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Introduction to Radar Meteorology

Introduction to Radar Meteorology. Leyda V. León-Colón, PhD Electrical and Computer Engineering Department. Types of radar. Ground Based Airborne Based Satellite Based Mobile. Radar Equation and Radar Reflectivity. Clear Air. Electromagnetic Waves and Polarization. Linear. Circular.

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Introduction to Radar Meteorology

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  1. Introduction to Radar Meteorology Leyda V. León-Colón, PhD Electrical and Computer Engineering Department

  2. Types of radar • Ground Based • Airborne Based • Satellite Based • Mobile

  3. Radar Equation and Radar Reflectivity

  4. Clear Air

  5. Electromagnetic Waves and Polarization Linear Circular Elliptical ** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTOR • LINEAR • VERTICAL • HORIZONTAL • CIRCULAR • Left Hand (LHC)-Counter Clockwise • Right Hand (RHC) - Clockwise • ELLIPTICAL

  6. Weather Radars: Why is it important? MECHANISM… HOW? Linear Tx Horizontal Rx Vertical Or any combination ZHH, ZHV,ZVV, ZVH Circular For Spheres: Tx RHC & Rx LHC For Irregular: Tx and Rx same power, i.e. Police Radars CDR: Circular Depolarization Ratio • (Tx) Transmit Power • (S) Power is Scattered over its path • (Rx) Scattered Power towards radar is measured

  7. Targets

  8. Dual Polarization in Weather Radars • Lineal Typical • Horizontal • Vertical • Dual polarization radars can estimate several return signal properties beyond those available from conventional, single polarization Doppler systems. • Hydrometeors: Shape, Direction, Behavior, Type, etc… • Events: Development, identification, extinction ZHH ZVV ZHV ZVH

  9. CSU-CHILL Radar V port H port Towards reflector Dual Polarized Doppler S-band

  10. CP2 Radar • Located at Brisbane, Australia • Single Polarized Doppler X-band • Dual polarized Doppler S-band

  11. SPOL & XPOL • NCAR’s SPOL • Dual Polarized, ZH • NOA’s XPOL (transportable) • Dual Polarized

  12. CASA and TropiNet Radars vs. NEXRAD WSR-88D: NEXRAD, all around the US Single Polarized, Doppler KOUN: NSSL’S Dual polarized Prototype • Dual polarized Doppler X-band

  13. Backscattered electric field from an individual scatterer is described by the scattering matrix. “S” values are complex numbers that depend on the scatterer shape, orientation and dielectric constant How are things done? Here, subscripts are transmit, receive from the particle viewpoint Largest terms are “co-polar” (repeated subscript) matrix elements Incident field due to transmitted radar pulse Backscattered electric field; contains both H and V components

  14. Some useful quantities that such a radar can measure are: • Ratio of the H and V signal powers (ZDR) • Phase difference between the H and V returns (fDP) • Degree of correlation between the H and V returns (rHV) • Ratio of orthogonal to “on channel” signal power (LDR)

  15. Inherent difference in Zdr characteristics of raindrops vs. hailstones

  16. Zdr observations in rain and hail Hail (~random orientation) dominates Z-weighted mean axis ratio: Zdr decreases to ~0 dB

  17. Differential Phase ΦDP vs. Specific Differential Phase KDP • Differential Phase doesn’t say anything by itself • BUT ITS CHANGE OVER SPACE and TIME DOES!!!! Wet Ice RAIN

  18. Negative KDPobserved in thunderstorm anvil For vertically-oriented particles, Svv> Shh; KDPnegative

  19. Dual-polarized Radars RAIN DP-based methods: Simple Attenuation Correction

  20. Co-polar H,V return signal correlation (rhv or rco) Numerator: Decreases when Shh and Svv are not uniformly correlated among the scatterers; (i.e., Svv is not always = .5 Shh for all scatterers in the pulse volume. When this uniformity does exist, rHVgoes to 1.0) Denominator: Normalizes the ratio into 0 to 1 range Factors that Reduce rHV (Balakrishnan and Zrnic 1990): Radar pulse volume variations in the distribution of scatterer: 1.Shapes, 2.Sizes, 3. d magnitudes (d is Mie-related differential phase shift on scattering) 4. canting angles 5. hydrometeor types (example: both liquid and frozen present) 6. hydrometeor shape irregularities (some rough aggregates, etc.)

  21. rHVreduced in hail area: Mixed precip types; rHVespecially reduced when Zrain=Zice Diverse shapes

  22. Melting level / bright band readily recognized by local rHVminimum. Reflectivity maximizes as frozen particles initially develop an outer water coating. With further descent / warming, smaller particles completely melt. Mixed frozen and completely melted layer gives lowest rHVvalues. (Enhanced Z is a few 100 m higher up) Blue contours are 20 and 40 dBZ

  23. rHV summary Typical Values Primarily useful to characterize variability of scatterer characteristics within the pulse volume. Drizzle / light rain > ~0.98 Convective (but no ice) rain > ~0.96 Hail / rain mixtures ~0.90 Bright band mixed rain and snow ~0.75 Tornado debris ~0.50 or less

  24. Linear Depolarization Ratio (LDR) • Is the ratio of the cross-polar to co-polar backscattered signal powers. Here the HV subscripts represent the receive and transmit polarizations respectively. • For cloud and precipitation targets, the cross polar signal level is typically only 10-2 – 10-3 of the co-polar level (LDR~ -20 to -30 dB)

  25. Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than rain drops. Red line ~upper LDR limit for rain Tropical” (ice-free) rain LDR observations: Upper LDR limit ~-24 to -25 dB. Note small LDR magnitudes. Snow LDR of -30 dB implies that cross polar signal from 30 dB snow echo is 0 dB. Noise can bias / obliterate such weak cross polar channel signals

  26. As with rHV, LDR maximizes in the melting level region where wet, non-spherical, gyrating ice particles exist.

  27. Hail areas present variable LDR levels. In this storm, the dBZ core area is characterized by LDR levels that are virtually all below -22 dB. Note also how LDR increases in clutter, noise, and many echo edge areas.

  28. Radar data values are used to develop a numerical score for each designated particle type. Identification is based on the highest-scoring type. Hydrometeor identification (HID)

  29. Hydrometeor classifications at 5.5 km MSL in a thunderstorm complex

  30. CASA:June 10th, 2007 PPI at 12.25 in elevation Cyril Zh(X) Corr. for Rain HID

  31. HID after attenuation correction DP-based SRT-modified

  32. Su, et al 2010, Bringi and Chandrasekar 2001 Hail Event on March 23rd, 2012 on SW Puerto Rico • ENDI News Report • Differential Hail Signal (HDR) • Dependent on ZHand Polarimetric Variable ZDR

  33. Detection on TropiNet: Cornelia HDR>10dB detect areas with hail High ZH collocates with HDR areas above 10dB

  34. Reflectivity and HDR Movie

  35. NO questions… Estoysaturada de Polarimetría!

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