profilers. wind profilers are phased array radars that measure the wind as a function of height...
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Profilers
Wind profilers are phased array radars that measure the windas a function of height above a fixed location.
Characteristics:
Wavelength: 33 cm to 6 mFrequency: 50 MHz to 900 MHzScattering elements: Variations in air moisture/density (best in moist turbulent air with no or uniform precip) (worst in dry stable air or variable precip)Wind detection: Doppler shift of frequency by moving
scatterers
1920s-70s Profilers were used for ionospheric researchLate 70s: First 50 MHz profiler tested for tropospheric researchMid 80s: Small network of 5 profilers deployed in Colorado1990-92: Demonstration network of profilers (blue) deployed in central US.Late 90s- Networks of profilers deployed by other agencies
Display of profiler data: A wind profile is generated every 6 minutesfrom the NOAA network. Data is often plotted every hour as below,But is available every six minutes (see COD site for best display)(http://weather.cod.edu/analysis/analysis.profiler.html)
Time reversed:Allows data to appear as it would in space for steady state passage of features
Cold frontal passagemarked by wind shift
Hourly wind profiles from Winchester, IL profiler
Colordenotesspeed
Noteboundary
Jetstream
Six minute wind profiles from Palestine, TX profiler
Note poor wind recovery
Phased array radar
A profiler with a radio acoustic sounding system (RASS)
RASS
Equipment cabinet
Close-up views
Upper arrayEast Beam
Lower arrayNorth BeamVertical Beam
Wavelengthdeterminedby array spacing
Each element of array transmits an electromagnetic pulse
The direction of the beam is determined by the lag between thetranmission time for the elements of the array
Vertical beam 75 degree beam
All elements transmit simultaneously
Longer cable lengthleads to lag in
transmission time across the array
Longer pulsesDifferent PRF
Nyquist velocity = 15 m/s
Nyquist velocity = 22.5 m/s
Radial velocity(not horizontal wind velocity!)
Radio Acoustic Sounding System
Horn that chirps
Antenna focusesSound chirp so thatIt propagates vertically
RASS - Calculation of virtual temperature
•The RASS unit sends out an acoustic (sound) wave in the vertical direction. •The propagation speed of the acoustic wave depends on the temperature and moisture composition of the atmosphere. The speed of the acoustic wave is related to the virtual temperature by Ca =
20.047(Tv1/2)
•The profiler then sends out it's own pulses which backscatter off of the RASS acoustic wave. It measures the Doppler shifted frequency of the acoustic wave (speed of sound), thus one can obtain Ca from the
Doppler shifted frequency. This is very similar to determining the radial velocity of precipitation targets with an 88-D by measuring the Doppler shifted frequency of the backscattered energy.
•Once Ca is known, then Tv = (Ca/20.047)2 .
•The RASS measurements of the vertical profile of Tv are generally
reported every hour, though it only takes a few minutes to obtain the measurements.
RASS data is generally only reliable in the boundary layer
Example of RASSvirtual temperatureretrieval
http://www.profiler.noaa.gov/jsp/profiler.jsp
RASS data
Profilers have sidelobes similar to traditional radars-- the intensity of the sidelobes related to the size of the array
The mechanism of scattering, Bragg scattering, depends on diffraction of electromagnetic waves caused by perturbations in the refractive index on a scale of half the wavelength of the radar
Water wave Diffraction pattern caused
by single object
Coherent wave reflection caused by objects spaced at half the wavelength of
impinging wave
Profilers measure winds in clear air
What causes variations in the refractive index at small scales?
Primarily variations in moisture (mixing ratio)
Do profilers sense precipitation (Rayleigh scattering)?
YES!
)sin()cos( eweuvre
Profiler equations: Clear air retrieval of winds
)sin()cos( ewevvrn wvup
)tan()sec( ewevu re )tan()sec( ewevv rn
Measured radial velocity
Solve for u and v components and winds
22 vuspeedwind
v
udirectionwind 1tan
Problem: When precipitation is occurring, vertical beam senses motion of precipitation (W = w + vt)
Consider two situations1: rain falling uniformly in all beams
Consider two situations2: rain falling only in tilted beamd
No rain Rain in all beams Rain in two slant beams
18.2 msvre10 msvrn10 msvup
110 msu10 msv
10 msw
10.2 msvre18.4 msvrn
15 msvup
110 msu10 msv
15 msw
10.2 msvre18.4 msvrn
10 msvup
11.7 msu11.17 msv
15 msw
Non-uniform rain contaminates wind recovery with profilers
Rain contamination
NOAA profilers use a 128-point fast-Fourier transform (FFT) to perform spectral analysis and determine velocity estimates.
Doppler spectrum
Doppler spectrum with noise floor removed and moments of spectrum (radial velocity and spectral width) estimated.
NOAA profilers are 404 MHz, the same frequency used by the SARSAT satellite that looks for signals from the “black box” of aircraft that have crash landed.
The profilers are required to shut down when the satellite can sense signals within an inhibit cone, as shown below
Each profiler is oriented differently so that the beams minimally interfere with satellite – net result:
NONE of the beams point north or east – if you use the raw data you must be aware of this and adjust your winds accordingly!!!
NOAA display with RASShttp://www.profiler.noaa.gov/jsp/profiler.jsp
RAP/NCAR profiler displayhttp://www.rap.ucar.edu/weather/upper/
College of DuPage Profiler Displayhttp://weather.cod.edu/analysis/analysis.profiler.html