Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 80

Fig. 4.3 Detection of MLH from ceilometer measurements in Augsburg, Germany on May 19,
2007 (time is given in GMT+2). Left : optical backscatter intensity, right : negative vertical gradient
of optical backscatter intensity. Dots indicate MLH determined from the gradient method
MLH. Thus the schemes listed below all resemble to the ARE methods for acoustic
remote sensing.
Figure 4.3 shows a sample measurement with a mono-axial ceilometer. The
left-hand frame displays the optical backscatter intensity and the right-hand frame
the negative vertical derivative of this intensity as time-height sections over one
day (from midnight to midnight) and over a height range of 2000 m. The data
were received on a clear day in spring, and the vertical structure of the ABL was
dominated by surface heating due to incoming solar radiation during daytime and
radiative surface cooling during night-time. In the morning hours until about 9 a.m.
a shallow stable nocturnal surface layer with a depth of about 200 m and a resid-
ual layer with a depth of about 1200-1400 m can be distinguished. From 9 a.m.
onwards the evolution of a daytime convective boundary layer with a maximum
depth of about 1400 m can be clearly seen. The dots in both frames of Fig. 4.3 indi-
cate the mixing-layer height determined with the gradient method described below
(see section “Gradient or Derivative Methods”). The right-hand frame in Fig. 4.3 ,
which shows the vertical derivative of the optical backscatter intensity, demonstrates
that the analyzed MLH values indeed coincide with maxima of the negative vertical
gradient of this backscatter intensity.
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 ). H 4 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 H 4 _n would require the definition of several thresholds, which
probably cannot be done a priory to the analysis. Therefore this will always lead to