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advantage of wavelet methods is the decomposition of the signal in both altitude as
well as vertical spatial scale of the structures in the backscatter signal.
The wavelet algorithm in de Haij et al. (
2006
) is applied to the 10 min aver-
aged range and overlap corrected backscatter profile
B
(
z
) within a vertical domain
of 90-3000 m. For each averaged profile the top of two significant aerosol layers
are detected in order to detect MLH as well as the top of a secondary aerosol layer,
like, e.g. an advected aerosol layer or the residual layer. This wavelet MLH method
uses the scale averaged power spectrum profile
W
B
(
z
) of the wavelet transform with
24 dilations between 15 and 360 m and step size 15 m. The top of the first layer,
H
4
_
1, is detected at the first range gate at which the scale averaged power spectrum
W
B
(
z
) shows a local maximum, exceeding a threshold value of 0.1. This thresh-
old value is empirically chosen, based on the analysis of several cases with both
well pronounced and less clearly pronounced mixing layer tops.
H
4
_
2 is optionally
determined in the height range between
H
4
_
1 and the upper boundary of detection.
A valid
H
4
_
2 is detected at the level with the strongest local maximum of W
B
(
z
)
provided that this maximum is larger than the W
B
(
z
)of
H
4
_
1. MLH is set equal to
H
4
_
1.
However, problems with this method arise e.g. in case of multiple (well defined)
aerosol layers, which renders the selection of the correct mixing layer top ambigu-
ous. Furthermore, in spring and summer the detection of the MLH for deep
(convective) boundary layers often fails. This is mostly due to the high variabil-
ity of the aerosol backscatter signal (see Section
4.2.2.5
) with height which limits
the range for MLH estimation in those conditions (de Haij et al.
2006
).
Variance Method
At the top of the convective boundary layer (CBL) we have entrainment of clear
air masses from the free troposphere into the ABL. The entrainment process is
temporarily variable and leads locally to considerable fluctuations in the aerosol
concentration. Therefore, the maximum in the vertical profile of the variance of the
optical backscatter intensity can be an indicator for an entrainment layer on top a
CBL (Hooper and Eloranta
1986
; Piironen and Eloranta
1995
). The method is called
variance centroid method in Menut et al. (
1999
). The variance method for the CBL
height is also described in Lammert and Bösenberg (
2006
). Due to the assumptions
made this method is suitable for daytime convective boundary layers only. An elu-
cidating comparison between the gradient method and the variance method can be
found in Martucci et al. (
2004
) although they used a Nd:YAG LIDAR at 532 nm
instead of a ceilometer and thus suffered from a high lowest range gate in the order
of 300 m.
4.2.1.3 RASS
The acoustic and optical methods for the determination of the mixing height, which
have been described so far, are all indirect methods that try to infer the mixing
height from other variables which usually adapt to the vertical structure of the
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