Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 73

of energy, momentum, moisture, or trace substances. Therefore, the detection of
the vertical layering is one of the principal tasks of experimental boundary layer
research.
We must distinguish between the mixing layer height, MLH (see Section 4.2.1 )
and the boundary layer height, z i (see Section 4.2.2 ). The boundary layer height
is the height up to which the influence of the presence of the lower surface is
detectable. The mixing layer height is the height up to which atmospheric proper-
ties or substances originating from the surface are dispersed by turbulent vertical
mixing processes. The mixing layer - if it is present at all - is a part of the
atmospheric boundary layer. Thus, the mixing layer height is usually shallower
than the boundary layer, but it fills the whole ABL in deep convective boundary
layers.
4.2.1 Mixing Layer Height
The mixing layer height is the height up to which atmospheric properties or sub-
stances originating from the Earth's surface or formed within this layer are dispersed
almost uniformly over the entire depth of this layer by turbulent vertical mixing pro-
cesses. Therefore, the existence and the height of a mixing layer can be analyzed
either from a detection of the presence of the mixing process, i.e. turbulence, or
from the verification that a given conservative atmospheric 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 mixing layer
and its heights are, e.g. potential temperature, specific humidity or aerosol particle
concentrations.
A rather complete overview of methods to determine the MLH from in situ mea-
surements and surface-based remote sensing has been given by Seibert et al. ( 2000 ).
Since then considerable development has taken place, especially concerning the
usage of surface-based remote-sensing methods (see the review paper by Emeis
et al. ( 2008 )). This subchapter will mainly follow this latter review.
Optical methods for MLH detection may be used to illustrate this recent progress.
Seibert et al. ( 2000 ) still classified LIDAR methods as expensive, not eye-save,
with a high lowest range gate, limited range resolution, and sometimes subject to
ambiguous interpretation. This has changed drastically in the past ten years when
better LIDARs have been built and ceilometers have been discovered to be a nearly
ideal boundary layer sounding instrument. Progress has been made in the field of
acoustic sounding as well. Similarly, algorithms for the determination of MLH from
vertical profiles of the acoustic backscatter intensity as described in Beyrich ( 1997 )
and Seibert et al. ( 2000 ) have been enhanced by using further variables available
from SODAR measurements such as the wind speed and the variance of the verti-
cal velocity component (Asimakopoulos et al. 2004 , Emeis and Türk 2004 ). Such
enhancements had been named as possible methods in Beyrich ( 1995 ) and Seibert
et al. ( 2000 ), but obviously no example was available at that time.