Doppler lidar measurements of vertical velocity spectra, length scales, and coherence in the convective planetary boundary layer

The HIgh Resolution 2 micrometer wavelength Doppler Lidar (HRDL) developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol particles in the convective boundary layer (PBL) over flat farmland in central Illinois in summer 1996. HRDL operated in the zenith-pointing mode continuously during daytime for several days during the Lidars in Flat Terrain (LIFT) experiment. We calculated profiles of vertical velocity (w) integral scales in both the alongwind and vertical directions from about 390 m height to the PBL top. In the middle of the mixed layer we found, from the ratio of the w integral scale in the vertical to that in the horizontal direction, that the w eddies are squashed by a factor of about 0.65 as compared to what would be the case for isotropic turbulence. Furthermore, there is a significant decrease of the vertical integral scale with height. The integral scale profiles and vertical coherence show that vertical velocity fluctuations in the convective PBL have a predictable anisotropic structure. The coherence along the vertical axis is consistently larger than predicted by a von Karman spectral model, which suggests that the von Karman spectrum does not adequately represent the actual spectra. The w spectra calculated from lidar time series are generally in agreement with previous observational results, but the increased vertical resolution allows an unprecedent view of how the spectra vary with height above 390 m in the PBL.