Limiting Forms of Reflection and Scatter from a Surface

1. Limiting Forms of Reflection and Scatter from a Surface specular reflector quasi-specular reflector Lambert reflector quasi-Lambert reflector

3. Doppler Radar From Josh Wurman NCAR S-POL DOPPLER RADAR

4. Doppler Shift: A frequency shift that occurs in electromagnetic waves due to the motion of scatterers toward or away from the observer. Analogy: The Doppler shift for sound waves is the frequency shift that occurs as race cars approach and then recede from a stationary observer Doppler radar: A radar that can determine the frequency shift through measurement of the phase change that occurs in electromagnetic waves during a series of pulses.

5. The electric field of a transmitted wave The returned electric field at some later time back at the radar The time it took to travel Substituting: The received frequency can be determined by taking the time derivative if the quantity in parentheses and dividing by 2p

6. Sign conventions The Doppler frequency is negative (lower frequency, red shift) for objects receding from the radar The Doppler frequency is positive (higher frequency, blue shift) for objects approaching the radar These “color” shift conventions are typically also used on radar displays of Doppler velocity Red: Receding from radar Blue: Toward radar

7. Note that Doppler radars are only sensitive to the radial motion of objects Air motion is a three dimensional vector: A Doppler radar can only measure one of these three components – the motion along the beam toward or away from the radar

8. Magnitude of the Doppler Shift Transmitted Frequency X band C band S band 9.37 GHz 5.62 GHz 3.0 GHz Radial velocity 62.5 Hz 37.5 Hz 20.0 Hz 625 Hz 375 Hz 200 Hz 3125 Hz 1876 Hz 1000 Hz 1 m/s 10 m/s 50 m/s These frequency shifts are very small: for this reason, Doppler radars must employ very stable transmitters and receivers

9. Clear-Air Radar

10. UHF 10 cm VHF

11. 10 cm VHF UHF

12. Clear-Air Turbulence

13. Clear-Air Wind Profilers

14. Wind Profiler Specifications

15. 50 MHz 915 MHz

17. Radial Velocity Positive Toward Surface

18. Signal Power

19. Bragg Diffraction (Scattering)

20. NWS-NOAA Profiler Network

21. SODAR

22. Performances of ARPL SODAR SODAR

23. Radio-Acoustic Sounding (RASS)

24. Beam Geometry

25. Pulsed Radar

26. Unambiguous Range

27. Coherent Integration