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1、Polarimetric Radar MeteorologyMOTIVATION:transitioning from conventional power-based measures of precipitation rate and coverage,to more accurate and complete dual-polarimetric estimates of precipitation types and amounts.lSCOPE:polarimetric theory,radar design,data processing,physical interpretatio
2、n,algorithms.lOBJECTIVE:through“hands-on”approach with data from research radars above,learn latest methods for quantifying precipitation types and mounts.Polarimetric Rain Rate EstimationPolarimetric Radar Rain Rates vs.Rain GaugesSimplified Block Diagram Doppler Radar Antenna Example of 3-D Beam P
3、atternlNCAR CP-2 X-band(Rinehart and Frush,1983)More sidelobes.The Radar Equation Refractive index and K-values as a function of phase and temperatureWater to Ice Transition in Tropical Convection Transition across melt level is 5-10 dBZ,as predicted by theoryConventional Doppler RadarWSR-88D(NEXRAD
4、)Doppler Velocity SpectrumVr examples:lTornadic Supercell Thunderstorm:May 29,2001 l如何识别降水类型？l如何精确测量降水量？l-极化雷达Radar Waves,Polarization,and ScatteringElectromagnetic SpectrumElectromagnetic Waves Brief Mathematical DescriptionPolarizationBackscattering MatrixCovariance MatrixRadar observablesElectrom
5、agnetic SpectrumElectromagnetic Waves:SpectrumElectromagnetic Waves Scattering and the Backscattering Matrix What are we measuring?Polarization,Dielectric,Refractive IndexRecall differences in returned power for ice and waterPolarization of matterRefractive IndexRelationship to Dielectric(or relativ
6、e permittivity)Recall Differences Between Ice and WaterRecall Differences Between Ice and WaterPolarizationRelating the dielectric constant,refractive index,and the dielectric factor(or how many ways can physicists say the same thing?)Refractive index and K-values as a function of phase and temperat
7、ureDifferential Propagation Phase Define Propagation phase shift Differential propagation phase(dp)Specific Differential phase shift(Kdp)Examples of dpand KdpKdp from Rayleigh-Gans theoryDependence on Number concentrationShape Dielectric Wavelength.Relationship to liquid(rain)water content and drop
8、diameterDifferential Propagation Phase(DP)and Specific Differential Phase(KDP)Phase Cont.Example of Differential Propagation Phase(dp)and Specific Differential Phase(Kdp)in rain at C-band(5.5 cm)Dual-polarized radar systemsIntroduction Polarization Radar SystemPolarization agility vs.polarization di
9、versityPolarization agile systemTransmit Block DiagramReceive Block DiagramCritical antenna componentsWaveguide SwitchOMT/FeedhornDishAntenna Requirements and effects on polarization measurementsZdr calibration*Introduction:Simplified Block Diagram of a Common Polarization Radar SystemDual-polarizat
10、ion radar system typesThere are two general system typesPolarization agility:Ability to change the transmitted polarization state between two orthogonal components(e.g.,linear horizontal and vertical polarization,Hand V,respectively)on a pulse-to-pulse basis.Polarization diversity:Ability to receive
11、 alternate orthogonal polarizations,but no alternate transmission of orthogonal components.Such a system transmits only a single elliptical orcircular polarization and then can receive co-polar and cross-polar components with dual receivers).We will focus primarily on polarization agile radar system
12、s.Polarization Agility Transmitted Waveform Schematic Simplified Block Diagram of Polarization Agile Radar Systems in Linear(H:Horizontal,V:Vertical)Polarization Basis-Transmit lTRANSMIT SIDESimplified Block Diagram of Polarization Agile Radar Systems in Linear(H:Horizontal,V:Vertical)Polarization B
13、asis ReceivelRECEIVE SIDEAssuming linear polarization basis and dual Receiver(e.g.,S-pol,CHILL)Polarization or Waveguide SwitchTo Switch or Not to SwitchCritical that switch isolate the H and V transmit/receive powers.Ferrite switches are not as robust,in this regard,as rotary switches.Further,Ferri
