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1、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 inte
2、rpretation, algorithms. OBJECTIVE: through “hands-on” approach with data from research radars above, learn latest methods for quantifying precipitation types and mounts.,2,3,Polarimetric Rain Rate Estimation,4,Polarimetric Radar Rain Rates vs. Rain Gauges,5,Simplified Block Diagram Doppler Radar,6,A
3、ntenna,7,Example of 3-D Beam Pattern,NCAR CP-2 X-band (Rinehart and Frush, 1983),8,More sidelobes.,9,The Radar Equation,10,11,Refractive index and K-values as a function of phase and temperature,12,Water to Ice Transition in Tropical Convection,Transition across melt level is 5-10 dBZ, as predicted
4、by theory,13,Conventional Doppler RadarWSR-88D (NEXRAD),14,Doppler Velocity Spectrum,15,Vr examples:,Tornadic Supercell Thunderstorm: May 29, 2001,16,如何识别降水类型？ 如何精确测量降水量？ - 极化雷达,17,Radar Waves, Polarization, and Scattering,Electromagnetic Spectrum Electromagnetic Waves Brief Mathematical Description
5、 Polarization Backscattering Matrix Covariance Matrix Radar observables,18,Electromagnetic Spectrum,19,Electromagnetic Waves: Spectrum,20,Electromagnetic Waves,21,Scattering and the Backscattering Matrix,22,What are we measuring?,23,24,25,Polarization, Dielectric, Refractive Index,Recall differences
6、 in returned power for ice and water Polarization of matter Refractive Index Relationship to Dielectric (or relative permittivity),26,Recall Differences Between Ice and Water,27,Recall Differences Between Ice and Water,28,Polarization,29,Relating the dielectric constant, refractive index, and the di
7、electric factor (or how many ways can physicists say the same thing?),30,Refractive index and K-values as a function of phase and temperature,31,Differential Propagation Phase, Define Propagation phase shift Differential propagation phase (dp) Specific Differential phase shift (Kdp) Examples of dpan
8、d Kdp Kdp from Rayleigh-Gans theory Dependence on Number concentration Shape Dielectric Wavelength. Relationship to liquid (rain) water content and drop diameter,32,Differential Propagation Phase (DP) and Specific Differential Phase (KDP),33,Phase Cont.,34,Example of Differential Propagation Phase (
9、dp) and Specific Differential Phase (Kdp) in rain at C-band (5.5 cm),35,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 Swi
10、tch OMT/Feedhorn Dish Antenna Requirements and effects on polarization measurements Zdr calibration*,36,Introduction: Simplified Block Diagram of a Common Polarization Radar System,37,Dual-polarization radar system types,There are two general system types Polarization agility: Ability to change the
11、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.
12、 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.,38,Polarization Agility Transmitted Waveform Schematic,39,Simplified Block Diagram of
13、Polarization Agile Radar Systems in Linear (H: Horizontal, V: Vertical) Polarization Basis -Transmit,TRANSMIT SIDE,40,Simplified Block Diagram of Polarization Agile Radar Systems in Linear (H: Horizontal, V: Vertical) Polarization Basis Receive,RECEIVE SIDE Assuming linear polarization basis and dua
14、l Receiver (e.g., S-pol, CHILL),41,Polarization or Waveguide Switch,42,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 lo
15、ss, 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 isolatio
16、n 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
17、 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.,43,Dual-polarization OMT/Feedhorn,44,Two examples of dual-polarized antennas,45,Antenna(feedhorn, orthomode transducer OMT, reflector),46,Beam P
18、attern Measurements CSU-CHILL,47,48,Zdr Calibration -Vertically Pointing Radar,49,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
19、 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 al
20、so 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-producin
21、g phenomena (e.g., strong reflectivity gradients; 20 dB/km).,50,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 corre
22、lation coefficient (hv) Apply 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 noise Fi
23、ltering or smoothing the range profile of dp Linear regression fit to the range profile of dp,51,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-hydro
24、meteors (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 characterizi
25、ng 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.,5
26、2,Wheres the ground clutter?,53,Wheres the ground clutter?,54,Elimination of non-hydrometeor echo: Polarimetric radar technique,55,Applying hv threshold,56,Elimination of non-hydrometeor echo: Polarimetric radar technique,57,Simple Suppression of Ground Clutter Using Polarimetric Radar Techniques,58
27、,23 July 02 N-pol DZ (unedited)(2012 UTC),59,23 July 02 N-pol DZ (edited)(2012 UTC),Thresholds: hv 0.7, (dp) 18,60,Summary for Non-hydrometeor Rejection by Polarimetric Methods,61,Estimation of Specific Differential Phase (Kdp),62,63,Range Filtering: Method #1,64,Example of Differential Propagation Phase (dp)and Specific Differential Phase (Kdp) Estimation in rain at C-band (5.5 cm),65,Linear Regression: Method #2,66,Standard Deviation of Kdp for method #2,