1. Introduction to Radar Meteorology
Leyda V. León-Colón, PhD
Electrical and Computer
Engineering Department

2. Types of radar
Ground Based
Airborne Based
Satellite Based
Mobile

3. Radar Equation and Radar Reflectivity

4. Clear Air

5. Electromagnetic Waves and Polarization
Linear
Circular
Elliptical
** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTOR
LINEAR
VERTICAL
HORIZONTAL
CIRCULAR
Left Hand (LHC)-Counter Clockwise
Right Hand (RHC) - Clockwise
ELLIPTICAL

7. Weather Radars: Why is it important?
MECHANISM…
HOW?
(Tx) Transmit Power
(S) Power is Scattered over its path
(Rx) Scattered Power towards radar is measured
Linear
Tx Horizontal
Rx Vertical
Or any combination
ZHH, ZHV,ZVV, ZVH
Circular
For Spheres: Tx RHC & Rx LHC
For Irregular: Tx and Rx same power, i.e. Police Radars
CDR: Circular Depolarization Ratio

8. Targets

9. Dual Polarization in Weather Radars
Dual polarization radars can estimate several return signal properties beyond those available from conventional, single polarization Doppler systems.
Hydrometeors: Shape, Direction, Behavior, Type, etc…
Events: Development, identification, extinction
Lineal Typical
Horizontal
Vertical
ZHH ZVV
ZHV ZVH

10. CSU-CHILL Radar Dual Polarized Doppler S-band V port H port Towards
reflector
Dual Polarized Doppler S-band

11. CP2 Radar Located at Brisbane, Australia
Single Polarized Doppler X-band
Dual polarized Doppler S-band

12. SPOL & XPOL NCAR’s SPOL Dual Polarized, ZH NOA’s XPOL (transportable)

13. CASA and TropiNet Radars vs. NEXRAD
Dual polarized Doppler X-band
WSR-88D: NEXRAD, all around the US
Single Polarized, Doppler
KOUN: NSSL’S Dual polarized Prototype

14. How are things done?
Backscattered electric field from an individual scatterer is described by the scattering matrix. “S” values are complex numbers that depend on the scatterer shape, orientation and dielectric constant
Incident field due to transmitted radar pulse
Backscattered electric field; contains both H and V components
Here, subscripts are transmit, receive from the particle viewpoint
Largest terms are “co-polar” (repeated subscript) matrix elements

15. Some useful quantities that such a radar can measure are:
Ratio of the H and V signal powers (ZDR)
Phase difference between the H and V returns (fDP)
Degree of correlation between the H and V returns (rHV)
Ratio of orthogonal to “on channel” signal power (LDR)

16. Inherent difference in Zdr characteristics of raindrops vs. hailstones

17. Zdr observations in rain and hail
Hail (~random orientation) dominates Z-weighted mean axis ratio: Zdr decreases to ~0 dB

18. Differential Phase ΦDP vs. Specific Differential Phase KDP
RAIN
Wet Ice
Differential Phase doesn’t say anything by itself
BUT ITS CHANGE OVER SPACE and TIME DOES!!!!

19. Negative KDP observed in thunderstorm anvil
For vertically-oriented particles,
Svv > Shh; KDP negative

20. RAIN Dual-polarized Radars
DP-based methods: Simple Attenuation Correction
RAIN

21. Co-polar H,V return signal correlation (rhv or rco)
Numerator:
Decreases when Shh and Svv are not uniformly correlated among the scatterers; (i.e., Svv is not always = .5 Shh for all scatterers in the pulse volume. When this uniformity does exist, rHV goes to 1.0)
Denominator:
Normalizes the ratio into 0 to 1 range
Factors that Reduce rHV (Balakrishnan and Zrnic 1990):
Radar pulse volume variations in the distribution of scatterer:
1.Shapes, 2.Sizes, 3. d magnitudes (d is Mie-related differential phase shift on scattering)
4. canting angles 5. hydrometeor types (example: both liquid and frozen present)
6. hydrometeor shape irregularities (some rough aggregates, etc.)

22. rHV reduced in hail area:
Mixed precip types; rHV especially reduced when Zrain=Zice
Diverse shapes

23. Melting level / bright band readily recognized by local rHV minimum
Melting level / bright band readily recognized by local rHV minimum. Reflectivity maximizes as frozen particles initially develop an outer water coating. With further descent / warming, smaller particles completely melt. Mixed frozen and completely melted layer gives lowest rHV values. (Enhanced Z is a few 100 m higher up)
Blue contours are 20 and 40 dBZ

24. rHV summary Typical Values
Primarily useful to characterize variability of scatterer characteristics within the pulse volume.
Drizzle / light rain > ~0.98
Convective (but no ice) rain > ~0.96
Hail / rain mixtures ~0.90
Bright band mixed rain and snow ~0.75
Tornado debris ~0.50 or less

25. Linear Depolarization Ratio (LDR)
Is the ratio of the cross-polar to co-polar backscattered signal powers. Here the HV subscripts represent the receive and transmit polarizations respectively.
For cloud and precipitation targets, the cross polar signal level is typically only 10-2 – 10-3 of the co-polar level (LDR~ -20 to -30 dB)

26. Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than rain drops.
Red line ~upper LDR limit for rain
Tropical” (ice-free) rain LDR observations: Upper LDR limit ~-24 to -25 dB.
Note small LDR magnitudes. Snow LDR of -30 dB implies that cross polar signal from 30 dB snow echo is 0 dB. Noise can bias / obliterate such weak cross polar channel signals

27. As with rHV, LDR maximizes in the melting level region where wet, non-spherical, gyrating ice particles exist.

28. Hail areas present variable LDR levels
Hail areas present variable LDR levels. In this storm, the dBZ core area is characterized by LDR levels that are virtually all below -22 dB.
Note also how LDR increases in clutter, noise, and many echo edge areas.

29. Hydrometeor identification (HID)
Radar data values are used to develop a numerical score for each designated particle type. Identification is based on the highest-scoring type.

30. Hydrometeor classifications at 5.5 km MSL in a thunderstorm complex

31. Cyril Zh(X) Corr. for Rain
CASA: June 10th, 2007
PPI at 12.25 in elevation
Cyril Zh(X) Corr. for Rain
HID

32. HID after attenuation correction
DP-based
SRT-modified

33. Hail Event on March 23rd, 2012 on SW Puerto Rico
ENDI News Report
Differential Hail Signal (HDR)
Dependent on ZH and Polarimetric Variable ZDR
Su, et al 2010, Bringi and Chandrasekar 2001

34. Detection on TropiNet: Cornelia
HDR>10dB detect areas with hail
High ZH collocates with HDR areas above 10dB

35. Reflectivity and HDR Movie

36. NO questions… Estoy saturada de Polarimetría!