Abstract
We utilized a Doppler lidar to measure integral scale and coherence of vertical velocity w in the daytime convective boundary layer (CBL). The high resolution 2 μm wavelength Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol particles. It operated continuously in the zenith-pointing mode for several days in the summer 1996 during the “Lidars in Flat Terrain” experiment over level farmland in central Illinois. We calculated profiles of w integral scales in both the alongwind and vertical directions from about 390 m height to the CBL 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 CBL have a predictable anisotropic structure. We found no significant tilt of the thermal structures with height in the middle part of the CBL.
Highlights
Turbulence is the main transport mechanism occurring within the convective boundary layer (CBL)
During August 1996, the National Center for Atmospheric Research’s Atmospheric Technology Division (NCAR/ATD) and NOAA’s Environment Technology Laboratory (ETL) deployed three lidars at the University of Illinois field site near Champaign, Illinois, USA, to observe the high resolution structure of aerosol, winds, and ozone in the lowest few kilometres of the atmosphere as the CBL evolved from early morning to late evening
This takes advantage of the lidar’s capability to obtain range-resolved radial measurements, from which a two-dimensional field of w can be obtained by use of Taylor’s hypothesis; that is by assuming that the field of turbulence is “frozen” as it advects past the lidar
Summary
Turbulence is the main transport mechanism occurring within the convective boundary layer (CBL). One way to investigate the structure of turbulence from observational studies is to consider the distance over which, on average, a variable remains correlated with itself Published estimates of this scale were obtained from airplane in situ measurements (e.g., Lenschow and Stankov, 1986; Durand et al, 2000) because of the ability of aircraft to probe from a few decametres above the surface to the free troposphere, and cover long distances in a relative short period of time. A major limitation, is that an aircraft can only probe one level at a time, and cannot measure two-point turbulence statistics on scales larger than the transverse dimensions of the aircraft An exception to this is the work of Lenschow and Kristensen (1988) and Kristensen et al (1989), who flew two identical aircraft in formation during the Dual Aircraft Formation Flight Experiment (DAFFEX) to obtain lateral two-point velocity statistics of all three wind components in the CBL. We compare our results with the predicted coherence for inertial subrange turbulence and for a von Kármán isotropic turbulence spectrum
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
