Radar Calibration: Vertical Pointing and Self-consistency Cals

1. Radar Calibration:Vertical Pointing and Self-consistency Cals

2. Calibrating ZDR • ZDR bias needs to be less than 0.1 dB • Particularly important for improving rainfall estimation over conventional radars (especially in light rain) • Particle classification • Microphysical process interpretation • Techniques • Engineering measurements • Measure gains at various measurement ports in radar system • Limited in accuracy by impedance mismatch between system and probe • Causes small reflections of power • Lost power is unmeasured over several measurement ports • Impacts differential calibration • usually within 0.25 dB or so.. • Need for additional calibration • Vertical pointing calibration • Cross-polar technique

3. Vertical Pointing Calibration of ZDR • Use a target with known intrinsic ZDR to calibrate ZDR • Rain/drizzle drops at vertical incident appear on average round – ZDR = 0 dB • Prefer light steady rain over strong convection to avoid mean canting of drops due to shear • Can also use ice, but ZDR distribution widens • Rotate antenna through 360 deg to average out any canting of particles

4. Vertical Pointing ZDR Calibration Time-height plots Z (dBZ) Can’t use all of the data Some problems ZDR (dB) Need some thresholds

5. Vertical Pointing ZDR Calibration: Thresholds • T/R Tube recovery • After transmit the system needs time to recover • For S-Pol about 2 km • Best in pure rain – no ground clutter contamination! • RHOHV > 0.98 • LDR < -25 dB • Receiver saturation • Received power < -50 dBm • Low SNR (Noise) • Received power > -100

6. Vertical Pointing ZDR Calibration: Thresholds Ka-band receiver curve Receiver saturation Noise

7. Vertical Pointing ZDR Calibration Time-height plots Z (dBZ) Histogram of ZDR 2200 Mean = 0.0 dB s = 0.18 dB 1200 ZDR (dB) -1 –0.6 –0.2 0 0.2 0.6 1 ZDR (dB) Thresholded ZDR (dB) The mean of measured ZDR is bias! Don’t forget to take means in linear units and convert back to dB!!

8. Absolute calibration of Z using self-consistency principle • The specific differential phase, KDP, IN RAIN can be related to a combination of Z and ZDR • Only valid for rain (no graupel, no brightband) • The relationship depends on Drop Size Distribution (DSD) and Drop shape assumptions • Calibration technique needs to take into account: • Attenuation by rain • Differential attenuation by rain • Attenuation by the atmosphere • Requires independent ZDR calibration! • Several self-consistency calibration algorithms exist

9. Absolute calibration of Z using self-consistency principle: Relationships • Simulate data using Gamma DSD • Three parameter fit to DSD • Shown to represent well natural variation in DSD’s (??)

10. Absolute calibration of Z using self-consistency principle: Relationships • Simulate data using Gamma DSD • Three parameter fit to DSD • Shown to represent well natural variation in DSD’s (??) • m = shape factor • L = slope factor • N0 = scaling factor • D = drop diameter

11. Absolute calibration of Z using self-consistency principle: Relationships • Simulate data using Gamma DSD • Three parameter fit to DSD • Shown to represent well natural variation in DSD’s (??) • Run simulations over naturally occurring values of m, L and N0 as determined by observations • Ground based disdrometer • Impact • Optical • Airborne particle probes • DSD observations have many problems of their own!!! • Splash • Particle break up from high-speed aircraft • Impact disdrometers have dead time issues…..

12. Absolute calibration of Z using self-consistency principle: Relationships • Must assume a drop shape – many to choose from

13. Absolute calibration of Z using self-consistency principle: Relationships

14. Absolute calibration of Z using self-consistency principle: Relationships

15. Absolute calibration of Z using self-consistency principle: Relationships

16. Absolute calibration of Z using self-consistency principle: Relationships • Different relationships for different assumptions: Gamma DSD Equilibrium shape Constrained Gamma DSD Less oblate shape Constrained Gamma DSD Equilibrium shape

17. Absolute calibration of Z using self-consistency principle: Data • Choose rays of data with long paths through rain

18. Absolute calibration of Z using self-consistency principle: Computation • Compute KDP from Z and ZDR using attenuation corrections • Integrate KDP in range to get estimated PHIDP change • Compare to measured PHIDP change • Scatterplot of estimated and measured PHIDP should lie on 1 to 1 line • Compute bias in Ze

19. Absolute calibration of Z using self-consistency principle: Computation TRMM-LBA Z bias = 0.1

20. Absolute calibration of Z using self-consistency principle: Computation TIMREX Z bias = 0.03

21. Absolute calibration of Z using self-consistency principle: Computation

22. Absolute calibration of Z using self-consistency principle • Used as part of the calibration “package” • Used to check to verify the engineering calibration • If a discrepancy would be found, would trigger search for calibration error • So far, so good