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METR 5004 A SERIES OF SHORT COURSES ON THE FUNDAMENTALS OF ATMOSPHERIC SCIENCE

METR 5004 A SERIES OF SHORT COURSES ON THE FUNDAMENTALS OF ATMOSPHERIC SCIENCE. THIS SHORT COURSE IS ON: BASICS OF POLARIMETRIC-DOPPLER RADAR AND WEATHER OBSERVATIONS Dr. Dick Doviak, NSSL/NOAA & THE UNIVERSITY OF OKLAHOMA Norman, Oklahoma. For theory and more information

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METR 5004 A SERIES OF SHORT COURSES ON THE FUNDAMENTALS OF ATMOSPHERIC SCIENCE

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  1. METR 5004A SERIES OF SHORT COURSES ON THE FUNDAMENTALS OF ATMOSPHERIC SCIENCE THIS SHORT COURSE IS ON:BASICS OF POLARIMETRIC-DOPPLER RADAR ANDWEATHER OBSERVATIONS Dr. Dick Doviak, NSSL/NOAA & THE UNIVERSITY OF OKLAHOMA Norman, Oklahoma METR 5004

  2. For theory and more information on weather radar: ACADEMIC PRESS, 2nd edition, 3rd & 4thPrts. or DOVER PUBLICATIONS INC June 2006 (Book has been translated into Russian and Chinese) Book errata and supplements at: www.nssl.noaa.gov/papers/books.html Questions? Dick.doviak@noaa.gov or Office at NWS Rm 4915 325-6587 METR 5004

  3. Radar Radio detecting and ranging of objects (Taylor and Furth US Navy 1940) Applications: Remote sensing (Air, Sea, and Land) Tracking of objects (aircraft, missiles, speeding cars, etc.) Astronomy (both active and passive, Doppler measurements) Medical, imaging objects in the ground, etc. Radar Meteorology exposes one to: The fundamental aspects of remote sensing using electromagnetic waves Random processes (fundamental to weather radar measurements) METR 5004

  4. Early development of Radar 1900: Tesla; Published the concept of radar. 1904: 1st demonstration of radio waves (continuous waves) to detect an object. 1925: 1st successful use of pulsed radio waves or RADAR to detect an object (i.e., an atmospheric layer 150 km AGL) by G. Breit and M. A. Tuve, Dept. of Terrestrial magnetism, Carnegie Institution Late 1930s and early 1940s: Explosive growth of radar for detecting and ranging aircraft. METR 5004

  5. The Spectrum of Electromagnetic Waves Microwave ovens First radar Weather radar Radio waves METR 5004

  6. Importance of Weather Radars • Electromagnetic waves can penetrate clouds and rain regions and thus reveal meteorological features inside storms! • Weather radars can provide quantitative and automated real-time information on storms, precipitation, hurricanes, tornadoes, etc. METR 5004

  7. Properties of Electromagnetic WavesChapter 2 METR 5004

  8. The Electric Field Equation (2.2b) Alternately: Wavenumber k = 2πf/c = 2π/λ Linear Polarization: E→Ex(orEy) = Ix+jQx Elliptical Polarization: Transmit both Exand Ey METR 5004

  9. Polarization(Fig. 2.2) METR 5004

  10. Dual-polarization Radar S K I P ? Y, or V X, or H Dual polarized waves METR 5004

  11. Vertically and Horizontally Polarized Waves Vertically polarized waves ( ): E vector lies in the verticalplane, but it has both a vertical and horizontal component!) Horizontally polarized waves ( ) E vector lies in the horizontal plane! Hydrometeor Properties: Electrical size Apparent canting angle Canting angle dispersion Eccentricity (shape) Circular polarization provides relatively simple formulas to measure directly these properties. But depolarization during propagation mitigates any advantage of circular polarization. (Doviak et al., JTECH 2000) METR 5004

  12. A Complex plane (Phasor diagram) Complex Numbers • Weather signal voltages (i.e., echoes) V are a field of complex numbers of the form V = I+jQ, where Iand Q are real numbers and j is the imaginary unit: • Component notation: • V = (I, Q) • I is the real or In-phase part, I = Re{V} • Q is the imaginary or Quadraturepart, Q = Im{V} • Polar notation: V= A(cosβ+ jsinβ) • Using Euler’s relation: V = Aexp[ j β] • Ais the amplitude: • βis the argument or phase: jQ(t) β I(t) METR 5004

  13. For typical atmospheric conditions (i.e., normal) the propagation path is a straight line if the earth has a radius 4/3rds times its true radius. Normal and Anomalous Propagation Sub refraction Free space Normal atmosphere (Rc≈ constant) Super refraction (anomalous propagation: Unusually cold moist air near the ground) METR 5004

  14. AP at KOUN (Norman, OK)Sept 09, 2004 - 1439 UTC Without GCF METR 5004

  15. AP at KOUN (Norman, OK) Sept 09, 2004 - 1439 UTC With GCF everywhere METR 5004

  16. Some improvements in weather warnings and examples of meteorological phenomena observed with Radar METR 5004

  17. (Thanks to Don Burgess of NSSL) METR 5004

  18. Evolution of the Boundary Layer (June 25, 1970) Range Arcs = 9.3 km λ=10 cm Wallops Is.,VA (Fig. 11.24) 7:36 am H = 6.1 km 12:53 am 10:23 am METR 5004

