see clear-air echo). The turbulent scattering structures are assumed to be moving with the same average velocity as the wind. The three-dimensional wind vector is determined by using the beam swinging technique of pointing the radar beam in at least three different directions. Another type of wind profiler radar that operates in the VHF–UHF band uses a technique called spaced antenna drift (SAD). SAD radars use a single vertical-beam transmitting antenna and three or more horizontally spaced, vertical-beam receiving antennas. The horizontal wind is determined from the cross correlation of the received echoes and the vertical wind is determined from the Doppler shift of the echoes. In addition to measuring the wind vector, wind profiler radars can also determine several other atmospheric quantities from the power, mean Doppler shift, and Doppler spectral width of the returned signal. These quantities include the strength of turbulence (parameterized by the refractive index structure constant Cn2), the eddy dissipation rate, atmospheric stability, momentum flux, virtual temperature and heat flux (using the RASS technique), and precipitation rates and drop- size distributions (from scatter from hydrometeors). Doppler lidars and acoustic sounders (sodars) can also be used as wind profilers. The scatterers for lidars are aerosols (for wavelengths around 10 μm) and molecules (for wavelengths less than 1 μm). Acoustic refractive index fluctuations caused by turbulence provide the scattering mechanism for sodars.
See also MST radar, boundary layer radar.
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