Abstract/Summary

Ceilometers are simple, relatively inexpensive vertically pointing lidars usually operating at wavelengths of 905-910 nm or 1064 nm that can function reliably unattended for long periods and, as the name suggests, have mainly been used for detecting cloud base at airports where they are valuable for air safety issues. In addition to detecting the large backscattered return from cloud base, they can provide vertical profiles of backscatter from both clouds and aerosols every 5-30 seconds with a range resolution of 10-20 m. This thesis presents a simple and robust method for calibrating ceilometers that has been tested in an operational environment. The method relies on using the integrated backscatter (B) from liquid clouds that totally extinguish the ceilometer signal; B is inversely proportional to the lidar ratio (S) of the backscatter to the extinction for cloud droplets. It is shown that for accurate calibration, care must be taken to exclude any profiles having targets with different values of S, such as drizzle drops and aerosol particles, profiles that do not totally extinguish the ceilometer signal, profiles with low cloud bases that saturate the receiver, and any profiles where the window transmission or the lidar pulse energy is low. A range dependent multiple scattering correction that depends on the ceilometer optics is applied to the attenuated backscatter profiles. A simple correction for water vapour attenuation for ceilometers operating at around 910 nm wavelength is applied to the signal using the vapour profiles from a forecast analysis. The consistency of profiles observed by a pair of co-located ceilometers in the UK Met Office network operating at around 910 nm and 1064 nm provides independent validation of the calibration technique. Secondly, an ice cloud forward model is used to predict attenuated backscatter, using profiles of ice water content (IWC) from the Met Office’s numerical weather prediction model, the UKV. A second moment approximation for particle area and mass is assumed, so that IWC becomes proportional to extinction. The lidar ratio for ice cloud is then used to calculate backscatter from the model derived extinction. The lidar ratio is calculated using ceilometer observations of thick attenuating ice cloud. Comparisons between the UKV derived attenuated backscatter and the attenuated backscatter observed by a ceilometer suggest that the UKV IWC is systematically too small in magnitude and too high in height.