Dual-Doppler Lidar Analysis
Dual-Doppler analysis of data from two coherent lidars during the Terrain-Induced Rotor Experiment (T-REX) allows the retrieval of flow structures, such as vortices, during mountain-wave events. The spatial and temporal resolution of this approach is sufficient to identify and track vortical motions on an elevated, cross-barrier plane in clear air. Assimilation routines or additional constraints such as two-dimensional continuity are not required. A relatively simple and quick least squares method forms the basis of the retrieval. Vortices are shown to evolve and advect in the flow field, allowing analysis of their behavior in the mountain–wave–boundary layer system. The locations, magnitudes, and evolution of the vortices can be studied through calculated fields of velocity, vorticity, streamlines, and swirl. Generally, observations suggest two classes of vortical motions: rotors and small-scale vortical structures. These two structures differ in scale and behavior. The level of coordination of the two lidars and the nature of the output (i.e., in range gates) creates inherent restrictions on the spatial and temporal resolution of retrieved fields. (Hill et al. 2009)
Figure. Rotor at 1104 PST (1904 UTC): (a) 2D velocity field and swirling strength contours; (b) streamlines (with arrows) and vorticity. Units of vorticity and swirling strength are s-1.
During the Joint Urban 2003 (JU2003) atmospheric field experiment in Oklahoma City, Oklahoma, of July 2003, lidar teams from Arizona State University and the Army Research Laboratory collaborated to perform intersecting range–height indicator scans. Because a single lidar measures radial winds, that is, the dot product of the wind vector with a unit vector pointing along the lidar beam, the data from two lidars viewing from different directions can be combined to produce horizontal velocity vectors. Analysis programs were written to retrieve horizontal velocity vectors for a series of eight vertical profiles to the southwest (approximately upwind) of the downtown urban core. This technique has the following unique characteristics that make it well suited for urban meteorology studies: 1) continuous vertical profiles from far above the building heights to down into the street canyons can be measured and 2) the profiles can extend to very near the ground without a loss of accuracy (assuming clear lines of site). The period of time analyzed spans from 1400 to 1730 UTC (0900–1230 local time) on 9 July 2003. Both shear and convective heating are important during the development of the boundary layer over this period of time. Differences in 10- and 20-min mean profiles show the effect of the variation of position approaching the urban core; for example, several hundred meters above the ground, velocity magnitudes for profiles separated by less than a kilometer may differ by over 1 m s−1. The effect of the increased roughness associated with the central business district can be seen as a deceleration of the velocity and a turning of the wind direction as the flow approaches the core, up to approximately 10° for some profiles. This effect is evident below 400–500 m both in the wind directions and magnitudes. Recommendations are given for how this type of data can be used in a comparison with model data. (Calhoun et al. 2006)