Remote Sensing of Boundary Layer Structure and Height - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 179

cases with both well pronounced and less clearly pronounced mixed layer tops.
H4_2 is optionally determined in the height range between H4_1 and the upper
boundary of detection. A valid H4_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
H4_1. MLH is set equal to H4_1.
However, problems with this method arise e.g., in case of multiple (well
defined) aerosol layers, which renders the selection of the correct mixed layer top
ambiguous. Furthermore, in spring and summer the detection of the MLH for deep
(convective) boundary layers often fails. This is mostly due to the high variability
of the aerosol backscatter signal with height which limits the range for MLH
estimation in those conditions (de Haij et al. 2006 ).
B.2.5 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 elucidating 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.
B.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 ABL. The only direct and key variable for the analysis of the presence of a
mixed layer is the vertical profile of virtual temperature. Temperature profiles can
directly be measured with a radio-acoustic sounding system (RASS). There is also
the option to derive vertical temperature profiles from Raman-LIDAR soundings
(Cooney 1972 ) and passive radiometer measurements but especially from passive
remote sensing the vertical resolution is usually not sufficient for boundary-layer
research.