Boundary Layer Clear-Air Radar Echoes: Origin of Echoes and Accuracy of Derived Winds

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Abstract
Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. 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 origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10°C. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado were made at radar wavelengths of 3, 5, and 10 cm. 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. The return signal in this layer is highly horizontally polarized with differential reflectivity ZDR values of 5–10 dB. This asymmetry causes the backscattering cross section to be considerably larger than one for a spherical water droplet of equal mass. At X band and possibly even at C and S hand the scattering enters the Mie region. It is concluded that insects are primarily responsible for the clear-air echo in the mixed boundary layer. At and above the top of the well-mixed boundary layer, Bragg scattering dominates and is frequently observed at S band. When insects and birds are not migrating, the Doppler velocities can be used to estimate horizontal winds in the boundary layer. Viewing angle comparisons of ZDR values were made to determine if migrations were occurring. Migrations were not observed in Florida and Colorado during summer daylight hours. Limited comparison of winds derived from Doppler radar with balloon-sounding winds showed good agreement. However, a more extensive study is recommended to determine the generality of this conclusion. Dual-Doppler analyses show that thin-line echoes are updraft regions. Comparison of these radar-derived vertical velocities with aircraft-measured vertical velocities showed a correlation coefficient of 0.79. In addition, the position of small-scale updraft maxima (1–2 km in diameter) along the sea-breeze front correspond to individual cumulus clouds. The good agreement between dual-Doppler-derived vertical motion fields and these other independent vertical velocity measurements provides evidence that the dual-Doppler-derived wind fields in the clear-air boundary layer are accurate and capable of providing details of the wind circulations associated with horizontal convective rolls and the sea breeze.

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