  19. Cirrus Cloud and Solar Emission Detected with the WSR-88D S K I P ? “Sun spike”: Solar emission detected with the WSR-88D METR 5004

  20. Wave Approaching Radar (~10 am) METR 5004

  21. Vr of the Undular Bore METR 5004

  22. Columbia’s debris field and other artifacts Seen with a WSR-88D weather radar near Shreveport, LA METR 5004

  23. Polarimetric-Doppler radar and its Environment (Chapter Three) METR 5004

  24. Doppler Radar (Fig. 3.1)A simplified block diagram METR 5004

  25. The WSR-88D Antenna METR 5004

  26. Radiation source (feed) for polarimetric parabolic reflector antenna Feed support spars H Radiation Source (feed horn) V METR 5004

  27. Wave Fronts-Surfaces of Constant Phase ψ(field near the antenna; broadside PA radiation) Parabolic Reflector Planar Phased Array . . . . . . . . Radiating element . . (Spherical wave front) Surfaces of constant phase (propagate at speed of light) Radiation source V H V H c = 3x108 m s-1 (SHV vs AHV) - 0 + Huygens Principle: “Each radiating element (or each point of a wave front) can be considered as the source of a secondary wave. The secondary waves then combine to form a new wave front, the new wave front being the envelope of the secondary waves”. METR 5004

  28. Angular Beam Formation(the transition from a circular beam of constant diameter to an angular beam of constant angular width) Fresnel zone Far field region METR 5004

  29. Doppler Radar (Fig. 3.1)A simplified block diagram Discuss beam characteristics Discuss beam width and sidelobes METR 5004

  30. Comparison of Theoretical and Measured Copolar One-way Horizontally Polarized Radiation Patterns for a WSR-88D (KOUN) θ1/2→ -3 dB level Theoretical Measured envelope of sidelobes Measurements from KOUN pattern data Power density below peak (dB) One-way Half-power beamwidth θ1=1.27λ/D (rad.) Half power (dB) = 10 log10 (1/2) = -3dB METR 5004

  31. Effects of WSR-88D Sidelobes on Radar Data (similar to Color plate 2b and Fig. 9.22) METR 5004

  32. Antenna (directive) Gaingt The defining equation: Eq. (3.4) (W m-2) = Incident power density r = range to measurement = radiation pattern = 1 on beam axis = transmitted power (W) METR 5004

  33. Doppler Radar (Fig. 3.1)A simplified block diagram Discuss the rf pulse METR 5004

  34. r=cτs/2 cτ λ Pulsed Radar Principle c = speed of microwaves = ch for H and = cv for V waves τ = pulse length λ = wavelength = λh for H and λv for V waves τs = time delay between transmission of a pulse and reception of an echo. METR 5004

  35. Doppler Radars • The Doppler effect (Austrian physicist, Christian Johann Doppler,1842) is the apparent change in frequency of a wave that is perceived by an observer moving relative to the source of the waves • Doppler radars use thisphenomenon to measurethe radial component of the velocity vector (towardor away from the radar) • Note that the radar always measures a velocity that isless than or equal to the true target velocity! METR 5004

  36. Propagation and backscattering by non spherical precipitation particles a b S K I P ? Spheroidal approximation METR 5004

  37. Wavenumber Phase of a propagating wave: ωt - kr Wavenumber: k = 2π/λ (i.e., k≡ phase shift per unit length) In vacuum: λ = c/f In rain: λr = cr/f λr < λ therefore kr> k In rain having oblate spheroidal shaped drops: λh= ch/f for H polarized waves λv= cv/f for V polarized waves METR 5004

  38. Wavenumbers for H, V Waves Horizontal polarization: Vertical polarization: where k = free space wavenumber = 3.6x106 (deg./km) (e.g., for R= 100 mm h-1, = 24.4okm-1, = 20.7okm-1) Therefore: ch< cv ; λh < λv ; kh>kv Specific differential phase: (for R = 100 mm h-1) (an important polarimetric variable related to rainrate) METR 5004

  39. Polarimetric Variables • Propagation - forward scattering • Kh and Kv - specific attenuations • Kdif- specific differential attenuation • ΦDP - differential phase • KDP - specific differential phase METR 5004

  40. Specific Differential Phase (Fig.6.17) Phase of H = h = 2khr Eq. (6.60) METR 5004

  41. H V time ΦDP H V time ΦDP Differential phase ΦDP ΦDP is not affected by radar mis-calibration, attenuation, and partial beam blockage METR 5004

  42. Backscattering Cross Section σb(Echoes from a single discrete scatterer) METR 5004

  43. Backscattering Cross Section, σbfor a Spherical Particle METR 5004

  44. Cross Section vs Diameter (Fig. 3.3) water ice S K I P ? METR 5004

  45. Backscattered Power Density Incident on Receiving Antenna METR 5004

  46. Echo Power PrReceived (3.20) Aeis the effective area of the receiving antenna for radiation from the θ,φ direction. It is shown that: (3.21) If the transmitting antenna is the same as the receiving antenna then: METR 5004

  47. The Radar Equation(point scatterer/discrete object) METR 5004

  48. Echo Power from Point Scatterers S K I P ? METR 5004

  49. Atmospheric Attenuation (Fig. 3.6) METR 5004

  50. Attenuation vs Rain Rate (Fig. 3.5) METR 5004

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