14、te switches experience a larger power insertion loss,the loss is not uniform between transmit and receive modes,and theyare very sensitive to temperature fluctuations.For high quality cross-polar measurements(e.g.,measuring depolarization)need an H/V or cross-polar isolation of at least30 dB(even lo
15、wer if possible;35 dB to 45 dB of isolation is preferable for effective hydrometeor identification).A single ferrite switch typically provides 20 to 25 dB of isolation(combinations of ferrite switches can reduce the isolation,but the insertion losses are markedly increase).Mechanical switch such as
16、S-POL provides 47 dB of isolationDual transmit system such as the CSU-CHILL does not use a switch and attains very low isolation(better than 45 dB).Drawback is increased cost and complexity.Dual-polarization OMT/FeedhornTwo examples of dual-polarized antennasAntenna(feedhorn,orthomode transducer OMT
17、,reflector)Beam Pattern Measurements CSU-CHILLZdr Calibration-Vertically Pointing RadarSystem/Antenna-ContinuedPossible to do a similar calibration by examining the statisticsof ZDR in a region of thunderstorm anvil-ice where little net orientation of the ice particles is expected(care must be taken
18、 if strong electric fields are present-these fields can and do orient the ice).Since fh,vare functions of-0and-0,it is clear that spatially inhomogeneous scatterers(e.g.,gradients across the beam)can produce antennapattern-related biases in ZDR-especially for poorly designed antennas!This is also tr
19、ue of other polarimetric variables such as LDR,and KdpMoral of the story-need a high quality antenna and need to know the characteristics of the antenna in great detail.Even with the best antenna,also need to apply caution when interpreting variables in the presence of certain bias-producing phenome
20、na(e.g.,strong reflectivity gradients;20 dB/km).Polarimetric Radar Data ProcessingElimination of non-hydrometeor radar echo(e.g.,ground clutter,anomalous propagation,clear air returns,non-meteorological targets)using polarimetric techniques.Apply simple threshold to the correlation coefficient(hv)Ap
21、ply simple threshold to the standard deviation of the differential phase(dp).Estimation of the specific differential phase(Kdp)Finite difference formula and standard deviation of Kdpgiven presence of measurement noise.Two techniques for reducing the effects of noiseFiltering or smoothing the range p
22、rofile of dpLinear regression fit to the range profile of dpElimination of non-hydrometeor radar echoStatement of the problem:For hydro-meteorological applications,it is desirable to isolate hydrometeors(i.e.,cloud and precipitation particles)from non-hydrometeors(e.g.,ground clutter and so-called“c
23、lear-air”returns,which is actually insects and sometimes birds).Non-polarimetric radar techniquesAnalyze elevation(or height)variation in echo structure.Problems with shallow systemsSubjectiveCreate a“clutter mask”by statistically characterizing ground clutter at a site using long periods of non-rai
24、ning data.Does not account for anomalous propagation.Doppler clutter filters typically eliminate radar echo with non-zero Doppler velocity and/or near-zero Doppler spectrum width.Works reasonably well but can eliminate precipitation echo.Wheres the ground clutter?Wheres the ground clutter?Eliminatio
25、n of non-hydrometeor echo:Polarimetric radar technique Applying hv thresholdElimination of non-hydrometeor echo:Polarimetric radar techniqueSimple Suppression of Ground Clutter Using Polarimetric Radar Techniques23 July 02 N-pol DZ(unedited)(2012 UTC)23 July 02 N-pol DZ(edited)(2012 UTC)lThresholds:
26、hv 0.7,(dp)18Summary for Non-hydrometeor Rejection by Polarimetric MethodsEstimation of Specific Differential Phase(Kdp)Range Filtering:Method#1Example of Differential Propagation Phase(dp)and Specific Differential Phase(Kdp)Estimation in rain at C-band(5.5 cm)Linear Regression:Method#2Standard Deviation of Kdp for method#2