A robust automated technique for operational calibration of ceilometers using the integrated backscatter from totally attenuating liquid clouds
A simple and robust method for calibrating ceilometers has been tested in an operational environment, demonstrating that the calibrations are stable to better than ±5 % over a period of a year. 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. The calibration technique involves scaling the observed backscatter so that B matches the predicted value for S of 18.8±0.8 sr for cloud droplets, at ceilometer wavelengths. For accurate calibration, care must be taken to only use profiles where the range correction is implemented and 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 for which the window transmission or the lidar pulse energy falls below 90 %. A range-dependent multiple-scattering correction that depends on the ceilometer optics should also be applied to the profile. For ceilometers operating at around 910 nm wavelength, a simple correction for water vapour attenuation is applied to the signal using the vapour profiles from a forecast analysis. For a generic ceilometer in the UK the 90 d running mean of the calibration coefficient over a period of 20 months is constant to within 3 % with no detectable annual cycle, thus confirming the validity of the humidity and multiple-scattering correction. For Gibraltar, where cloud cover is less prevalent than in the UK, the 90 d running mean calibration coefficient was constant to within 4 %. The more sensitive ceilometer model operating at 1064 nm is unaffected by water vapour attenuation but is more prone to saturation in liquid clouds; such profiles can be recognised and rejected and, despite the more restricted sample of cloud profiles, a robust calibration is readily achieved. In the UK, the running mean 90 d calibration coefficients varied by about 4 % over a period of 1 year. The consistency of profiles observed by nine pairs of co-located ceilometers in the UK Met Office network operating at around 910 and 1064 nm provided independent validation of the calibration technique. In all cases, if quantitative and reliable backscatter observations are to be obtained it is essential to keep the window clean. This may be a challenge in dusty locations. EUMETNET is currently networking 700 European ceilometers so they can provide ceilometer profiles in near real time to European weather forecast centres and has adopted the cloud calibration technique described in this paper for ceilometers with a wavelength of around 910 nm.