《雷达新技术》ppt课件

1、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 interpretation, algorithms.OBJECTIVE:

2、through “hands-on approach with data from research radars above, learn latest methods for quantifying precipitation types and mounts.lNCAR CP-2 X-band (Rinehart and Frush, 1983) Transition across melt level is 5-10 dBZ, as predicted by theorylTornadic Supercell Thunderstorm: May 29, 2001l如何识别降水类型？l如

3、何准确丈量降水量？l- 极化雷达Electromagnetic SpectrumElectromagnetic Waves Brief Mathematical DescriptionPolarizationBackscattering MatrixCovariance MatrixRadar observablesRecall differences in returned power for ice and waterPolarization of matterRefractive IndexRelationship to Dielectric (or relative permittiv

4、ity) 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 diameterIntroduction Po

5、larization Radar SystemPolarization agility vs. polarization diversityPolarization agile systemTransmit Block DiagramReceive Block DiagramCritical antenna componentsWaveguide SwitchOMT/FeedhornDishAntenna Requirements and effects on polarization measurementsZdr calibration*There are two general syst

6、em 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 alternate orthogonal polarizations, but

7、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.lTRANSMIT SIDElRECEIVE SIDElAssuming

8、linear polarization basis and dual Receiver (e.g., S-pol, CHILL)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 betwee

9、n 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 hydr

10、ometeor 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 isolationDual transmit system such as the CSU-CHI

11、LL does not use a switch and attains very low isolation (better than 45 dB). Drawback is increased cost and complexity. lPossible 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 mu

12、st be taken if strong electric fields are present-these fields can and do orient the ice).lSince 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 antenn

13、as! This is also true of other polarimetric variables such as LDR, and KdplMoral 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

14、 bias-producing phenomena (e.g., strong reflectivity gradients; 20 dB/km).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 (hv)App

15、ly 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

16、profile of dpLinear regression fit to the range profile of dpStatement 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 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-rain