5Electromagnetic Waves and Polarization LinearCircularElliptical** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTORLINEARVERTICALHORIZONTALCIRCULARLeft Hand (LHC)-Counter ClockwiseRight Hand (RHC) - ClockwiseELLIPTICAL
7Weather Radars: Why is it important? MECHANISM…HOW?(Tx) Transmit Power(S) Power is Scattered over its path(Rx) Scattered Power towards radar is measuredLinearTx HorizontalRx VerticalOr any combinationZHH, ZHV,ZVV, ZVHCircularFor Spheres: Tx RHC & Rx LHCFor Irregular: Tx and Rx same power, i.e. Police RadarsCDR: Circular Depolarization Ratio
9Dual 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, extinctionLineal TypicalHorizontalVerticalZHH ZVVZHV ZVH
10CSU-CHILL Radar Dual Polarized Doppler S-band V port H port Towards reflectorDual Polarized Doppler S-band
11CP2 Radar Located at Brisbane, Australia Single Polarized Doppler X-bandDual polarized Doppler S-band
13CASA and TropiNet Radars vs. NEXRAD Dual polarized Doppler X-bandWSR-88D: NEXRAD, all around the USSingle Polarized, DopplerKOUN: NSSL’S Dual polarized Prototype
14How 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 constantIncident field due to transmitted radar pulseBackscattered electric field; contains both H and V componentsHere, subscripts are transmit, receive from the particle viewpointLargest terms are “co-polar” (repeated subscript) matrix elements
15Some 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)
16Inherent difference in Zdr characteristics of raindrops vs. hailstones
17Zdr observations in rain and hail Hail (~random orientation) dominates Z-weighted mean axis ratio: Zdr decreases to ~0 dB
18Differential Phase ΦDP vs. Specific Differential Phase KDP RAINWet IceDifferential Phase doesn’t say anything by itselfBUT ITS CHANGE OVER SPACE and TIME DOES!!!!
19Negative KDP observed in thunderstorm anvil For vertically-oriented particles,Svv > Shh; KDP negative
21Co-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 rangeFactors 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.)
22rHV reduced in hail area: Mixed precip types; rHV especially reduced when Zrain=ZiceDiverse shapes
23Melting 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
24rHV summary Typical Values Primarily useful to characterize variability of scatterer characteristics within the pulse volume.Drizzle / light rain > ~0.98Convective (but no ice) rain > ~0.96Hail / rain mixtures ~0.90Bright band mixed rain and snow ~0.75Tornado debris ~0.50 or less
25Linear 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)
26Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than rain drops.Red line ~upper LDR limit for rainTropical” (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
27As with rHV, LDR maximizes in the melting level region where wet, non-spherical, gyrating ice particles exist.
28Hail 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.
29Hydrometeor 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.
30Hydrometeor classifications at 5.5 km MSL in a thunderstorm complex
31Cyril Zh(X) Corr. for Rain CASA: June 10th, 2007PPI at 12.25 in elevationCyril Zh(X) Corr. for RainHID
32HID after attenuation correction DP-basedSRT-modified
33Hail Event on March 23rd, 2012 on SW Puerto Rico ENDI News ReportDifferential Hail Signal (HDR)Dependent on ZH and Polarimetric Variable ZDRSu, et al 2010, Bringi and Chandrasekar 2001
34Detection on TropiNet: Cornelia HDR>10dB detect areas with hailHigh ZH collocates with HDR areas above 10dB