BOUNDARY-LAYER CLEAR-AIR RADAR ECHOES - ORIGIN OF ECHOES AND ACCURACYOF DERIVED WINDS

Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and res earch meteorologists are using these Doppler velocities to derive wind s. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelen gth and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimat e winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-in dex gradient (Bragg scattering) and insects and birds (particulate sca ttering). These echoes are most commonly observed over land from sprin g through autumn. Seldom do they occur over large bodies of water. Wid espread clear-air echoes have also been observed in winter when temper atures are above 10 degrees 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 theoret ical backscattering model, indicate that the echoes result from both p articulate 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 Z(DR) val ues of 5-10 dB. This asymmetry causes the backscattering cross section to be considerably larger than one for a spherical water droplet of e qual mass. At X band and possibly even at C and S band the scattering enters the Mie region. It is concluded that insects are primarily resp onsible for the clear-air echo in the mixed boundary layer. At and abo ve the top of the well-mixed boundary layer, Bragg scattering dominate s 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 Z(DR) values were made to determine if migrations were occurring. Migrations were not observed in Florida and Colorado during summer daylight hours. Lim ited comparison of winds derived from Doppler radar with balloon-sound ing winds showed good agreement. However, a more extensive study is re commended to determine the generality of this conclusion. Dual-Doppler analyses show that thin-line echoes are updraft regions. Comparison o f these radar-derived vertical velocities with aircraft-measured verti cal 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 the se other independent vertical velocity measurements provides evidence that the dual-Doppler-derived wind fields in the clear-air boundary la yer are accurate and capable of providing details of the wind circulat ions associated with horizontal convective rolls and the sea breeze.