND FREQUENCIES
8/4/2014

2. Summary
• Utilize CHILL Radar’s dual-frequency/dual-polarization
to detect Mie scattering
–Occurrence and size of hail
• Mie scattering occurs at lower hydrometeor diameters
at the X-Band
–Causes differences in radar moments between bands

3. Introduction
• Problem
– Hail is damaging and costly
– Small to moderate sized hail not as easy to detect
– Previous detection methods use DF or DP, not both
• Explore differences between DP products arising from Mie
scattering
– Mie scattering at X-band, Rayleigh at S-band
• End goal is to improve hail detection

4. (Skolnik 1980)

5. Background: Mie Backscattering
• Evidence of Mie scattering in three moments
• dBZ
– Decrease in dBZ due to initial lowering
in backscatter power in Mie regime
• ρhv
– Measure of the level of correlation
between the co-polar return signals
– Large change in backscatter with small diameter
change in Mie regime (Shoichiro and Kyosuke 2013)

6. Background: Mie Backscattering
• ϕdp
– Phase difference between the
co-polar signals
– Radar measures
ΨC = ϕdp (diff. prop. phase)
+ δ (diff. backscat. phase)
– Significant non-zero values of δ (> 5°) occur in Mie scattering
(Hubbert and Bringi 1995, Trömel et. al. 2013).
– Mie scattering causes “bumps” in ϕdp profile
Zrnić et. al. 1993

7. Methods
• Gather case studies, using VCHILL to get a first look
• Storm reports (SPC, CoCoRaHS, MPING)
• Load and manipulate radar files with MATLAB
• Look at 3 different parameters over a
time series of radar scans
– Dual frequency ratio (DFR)
– ρhv
– φdp
• Raw – Smoothed
• “5&5” test
• Use hydrometeor identification algorithm to outline areas of hail
• Compare values of parameters inside and outside hail region

8. Methods: DFR
• DFR: dBZSB – dBZXB
• Attenuation corrected
• Two methods
1. Calculate DFR each time step, then take temporal maximum of that
at each gate
2. Calculate temporal maximum reflectivity at each gate for each
band, then take difference between those
• Method 2 found to be less noisy

9. Method 1 Method 2

10. Methods: ρhv
• “Min-hold” technique
– Keep track of minimum ρhv that occurred over time at each gate
– Plot all of these minimum values at once
– Compare values inside and outside HID hail contour (histograms)
• X-band ρhv always lower than S-band, even without Mie
scattering
– Antenna/hardware artifact
– Correct for by looking for areas of light rain
• determine correction factor needed to match S-band in this area

11. UNCORRECTEDCORRECTED

12. Methods: φdp
• Use DROPS program to give smoothed φdp
– Also provides us with our HID
• Method 1
– Absolute difference between smoothed and raw φdp
– Max-hold
– Compare values inside and outside HID hail contour (histograms)
• Method 2
– Take difference between smoothed and raw φdp
– Look for stretches where difference remains the same sign and above a certain
magnitude for a certain number of gates
• S-band: >5 degrees for 3 gates (“5&3” test)
• X-band: >5 degrees for 5 gates (“5&5” test)
– Also includes a dBZ threshold (> 25 dBZ)

13. Methods: φdp
• Method 2
– S-band: >5 degrees for 3 gates (“5&3” test)
– X-band: >5 degrees for 5 gates (“5&5” test)
Range
φdp

14. Case Studies
• Two 0.5” – 1.5” hail storms
– Loveland, CO 06/03/2015
– Fort Collins, CO 06/04/2015
– Hail large enough to cause Mie scattering at X-band, not at S.
• One giant hail case (>4”)
– Minooka, IL 06/10/2015
– NEXRAD WSR-88D data from KLOT (no X-Band)
– Hail large enough to show Mie scattering at S-band

15. Event 1: Loveland, CO
• Hail up to 1.5” in Loveland
• 8 radar scans ~0100 – 0118 UTC
• Warned for ping-pong ball sized
hail as it moved through Loveland
• 12 storm reports gathered

18. ρhv
S-Band X-Band

19. S-BANDX-BAND
S-BANDX-BAND
0.8423 0.9589
0.0484
0.970 0.984
0.014

20. φdp (Raw - Smoothed)
S-Band X-Band

21. S-BANDX-BAND
S-BANDX-BAND
1.39

22. φdp (5&3/5&5 test)
S-Band X-Band

23. Event 2: Fort Collins, CO
• Hail up to 1.25” in Fort Collins
• 19 radar scans ~2340 – 2355 UTC
• Warned for half-dollar sized hail
at 23:52 UTC
• 20 storm reports gathered

25. ρhv
S-Band X-Band

26. S-BANDX-BAND
S-BANDX-BAND
0.9590 0.9820
0.0230
0.9002 0.9538
0.0536

27. φdp (Raw - Smoothed)
S-Band X-Band

28. S-BANDX-BAND
S-BANDX-BAND
1.29

29. φdp (5&3/5&5 test)
S-Band X-Band

30. φdp (5&3/5&5 test)
DFR X-Band

31. Events 1 & 2 Summary
• All methods could detect hail/Mie scattering to some extent
• Median ρhv was reduced by ~0.05 in HID hail region in X-band
– Compared to ~0.015 in S-Band
• Absolute difference between raw and smoothed φdp increased
by ~1.35° in X-band
– Compared to no change in the S-Band
– Plot very noisy looking
• 5&5 method able to successfully hone in on HID hail region
better, got rid of noise
– Detected extended “bumps” in φdp profile

32. Event 3: Minooka, IL
• Hail up to 4.75” in Minnoka, IL
– Second largest ever recorded in IL
• KLOT WSR-88D data
– No X-band -> no DFR
• Looking for same Mie scattering
indications that previous cases
showed at X-band http://www.weather.gov/lot/15jun10
Photo shared to the NWS by Lauren Geiselman

33. dBZRaw–Smoothedφdp
ρhv5&3Test
4.75” Hail

34. Event 3 Summary
• Same Mie scattering effects observed as were at X-Band for
smaller hail
– Reduction in ρhv
– Increase in magnitude of difference between raw and smoothed φdp
• 5&3 method unable to localize on Mie scattering region
– Flagged gates throughout entire storm
– Very unlikely hail large enough for S-Band Mie scattering was that
prevalent

35. Conclusions
• Using ρhv and φdp to detect Mie scattering appears to be a
viable strategy
– Can be seen at both bands
• Mie scattering effects are observed at smaller diameter hail at
X-band
– Could lead to earlier radar detection/warnings
• “5&3” detection method needs further refinement at S-Band

36. Future Work
• Improve 5&3 (S-band) test
• Perform more statistical analysis
(i.e. statistical significance of ρhv reduction, etc)
• Zdr in relation to hail size
– Large hail -> negative S-band Zdr?
• Eventually a more advanced algorithm could be developed
– Like DFR (compare between two radar bands), but with polarimetric
parameters

37. Questions?
Loveland Storm Fort Collins Storm
S-Band X-Band S-Band X-Band
ρhv
Difference in median value
inside/outside hail region
0.014 0.0484 0.023 0.0536
Raw – Smoothed ϕdp
Difference in median value
inside/outside hail region
.09 1.39 0.10 1.29
“5&3”/ “5&5” test
Percentage of gates within
hail region that were
flagged as Mie scattering
0.6 13.4 1.8 23.6

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