CAPPI scans are formed from boundary layer volume scans of the VIL. Figure 24 illustrates a lidar backscatter profile in a lidar-centered cartesian coordinate system. The direction of is parallel to the center azimuth angle of the CAPPI scan.
Figure 24: The backscatter signal geometry in cartesian coordinates. The lidar is located at the origin of coordinate system. Sample point n is moved upwind to correct a distortion due to wind during scanning. Line Rijk is the distance from the corrected sample point to grid point . is the azimuth scan angle; is the elevation scan angle.
Since wind moves aerosol structures during scanning, each backscatter signal location is corrected by moving it a distance upwind, where is the mean wind vector and t is the time elapsed from the first sample in the volume scan. A priori mean wind information for the correction is not required, since the analysis can be repeated using the result from the previous analysis loop as the mean wind estimate . Usually, when no correction for the wind (i.e. zero wind speed) is performed in the first iteration, no more than one iteration of the wind analysis loop is required . After the correction, the signal amplitude of each data point is averaged to the eight closest grid points with weights as defined by the following
where Rijk is the distance between the corrected signal point and the grid point . is the distance between the opposite corner points and . The signal intensity in each point in a CAPPI scan at altitude level k is expressed as
When using a dense grid spacing, some grid cells at long ranges remain unsampled. These holes are filled by linearly interpolating the closest sampled cells as follows. First, single, empty pixels are filled by the average of their neighboring sampled pixels. Then, empty lines in -direction are linearly interpolated between the sampled border pixels, unless the length of the empty line exceeds 150 m. Finally, empty lines in -direction are interpolated similarly. Several other interpolation schemes were also studied, but the one above proved to be fast and preserved the coherence between subsequent CAPPI planes. If sparser grid spacing was used to avoid empty pixels, the spatial resolution would be reduced at the closer range where the quality of the signal is best.