《雷达新技术》PPT课件.ppt

1、Polarimetric Radar Meteorology,MOTIVATION: transitioning from conventional power-based measures of precipitation rate and coverage, to more accurate and complete dual-polarimetric estimates of precipitation types and amounts. SCOPE: polarimetric theory, radar design, data processing, physical interp

2、retation, algorithms. OBJECTIVE: 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,Examp

3、le of 3-D Beam Pattern,NCAR CP-2 X-band (Rinehart and Frush, 1983),More sidelobes.,The 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 Dopp

4、ler RadarWSR-88D (NEXRAD),Doppler Velocity Spectrum,Vr examples:,Tornadic Supercell Thunderstorm: May 29, 2001,如何识别降水类型？ 如何精确测量降水量？ - 极化雷达,Radar Waves, Polarization, and Scattering,Electromagnetic Spectrum Electromagnetic Waves Brief Mathematical Description Polarization Backscattering Matrix Covari

5、ance 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 Refrac

6、tive 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 index, and the dielectric factor (or how many ways can physicists say the same thing?),Refracti

7、ve index and K-values as a function of phase and temperature,Differential Propagation Phase, Define Propagation phase shift Differential propagation phase (dp) Specific Differential phase shift (Kdp) Examples of dpand Kdp Kdp from Rayleigh-Gans theory Dependence on Number concentration Shape Dielect

8、ric Wavelength. Relationship to liquid (rain) water content and drop diameter,Differential 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 sys

9、tems,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 calibrat

10、ion*,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

11、 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-pola

12、r 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 Agile Radar Systems in Linear (H: Horizontal, V: Vertical) Polarization Basis -Transmit,TRANSMI

13、T SIDE,Simplified Block Diagram of Polarization Agile Radar Systems in Linear (H: Horizontal, V: Vertical) Polarization Basis Receive,RECEIVE SIDE Assuming linear polarization basis and dual Receiver (e.g., S-pol, CHILL),Polarization or Waveguide Switch,To Switch or Not to Switch,Critical that switc

14、h 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 fluctuation

15、s. 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

16、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). Draw

17、back 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 calibr

18、ation 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 -0and -0, it is clear that spatial

19、ly 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 character

20、istics 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

21、(e.g., ground clutter, anomalous propagation, clear air returns, non-meteorological targets) using polarimetric techniques. Apply simple threshold to the correlation coefficient (hv) Apply simple threshold to the standard deviation of the differential phase (dp). Estimation of the specific different

22、ial 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 dp Linear regression fit to the range profile of dp,Elimination of non-hydrometeor radar echo,State

23、ment 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 techniqu

24、es 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