In the warm season the optically clear boundary layer often contains scatterers that can be detected by sensitive radars to distances of 50–100 km. Inhomogeneities in the field of reflectivity lead to echo patterns that have some persistence over periods as long as 5–10 minutes. These echo patterns translate with the local wind and can, for example, be easily followed by eye on PPI playback loops reviewing 15–20 minutes of data. In this paper a technique is discussed that determines the wind field by objectively identifying and tracking local echo. patterns. The technique, called TREC (Tracking Radar Echoes by Correlation), involves the cross-correlation of the echo features measured at two times a few minutes apart. The translation of a local feature during the measurement interval then determines the local wind. Use of this technique in the clear boundary layer means that problems associated with noisy data (reflectivities just above the minimum detectable signal) and ground clutter will be common. A number of methods for dealing, with these matters are presented. The use of TREC in clear air rather than in storms carries the advantage that misleading results associated with the sedimentation of hydrometeors in a sheared flow are avoided.
In this study TREC is applied to data collected during the Convective Initiation and Downburst Experiment (CINDE) in northeastern Colorado. The method is shown to provide horizontal winds in the boundary layer (the region of significant clear-air echo) over areas 100 to 150 km on a side with a resolution of about 10 km. TREC can be used with either conventional or Doppler radars sensitive enough to detect clear-air echo, though there are some advantages in using single Doppler measurements to improve the reliability of the technique. Applications of the present method are anticipated in a number of research and forecast areas.