Viewing angle comparisons of Z DR values were made to determine if migrations were occurring. When insects and birds are not migrating, the Doppler velocities can be used to estimate horizontal winds in the boundary layer. At and above the top of the well-mixed boundary layer, Bragg scattering dominates and is frequently observed at S band. It is concluded that insects are primarily responsible for the clear-air echo in the mixed boundary layer. At X band and possibly even at C and S hand the scattering enters the Mie region.
This asymmetry causes the backscattering cross section to be considerably larger than one for a spherical water droplet of equal mass.
The return signal in this layer is highly horizontally polarized with differential reflectivity Z DR values of 5–10 dB. These comparisons, when analyzed along with a theoretical backscattering model, indicate that the echoes result from both particulate and Bragg scattering with particulate scattering dominating in the well-mixed boundary layer. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado were made at radar wavelengths of 3, 5, and 10 cm. Widespread clear-air echoes have also been observed in winter when temperatures are above 10☌. Seldom do they occur over large bodies of water. These echoes are most commonly observed over land from spring through autumn. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). The origin of these echoes has been an ongoing controversy in radar meteorology. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. Operational and research meteorologists are using these Doppler velocities to derive winds. Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D.
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