ALCF

Documentation

Calibration

Depending on the instrument, calibration in the ALCF may be an essential part of processing ALC data. Ceilometers often report backscatter values in arbitrary units which need to be converted to m^-1.sr^-1 for a reliable comparison with the COSP simulator. The recommended calibration method in the ALCF is O’Connor et al. (2004). The method is based on the fact that the lidar ratio in fully opaque liquid stratocumulus profiles is approximately constant, depending only on the wavelength of the lidar, assuming the cloud droplet size of the stratocumulus cloud is within a certain typical range.

The calibration steps are outlined below:

1. Process and plot backscatter profiles with the lidar ratio, assuming the default calibration coefficient.

   alcf auto lidar <type> <input> <output> --lr

2. Identify time periods with stratocumulus cloud.

   Go through the backscatter profile plots in <output>/plot/backscatter/ and note the time periods with stratocumulus cloud in a text file calibration_time_periods.txt in the format:

    <start> <end>
    <start> <end>
    ...
   

   where start and end are in the format YYYY-MM-DDTHH:MM. Avoid profiles with multiple layers or cloud close to the surface. See O’Connor et al. (2014) or Hopkin et al. (2019) for more information.

   Figure 1 shows an example of a stratocumulus cloud in the time period:

    2018-02-18T13:00 2018-02-18T15:00
   

   
   Figure 1: Stratocumulus layer between 13:00 and 15:00 UTC suitable for calibration.

3. Run alcf calibrate with a file listing the time periods to calculate the calibration coefficient.

   alcf calibrate <type> <input> calibration.txt calibration_time_periods.txt

   The coefficient is saved in calibration_time_periods.txt.

4. Process and plot backscatter profiles with the calculated calibration coefficient.

   alcf auto lidar <type> <input> <output> --lr calibration_file: calibration.txt

5. Check that the plotted lidar ratio in the stratocumulus profiles is approximatly 18.8 sr.