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

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Presentation on theme: "Introduction to Radar Meteorology Leyda V. León-Colón, PhD Electrical and Computer Engineering Department."— Presentation transcript:

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 ** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTOR LINEAR VERTICAL HORIZONTAL CIRCULAR Left Hand (LHC)-Counter Clockwise Right Hand (RHC) - Clockwise ELLIPTICAL LinearCircularElliptical


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

8 Targets

9 Dual Polarization in Weather Radars 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 Z HH Z VV Z HV Z VH Lineal Typical Horizontal Vertical

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

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

12 SPOL & XPOL NCARs SPOL Dual Polarized, Z H NOAs XPOL (transportable) Dual Polarized

13 CASA and TropiNet Radars vs. NEXRAD Dual polarized Doppler X-band WSR-88D: NEXRAD, all around the US Single Polarized, Doppler KOUN: NSSLS Dual polarized Prototype

14 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 Incident field due to transmitted radar pulse Backscattered electric field; contains both H and V components Here, subscripts are transmit, receive from the particle viewpoint Largest terms are co-polar (repeated subscript) matrix elements How are things done?

15 Some useful quantities that such a radar can measure are: Ratio of the H and V signal powers (Z DR ) Phase difference between the H and V returns ( DP ) Degree of correlation between the H and V returns ( HV ) Ratio of orthogonal to on channel signal power (LDR)

16 Inherent difference in Z dr characteristics of raindrops vs. hailstones

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

18 Differential Phase Φ DP vs. Specific Differential Phase K DP Differential Phase doesnt say anything by itself BUT ITS CHANGE OVER SPACE and TIME DOES!!!! RAIN Wet Ice

19 Negative K DP observed in thunderstorm anvil For vertically-oriented particles, S vv > S hh ; K DP negative


21 Numerator: Decreases when S hh and S vv are not uniformly correlated among the scatterers; (i.e., S vv is not always =.5 S hh for all scatterers in the pulse volume. When this uniformity does exist, HV goes to 1.0) Denominator: Normalizes the ratio into 0 to 1 range Factors that Reduce HV (Balakrishnan and Zrnic 1990): Radar pulse volume variations in the distribution of scatterer: 1.Shapes, 2.Sizes,3. magnitudes ( 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.) Co-polar H,V return signal correlation ( hv or co )

22 HV reduced in hail area: Mixed precip types; HV especially reduced when Z rain =Z ice Diverse shapes

23 Melting level / bright band readily recognized by local HV minimum. 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 HV values. (Enhanced Z is a few 100 m higher up) Blue contours are 20 and 40 dBZ

24 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 HV summary Typical Values

25 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) Linear Depolarization Ratio (LDR)

26 Red line ~upper LDR limit for rain Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than rain drops. 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 Tropical (ice-free) rain LDR observations: Upper LDR limit ~-24 to -25 dB.

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

28 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.

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

30 Hydrometeor classifications at 5.5 km MSL in a thunderstorm complex

31 CASA: June 10 th, 2007

32 HID after attenuation correction

33 Hail Event on March 23 rd, 2012 on SW Puerto Rico ENDI News Report Differential Hail Signal ( HDR ) Dependent on Z H and Polarimetric Variable Z DR Su, et al 2010, Bringi and Chandrasekar 2001

34 Detection on TropiNet: Cornelia HDR>10dB detect areas with hail High Z H collocates with HDR areas above 10dB

35 Reflectivity and HDR Movie

36 NO questions… Estoy saturada de Polarimetría!

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