To eliminate the effects of variable atmospheric attenuation, scan angle dependent background signal level, and shot-to-shot variations in energy normalization, the lidar backscatter profiles are median high-pass-filtered. The length of the filter must be longer than an average aerosol structure size to avoid the distortion of aerosol structures. In this study, a 2 km long filter is used. The first and last 1 km of each data record is median filtered by adjusting the length so as many data points as possible are used. Median high pass filtering the signal in equation (2) leads to
where k is the half width of the filter. The median filter is optimal for this application, since the position deformation of aerosol structure edges should be kept minimal. Signals above optically thick clouds are not used to prevent correlations between cloud shadows. Figure 23 shows an example of a pre-filtered backscatter profile.
Figure 23: A raw backscatter signal and median high-pass-filtered signal as functions of the range on August 1, 1989, at 14:01:48 CDT. The inside graphs represent raw (upper graph) and filtered (lower graph) signal from the boundary layer aerosols magnified by ten. The elevation angle is 5.32. The background signal level and varying extinction are reduced in the filtering process, while aerosol structures and their positions remain unchanged.