Geoscience Reference
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place near the surface as well. Making the assumption that the vertical particle
distribution adapts rapidly to the changing thermal structure of the boundary layer,
MLH can be determined from the analysis of the vertical aerosol distribution. This
also includes the assumption that the vertical aerosol distribution is not dominated
by horizontally advected aerosol plumes or layers. The heights of near surface
aerosol layers (H4_n) can be analysed from optical vertical backscatter profiles
obtained by optical remote sensing. Several methods have been developed, the
most prominent of these being: (1) the threshold method, (2) the gradient or
derivative method, (3) the idealised gradient method, (4) the wavelet method, and
(5) the variance method. In addition, the abovementioned horizontal wind speed
method (
Sect. B.1.2
) and vertical wind variance method (
Sect. B.1.3
) are available
to derive the vertical structure of the boundary layer from Doppler wind lidar data.
The application of optical remote sensing for MLH determination has focussed
on the use of ceilometers in recent years but small wind lidars usually provide this
information as well. In contrast to wind lidars, ceilometers do not determine the
Doppler shift of the backscattered signal. For the detection of MLH below
150-200 m a ceilometer with one optical axis for the emitted and the received
beam should be used. Due to the thin light beams the overlap of the emitted and
received beam from a ceilometer with two parallel optical axes can be insufficient
in this height range.
B.2.1 Threshold Method
Melfi et al. (
1985
) and Boers et al. (
1988
) used simple signal threshold values,
though this method suffers from the need to define them appropriately (Sicard et al.
2006
). H4 is defined here as the height within the vertical profile of the optical
backscatter intensity where the backscatter intensity first exceeds a given threshold
when coming downward from the free unpolluted troposphere. The determination
of several heights H4_n would require the definition of several thresholds which
probably cannot be done a priory to the analysis. Therefore this will always lead to
a subjective analysis of MLH.
B.2.2 Gradient or Derivative Methods
Hayden et al.
(1997)
and Flamant et al. (
1997
) proposed to use the largest negative
peak of the first derivative of the optical attenuated backscatter intensity (B(z)) for
the detection of H4 from LIDAR data (height of gradient minimum H4
GM
):
H4
GM
¼
min oB
ð
z
Þ=
oz
ð
Þ
ð
B
:
1
Þ
Likewise Wulfmeyer
(1999)
used the first minimum of the slope to detect the
top of a convective boundary layer from DIAL data. Münkel and Räsänen (
2004
),
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