Using Doppler lidar measurements from a suite of scanning strategies to determine the boundary layer height

One of the most basic and essential characteristics that describes the atmospheric boundary layer is its height. Profiles of many other quantities such as mean velocity, velocity variances, temperature, and humidity are related to and often scaled by the boundary layer height. The boundary layer height also provides information about the depth over which conserved quantities are well-mixed. Hence, accurate measurements of the boundary layer height are necessary to understand and quantity the dispersion of pollutants, aerosols, trace gases, and other anthropogenic emissions.

Previously, various types of lidars have been used to determine the boundary layer height as sharp vertical gradients in backscatter or as the height where vertical velocity variance decreases below a predetermined threshold. However, these methods have limitations. During the nighttime and morning hours, often a large gradient in backscatter is apparent at the top of a residual layer, not necessarily at the top of the boundary layer itself, which may lead to overestimates of the boundary layer height. Using vertical velocity variance alone comes with its own issues, as high values of vertical velocity variance can be associated with waves and other features not necessarily due to turbulent mixing within the boundary layer. Additionally, these methods cannot be used to detect a shallow boundary layer when its height is less than the minimum range of the lidar, since these methods typically are used when the lidar is zenith pointing.

To overcome these limitations, Doppler lidar measurements from a suite of scanning strategies are used here to determine the boundary layer height under a wide range of conditions. For the Indianapolis Flux Experiment (INFLUX), a Doppler lidar has been deployed for more than two years providing continuous measurements of mean wind profiles and boundary layer height. Every 20 minutes, the lidar completes a sequence of measurements including velocity azimuth display scans, shallow range height indicator scans, and zenith stares. Within this presentation, we will discuss how information from all of these scans is jointly used to automatically determine the boundary layer height.