The height of the atmospheric boundary layer is a fundamental parameter in boundary-layer studies, including those conducted for wind energy applications. Unfortunately, it can often be a difficult parameter to measure. During daytime conditions with strong solar heating of the surface, the boundary-layer height is driven by convection from the surface. In midlatitudes with little cloud cover, the boundary-layer height typically ranges between 1 and 3 km (Stull 1988; Wallace and Hobbs 2006). In contrast, nocturnal boundary layers are much more shallow (20–300 m), driven by mechanical forcing and buoyant suppression of turbulence. Unfortunately, observations of stable boundary layer height are rarely straightforward, as a result of the low levels of turbulence and frequent intermittency in the stable boundary layer (Steeneveld et al. 2007). In one study based on the CASES-99 stable boundary layer experiment, a classical stable boundary layer height could be defined only 22% of the time (Vickers and Mahrt 2004).
The standard approach to directly measuring the boundary-layer height is to assess a profile of potential temperature (or virtual potential temperature) from a radiosonde (or tethered airborne system [remotely piloted aircraft system]), recognizing that the boundary layer is topped by a strong inversion (Schmid and Niyogi 2012). Other remote-sensing instruments, such as radar wind profilers (Angevine et al. 1994; Coulter and Holdridge 1998; Cohn and Angevine 2000) can be used to identify the inversion height (and therefore the boundary-layer top) via a maximum in range-corrected SNR ratio. A comparison of the radiosonde and radar wind profile approach appears in Figure 17.
If the instruments have suitable range, lidars may be used to identify the top of the boundary layer. Because lidars rely on the presence of aerosols in the atmosphere for backscatter, and aerosols usually have a source at the surface, a strong contrast in aerosol density would indicate the entrainment zone separating the convective boundary layer from the free troposphere (Cohn and Angevine 2000). At night, the height of the stable boundary layer is often (but not always) coincident with the nocturnal low-level jet (when it forms) (Banta et al. 2002), and so wind profiles from lidar could potentially be used to identify the depth of the stable boundary layer.
Microwave radiometers, as they measure profiles of temperature and moisture, should be able to provide an assessment of boundary layer height. Based on 1 year of comparisons between microwave radiometers and radiosonde profiles, Collaud Coen et al. (2014) found reasonable agreement between all of the boundary-layer height estimates. Ceilometers (otherwise not discussed in this document) measure aerosol backscatter, and can also be used to identify the entrainment zone and the top of the daytime convective boundary layer. Based on 40 profiles and comparison between radiosondes and ceilometer-based aerosol methods, reasonable agreement was found (Caicedo et al. 2016). Ceilometers struggle with identifying the top of the stable boundary layer because there is no expectation of aerosol gradient at the top of the stable boundary layer.
Typically measured in m or km.