Nocturnal boundary layer height estimate from Doppler lidar measurements

Accurate estimation of the stable boundary layer (SBL) height and its evolution through the night is very important for better understanding of nighttime mixing processes, and application in numerical weather prediction and air quality models. The boundary layer (BL), including SBL, is defined as the atmospheric layer adjacent to the Earth's surface that interacts with the surface via turbulent processes. The top of this layer h is therefore a level where turbulence quantities drop to a minimum value. Because profiles of turbulence quantities are often unavailable, it is desirable to be able to express h in terms of mean variables. In this study we use profiles of mean wind speed U and the streamwise velocity variance (the largest component of the TKE) computed from Doppler lidar scan data to determine h. Analysis of data from two nocturnal BL field campaigns over the U.S. Great Plains has indicated that h is closely associated with the height of the low- level jet (LLJ) maximum. However, for certain profiles this technique may be difficult to implement. In this study we investigate 10-min wind profiles calculated from lidar vertical-slice scans with an improved method to estimate h and its relation with the BL height determined from the variance profiles. The improved method, which finds the peak negative value of the second derivative (curvature) of U(z), does a superior job of identifying the top of the BL. We also tried using this technique with lidar aerosol backscatter data to find the afternoon unstable BL height. Analysis of lidar vertical staring scans in late afternoon shows that minimum value of second derivative in aerosol backscatter profile can be a measure of BL mixing height.