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. A simple and robust method for calibrating ceilometers has beentested in an operational environment, demonstrating that the calibrations arestable to better than ±5 % over a period of a year. The methodrelies on using the integrated backscatter (B) from liquid clouds thattotally extinguish the ceilometer signal; B is inversely proportional to thelidar ratio (S) of the backscatter to the extinction for cloud droplets. Thecalibration technique involves scaling the observed backscatter so that Bmatches the predicted value for S of 18.8±0.8 sr for cloud droplets,at ceilometer wavelengths. For accurate calibration, care must be taken toonly use profiles where the range correction is implemented and to excludeany profiles having targets with different values of S, such as drizzledrops and aerosol particles, profiles that do not totally extinguish theceilometer signal, profiles with low cloud bases that saturate the receiver,and any profiles for which the window transmission or the lidar pulse energyfalls below 90 %. A range-dependent multiple-scattering correction thatdepends on the ceilometer optics should also be applied to the profile. Forceilometers operating at around 910 nm wavelength, a simple correction forwater vapour attenuation is applied to the signal using the vapour profilesfrom a forecast analysis. For a generic ceilometer in the UK the 90 drunning mean of the calibration coefficient over a period of 20 months isconstant to within 3 % with no detectable annual cycle, thus confirmingthe validity of the humidity and multiple-scattering correction. ForGibraltar, where cloud cover is less prevalent than in the UK, the90 drunning mean calibration coefficient was constant to within 4 %. The moresensitive ceilometer model operating at 1064 nm is unaffected by watervapour attenuation but is more prone to saturation in liquid clouds; suchprofiles can be recognised and rejected and, despite the more restrictedsample of cloud profiles, a robust calibration is readily achieved. In theUK, the running mean 90 d calibration coefficients varied by about 4 %over a period of 1 year. The consistency of profiles observed by ninepairs of co-located ceilometers in the UK Met Office network operating ataround 910 and 1064 nm provided independent validation of the calibrationtechnique. In all cases, if quantitative and reliable backscatterobservations are to be obtained it is essential to keep the window clean.This may be a challenge in dusty locations. EUMETNET is currently networking700 European ceilometers so they can provide ceilometer profiles in nearreal time to European weather forecast centres and has adopted the cloudcalibration technique described in this paper for ceilometers with awavelength of around 910 nm.

A robust automated technique for operational calibration of ceilometers using the integrated backscatter from totally attenuating liquid clouds