Clear-Air Radar - Rutgers University

Clear-Air Radar - Rutgers University

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

Bragg Diffraction (Scattering) Pulsed Radar Doppler Radar From Josh Wurman NCAR S-POL DOPPLER RADAR

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.

The electric field of a transmitted wave Et t E0 cos 2pf t t 0 The returned electric field at some later time back at the radar Et t E1 cos 2pf t t t 1

The time it took to travel Substituting: 2r t c 2r

Et t E1 cos 2pf t t 1 c The received frequency can be determined by taking the time derivative if the quantity in parentheses and dividing by 2p

1 d 2 f dr 2f v 2r fr 2pf t t 1 f t t ft t r ft f d 2p dt c c dt

c 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 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 Clear-Air Radar

4 2 6 N 6 n

a ii i 1 10 cm UHF Cn2 1/3

VHF Clear-Air Turbulence Clear-Air Wind Profilers Wind Profiler Specifications Frequency (MHz)

Wavelength (m) Maximum Altitude (km) 20 15 Antenna

Size (m) 100 x 100 15 x 15 50 449 6 0.75

915 ~0.3 5-6 5x5 1036

~0.3 5.5-6 5x5 Target Clear Air Clear Air

and Heavy Precipitation Clear Air and Precipitation Clear Air and Precipitation Band

Designation VHF UHF UHF UHF 50 MHz 915 MHz Positive Toward

Surface Radial Velocity Signal Power NWS-NOAA Profiler Network Beam Geometry

Unambiguous Range Coherent Integration

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