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