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

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Doppler Radar From Josh Wurman NCAR S-POL DOPPLER RADAR

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Doppler Shift: A frequency shift that occurs in electromagnetic waves due to the motion of scatterers toward or away from the 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. Analogy: The Doppler shift for sound waves is the frequency shift that occurs as race cars approach and then recede from a stationary observer

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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 2

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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 Blue: Toward radar Red: Receding from radar

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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

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

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Clear-Air Radar

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VHF UHF 10 cm

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VHF UHF 10 cm

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Clear-Air Turbulence

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Clear-Air Wind Profilers

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Wind Profiler Specifications Frequency (MHz) Wavelength (m) Maximum Altitude (km) Antenna Size (m) TargetBand Designation 50620100 x 100Clear AirVHF 4490.751515 x 15Clear Air and Heavy Precipitation UHF 915~0.35-65 x 5Clear Air and Precipitation UHF 1036~0.35.5-65 x 5Clear Air and Precipitation UHF

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915 MHz 50 MHz

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Radial Velocity Positive Toward Surface

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Signal Power

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Bragg Diffraction (Scattering)

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NWS-NOAA Profiler Network

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SODAR

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ParameterPerformance Horizontal wind speed componentsRange 0-20 m/s, accuracy 0.2 m/s Horizontal wind speed vectorsRange 0 - 25 m/s Vertical wind speed componentsRange 0 - 10 m/sec, accuracy 0.1 m/s Horizontal wind Direction0 - 359 degrees Resolution of reading0.1 m/s Sampling Height50 m - 900 m AGL Environmental Conditions-10C to + 40C, 0 - 100% humidity Acoustic Frequency1525 - 2225 Hz (selectable) Performances of ARPL SODAR

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Radio-Acoustic Sounding (RASS)

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Beam Geometry

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Pulsed Radar

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Unambiguous Range

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Coherent Integration

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