at various elevations above the ground. Readings are made at each kilometer above sea level, up to the extent of the troposphere (up to the limit of the earths atmosphere, or 17 kilometers above mean sea level). Above this level there is inadequate water vapor present to produce a radar "bounce." The data produced is very useful to meteorological and aviation interests and is available through NOAA websites.
Radar Wind Profiler
Pulse-Doppler radarwind profilers operate using principles similar to those used by Dopplersodars, except that electromagnetic (EM) signals are used rather than acoustic signals to remotelysense winds aloft. In a typical implementation, the radar can sample alongeach of five beams: one is aimed vertically to measure vertical velocity, and four are tilted offvertical and oriented orthogonal to one another to measure the horizontal components of the air'smotion. A UHF profiler includes subsystems to control the radar's transmitter, receiver, signalprocessing, and RASS((Radio Acoustic Sounding System), if provided, as well as data telemetry and remote control.
The radar transmits an
electromagneticpulse along each of the antenna's pointing directions. The duration of thetransmission determines the length of the pulse emitted by the antenna, which in turn correspondsto the volume of air illuminated (in electrical terms) by the radar beam. Small amounts of thetransmitted energy are scattered back (referred to as backscattering) toward and received by theradar. Delays of fixed intervals are built into the data processing system so that the radarreceives scattered energy from discrete altitudes, referred to as range gates. The Dopplerfrequency shift of the backscattered energy is determined, and then used to calculate the velocityof the air toward or away from the radar along each beam as a function of altitude. The source ofthe backscattered energy (radar “targets”) is small-scale turbulent fluctuations that induceirregularities in the radio refractive indexof the atmosphere. The radar is most sensitive toscattering by turbulent eddies whose spatial scale is ½ the wavelength of the radar, orapproximately 16 centimeters (cm) for a UHF profiler.
A profiler's (and sodar's) ability to measure winds is based on the assumption that theturbulent eddies that induce scattering are carried along by the mean wind. The energy scatteredby these eddies and received by the profiler is
orders of magnitudesmaller than the energytransmitted. However, if sufficient samples can be obtained, then the amplitude of the energyscattered by these eddies can be clearly identified above the background noise level, then themean wind speed and direction within the volume being sampled can be determined.The radial components measured by the tilted beams are the vector sum of the horizontalmotion of the air toward or away from the radar and any vertical motion present in the beam.Using appropriate trigonometry, the three-dimensional meteorological velocity components(u,v,w) and wind speed and wind direction are calculated from the radial velocities withcorrections for vertical motions.
A boundary-layer radar wind profiler can be configured tocompute averaged wind profiles for periods ranging from a few minutes to an hour.Boundary-layer radar wind profilers are often configured to sample in more than onemode. For example, in a “low mode,” the pulse of energy transmitted by the profiler may be 60m in length. The pulse length determines the depth of the column of air being sampled and thusthe vertical resolution of the data. In a “high mode,” the pulse length is increased, usually to 100m or greater. The longer pulse length means that more energy is being transmitted for eachsample, which improves the
signal-to-noise ratio(SNR) of the data. Using a longer pulse lengthincreases the depth of the sample volume and thus decreases the vertical resolution in the data.The greater energy output of the high mode increases the maximum altitude to which the radarwind profiler can sample, but at the expense of coarser vertical resolution and an increase in thealtitude at which the first winds are measured. When radar wind profilers are operated inmultiple modes, the data are often combined into a single overlapping data set to simplify postprocessingand data validation procedures. [cite book
last = Bailey
first = Desmond T.
others = John Irwin
title = Meteorological Monitoring Guidance for Regulatory Modeling Applications
origyear = 1987
origmonth = 6
url = http://www.epa.gov/scram001/guidance/met/mmgrma.pdf
year = 2000
month = 2
United States Environmental Protection Agency
location = Research Triangle Park, NC
id = EPA-454/R-99-005
pages = pp. 9-9 to 9-11
chapter = Upper-air Monitoring]
* Official NOAA wind profiler search page [http://www.profiler.noaa.gov/npn/profiler.jsp] See real time (and 12 hour history) graphic displays of wind direction and speed from ground level up to 17 KM above sea level (at 1 KM intervals). Click on any star or dot, then click on "get plot" at left.
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