第九章雷达新技术1ppt课件.ppt

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 interpretation, algorithms.lOBJECTIVE: through “hands-on” approach with data from research radars above, learn latest methods for quantifying precipitation types and mounts...Polarimetric Rain Rate Estimation.Polarimetric Radar Rain Rates vs. Rain Gauges.Simplified Block Diagram Doppler Radar .Antenna .Example of 3-D Beam PatternlNCAR CP-2 X-band (Rinehart and Frush, 1983).More sidelobes……..The Radar EquationThe Radar Equation ..Refractive index and K-values as a function of phase and temperature.Water to Ice Transition in Tropical Convection Transition across melt level is ~ 5-10 dBZ, as predicted by theory.Conventional Doppler RadarWSR-88D (NEXRAD).Doppler Velocity Spectrum.Vr examples:lTornadic Supercell Thunderstorm: May 29, 2019. 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 Scattering•Electromagnetic Spectrum•Electromagnetic Waves –Brief Mathematical Description•Polarization•Backscattering Matrix•Covariance Matrix•Radar observables.Electromagnetic Spectrum.Electromagnetic Waves: Spectrum.Electromagnetic Waves .Scattering and the Backscattering Matrix .What are we measuring?...Polarization, Dielectric, Refractive Index•Recall differences in returned power for ice and water•Polarization of matter•Refractive Index–Relationship to Dielectric (or relative permittivity).Recall Differences Between Ice and Water.Recall Differences Between Ice and Water.Polarization.Relating the dielectric constant, refractive Relating the dielectric constant, refractive index, and the dielectric factorindex, and the dielectric factor ( (or how many ways or how many ways can physicists say the same thing?)can physicists say the same thing?).Refractive index and K-values as a function of phase and temperature.Differential Propagation Phase• Define –Propagation phase shift –Differential propagation phase (φdp) –Specific Differential phase shift (Kdp)•Examples of φdpand Kdp•Kdp from Rayleigh-Gans theory–Dependence on •Number concentration•Shape •Dielectric •Wavelength. –Relationship to liquid (rain) water content and drop diameter.Differential Propagation Phase (ΦDP) and Specific Differential Phase (KDP).Phase Cont………..Example of Differential Propagation Phase (φdp) and Example of Differential Propagation Phase (φdp) and Specific Differential Phase (Kdp) in rain at C-band (5.5 Specific Differential Phase (Kdp) in rain at C-band (5.5 cm)cm).Dual-polarized radar systems•Introduction Polarization Radar System•Polarization agility vs. polarization diversity•Polarization agile system–Transmit Block Diagram–Receive Block Diagram•Critical antenna components–Waveguide Switch–OMT/Feedhorn–Dish•Antenna Requirements and effects on polarization measurements–Zdr calibration*.Introduction: Simplified Block Diagram of a Common Polarization Radar System.Dual-polarization radar –system types•There are two general system types–Polarization 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 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 systems..Polarization Agility –Transmitted Waveform Schematic .Simplified Block Diagram of Polarization Simplified Block Diagram of Polarization Agile Radar Systems in Linear (Agile Radar Systems in Linear (H: Horizontal, H: Horizontal, V: VerticalV: Vertical) Polarization Basis -Transmit) Polarization Basis -Transmit lTRANSMIT SIDE.Simplified Block Diagram of Polarization Agile Simplified Block Diagram of Polarization Agile Radar Systems in Linear (H: Horizontal, V: Vertical) Radar Systems in Linear (H: Horizontal, V: Vertical) Polarization Basis –ReceivePolarization Basis –ReceivelRECEIVE SIDEAssuming linear polarization basis and dual Receiver (e.g., S-pol, CHILL).Polarization or Waveguide Switch.To Switch or Not to Switch•Critical that switch isolate the H and V transmit/receive powers. –Ferrite switches are not as robust, in this regard, as rotary switches. –Further, Ferrite 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 lower 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 S-POL provides 47 dB of isolation–Dual 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/Feedhorn.Two examples of dual-polarized antennas.Antenna(feedhorn, orthomode transducer [OMT], reflector).Beam Pattern Measurements –CSU-CHILL..Zdr Calibration -Vertically Pointing Radar.System/Antenna-Continued•Possible 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 if strong electric fields are present-these fields can and do orient the ice).•Since fh,vare functions of θ-θ0and Φ-Φ0, 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 true of other polarimetric variables such as LDR, and Kdp•Moral 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 phenomena (e.g., strong reflectivity gradients; >20 dB/km)..Polarimetric Radar Data Processing•Elimination 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 (ρhv)–Apply simple threshold to the standard deviation of the differential phase [σ(Ψdp)].•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 noise•Filtering or smoothing the range profile of Ψdp•Linear regression fit to the range profile of Ψdp.Elimination of non-hydrometeor radar echo•Statement 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 “clear-air” returns, which is actually insects and sometimes birds).•Non-polarimetric radar techniques–Analyze elevation (or height) variation in echo structure.•Problems with shallow systems•Subjective–Create a “clutter mask” by statistically characterizing ground clutter at a site using long periods of non-raining 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..Where’s the ground clutter?.Where’s the ground clutter?.Elimination of non-hydrometeor echo: Polarimetric radar technique .Applying ρhv threshold.Elimination of non-hydrometeor echo: Polarimetric radar technique.Simple Suppression of Ground Clutter Using Polarimetric Radar Techniques.23 July 02 N-pol DZ (unedited)(2019 UTC).23 July 02 N-pol DZ (edited)(2019 UTC)lThresholds: ρhv> 0.7, σ(Ψdp) < 18°.Summary for Non-hydrometeor Rejection by Polarimetric Methods.Estimation of Specific Differential Phase (Kdp)..Range Filtering: Method #1.Example of Differential Propagation Phase Example of Differential Propagation Phase (φdp)and Specific Differential Phase (Kdp) (φdp)and Specific Differential Phase (Kdp) Estimation in rain at C-band (5.5 cm)Estimation in rain at C-band (5.5 cm).Linear Regression: Method #2.Standard Deviation of Kdp for method #2 .。