1 / 29

Weather Signal Characteristics

WEATHER SIGNALS Chapter 4 (Focus is on weather signals or echoes from radar resolution volumes filled with countless discrete scatterers---rain, insects, perturbations in atmospheric refractive index, etc.). Weather Signal Characteristics. Large dynamic range (100 million!)

acoffin
Download Presentation

Weather Signal Characteristics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. WEATHER SIGNALSChapter 4(Focus is on weather signals or echoes from radar resolution volumes filled with countless discrete scatterers---rain, insects, perturbations in atmospheric refractive index, etc.) METR 5004

  2. Weather Signal Characteristics • Large dynamic range (100 million!) • Signals are semi coherent • Numerous scatterers in the radar resolution volume The choice instrument for weather surveillance is the pulsed polarimetric Doppler weather radar. Extracting information (i.e., fields of echo H, V power, Doppler velocity, and correlation of H, V echoes) involves processing of echoes that randomly fluctuate. METR 5004

  3. Echoes (I or Q) from Distributed Scatterers (Fig. 4.1) c(s)≈ t (t = transmitted pulse width) mTs Weather signals (echoes) METR 5004

  4. Resolution Volumes Range resolution 150 m Lightning detector METR 5004

  5. Echo samples from 16 Resolution Volumes (Fig. 4.3) Ts = 768 ms

  6. Gate 12 Signal Spectrum (Fig. 8.34) MTs = 128x0.768 ms = 98.3 ms 12 spectra are averaged METR 5004

  7. Repeat of Fig. 4.3 randomness c(mTs) ≈ 2-4 ms

  8. Statistics of Weather Signals I and Q are uncorrelated zero mean random variables with Gaussian probability density function (pdf) Amplitude |V| has a Rayleigh pdf P has an exponential pdf β has a uniform pdf β Thanks to Dr. Sebastian Torres

  9. Weather Echo Statistics (Fig. 4.4) METR 5004

  10. Weighting Functions for Scatterers and the Weather Radar Equation (4.12) METR 5004

  11. Drop Size Distributions (Fig. 8.3b) (1948) (1943) (From drop sizes between 1 and 3 mm) METR 5004

  12. The Angular Weighting Function METR 5004

  13. The Measured Range Weighting Function for two Receiver Bandwidths (Fig. 4.6) METR 5004

  14. Range Weighting Function for Echoes Samples at Range Time τs(Fig.4.7) 250 m METR 5004

  15. The Resolution Volume V6 Range weighting function Fig. 5.11 Angular weighting function METR 5004

  16. Spectrum of a transmitted rectangular pulse ftf If receiver frequency response is matched to the spectrum of the transmitted pulse (an ideal matched filter receiver), some echo power will be lost. This is called the finite bandwidth receiver loss Lr. For and ideal matched filter Lr = 1.8 dB. METR 5004

  17. Receiver Loss Factor (Fig. 4.8) for a Gaussian receiver response and rectangular pulse(in general a matched condition is when B6 =1) Lr = 1.8 dB for an ideal matched receiver B6= 6 dB bandwidth of the receiver’s frequency response.  = transmitted pulse width Eq.(4.28) METR 5004

  18. Reflectivity Factor Z(Spherical scatterers; Rayleigh condition: D≤ λ/16) METR 5004

  19. Reflectivity Factor of Spheroids (Horizontally Polarized Waves) S K I P ? p[De,e,ξ] = probability density σh[De,e,ξ] = backscatter cross section for H pol. De = equivalent volume diameter e = eccentricity of the spheroid scatterer ξ = angle between the symmetryaxis and the electric field direction N0 = the number density per unit diameter (m-4) METR 5004

  20. Differential Reflectivity in dB units: ZDR(dB) = Zh(dBZ) - Zv(dBZ) in linear units: Zdr = Zh(mm6m-3)/Zv(mm6m-3) - is independent of drop concentration N0 - depends on the shape of scatterers METR 5004

  21. Shapes of raindrops falling in still air and experiencing drag force deformation. De is the equivalent diameter of a spherical drop.ZDR (dB) is the differential reflectivity in decibels (Rayleigh condition is assumed). Adapted from Pruppacher and Beard (1970) METR 5004

  22. The Weather Radar Equation METR 5004

  23. Acquisition, Processing and Display of Weather radar data Volume Coverage Pattern Azimuthal Scan (constant elevation) Product displays (e.g., CAPPI, etc.) 1 2 Base Data: Ph, Pv,v, SW, etc. M Range-time s Time series of M power samples Data radial s (I2 + Q2) Radial of reflectivity factor Z Sample-time Range r = cs/2 (M power samples are processed to produce one Z estimate at r)

  24. WSR-88D Thresholding Data Fieldsbased on Signal to Noise Ratios Locations with non-significant powers are censored: -Non significant returns have a SNR below a user-defined threshold -The system allows a different threshold for each spectral moment METR 5004

  25. Z (dBZ) SNR> -3dB

  26. Z (dBZ) SNR> 2dB

  27. SNR> -0.5 dB

  28. SNR> 3.5 dB

  29. Weather Echo Power vs Range (WSR-88D) Pr=Pn≈6x10-15 Watts METR 5004

More Related