Radar wind profilers
Radar wind profilers (RWPs) also rely on the Doppler effect as they transmit pulses of electromagnetic radiation vertically and in at least two slightly off-vertical (~75-degree elevation) directions to resolve the three components of the wind (Ecklund et al. 1988). RWPs sample backscatter returns from atmospheric features, such as turbulence, clouds, or precipitation, but they also measure returns from nonatmospheric features, notably insects and birds. Signal processing software enables a proper focus on the atmospheric returns (Wilczak et al. 1995).
Just as with DBS lidar scans, line-of-sight velocities for each of the transmitted beam directions (three in this case) are assumed to represent horizontally homogeneous winds so that the three components of the flow can be reconstructed. For an RWP, a single wind profile is produced over an averaging period of 30 to 90 s, but these individual profiles are typically averaged together into a larger averaging period of 15-60 min. Depending on the frequency of the radio emission, RWP have different vertical resolutions and lowest measurement altitudes. 915 MHz wind profilers usually have nominally 60-m vertical resolution, with the lowest level approximately 100 m above the surface. 449 MHz wind profilers usually have nominally 100-m vertical resolution, with the lowest level approximately 200 m above the surface. RWP technology is commercially-available at TRL > 8 and has been commercially available for the last 20 years or more.
High-resolution RWPs can provide information on the height of the daytime convective atmospheric boundary layer (Angevine et al. 1994; Cohn and Angevine 2000). Because clear-air radars of this sort rely on inhomogeneities in the radio refractive index structure parameter C_N^2, and that parameter is a maximum at the inversion atop the convective boundary layer, the peak in the range-corrected SNR ratio of the RWP identifies the top of the inversion. Of course, the resolution of the height of the boundary layer is limited by the vertical resolution of the radar. Further, nocturnal stable boundary layer heights tend to be much lower than daytime convective boundary layers, often below the lowest measurement height of an RWP.
Radar wind profilers are generally considered to have a TRL of 8.