Two-component horizontal motion vectors from scanning eye-safe aerosol lidar and the correlation method

Two-component horizontal motion vectors are derived from sequences of PPI scans collected by the Raman-shifted Eye-safe Aerosol Lidar (REAL) at the 2007 Canopy Horizontal Array Turbulence Study (CHATS). The method used to derive the vectors is the correlation method. The vectors are representative of the predominant movement of aerosol backscatter perturbations within a defined region of the scan (ranging from 250 x 250 square meters to 1000 x 1000 square meters) and over the time between two scans (ranging from 10 to 30 seconds). To explore whether the motion vectors are representative of the wind, time-series comparisons of the vectors from an area surrounding an instrumented tower are compared with sonic anemometer measurements. Very good agreement is found at night when the atmosphere is stable with light winds. However, during the day, when the atmosphere is unstable and turbulent, the agreement becomes poorer—presumably the result of a more inhomogeneous velocity field and the two very different sampling and measurement methods. The lidar vectors are the result of relatively large aerosol features moving across an area while the sonic anemometer measurements are of the air motion at a point. In addition to the vectors, several quantities intrinsic to the lidar data such as signal-to-noise ratio, and quantities independent of the lidar such as turbulent kinetic energy, are calculated for each vector. A goal of the project is to develop the ability to estimate the representativeness of a lidar derived vector to a standard wind measurement (such as the sonic anemometer) without requiring the presence of a tower and auxiliary micrometeorological measurements. Overall, the results thus far are encouraging because they suggest that two-component vector fields can be extracted from the images produced by a single, eye-safe, scanning aerosol lidar often and, in fact, over a majority of the time with increased averaging. This capability may be of particular value in applications where it is more important to obtain coarse fields of two wind components rather than higher resolution fields of only a single wind component as is available from current Doppler lidars. In addition, the motion vectors are likely to be of value in predicting the dispersion of aerosol plumes and compliment the high resolution aerosol imagery produced by high-performance direct-detection elastic backscatter lidar systems. For more details on the lidar system, please see: http://www.phys.csuchico.edu/lidar