Remote Sensing of Boundary Layer Structure and Height - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 170

Appendix B
Remote Sensing of Boundary Layer Structure
and Height
The mixed layer height (MLH) and the boundary layer height appear as height
scales in several approaches for the description of vertical wind and turbulence
profiles in Chap. 3 . MLH is the height up to which atmospheric properties or
substances originating from the Earth's surface or formed within the surface layer
are dispersed almost uniformly over the entire depth of the mixed layer by tur-
bulent vertical mixing processes. Therefore, the existence and the height of a
mixed layer can either be analyzed from detecting the presence of the mixing
process, i.e., turbulence, or from the verification that a given conservative atmo-
spheric variable is distributed evenly over a certain height range. The level of
turbulence can for instance be derived from fluctuations of the wind components or
from temperature fluctuations. Suitable conservative atmospheric variables for the
identification of the mixed layer and its height are, e.g., potential temperature,
specific humidity or aerosol particle concentrations (Fig. B.1 ).
Figure B.1 shows two snapshots from a diurnally varying boundary layer under
clear-sky conditions as depicted in Fig. 3.2 . The left frame in Fig. B.1 is valid
around noon, the right frame around midnight. For a convective boundary layer at
noon, MLH and boundary-layer height are more or less identical, the vertical
mixing is thermally driven and reaches right to the top of the boundary layer. At
night, when mechanically produced turbulence is present only, MLH is usually
identical with the much lower height of the stable surface layer. The nocturnal
boundary-layer height is usually identical to the top height of the residual layer,
which is a remnant from the daytime convective layer. Because MLH is not a
primary atmospheric variable and cannot be determined from in situ measurements
at the surface, this Appendix B has been added here in order to illustrate the
necessary measurement efforts to determine this parameter. Figure B.1 shows that
distinct features in the vertical profiles of atmospheric turbulence, temperature,
specific humidity and the aerosol content are appropriate to determine the mixed
layer height and the boundary layer height.
In situ measurements of the abovementioned conservative variables can be
done over the necessary vertical height range of up to 1 or 2 km only by launching