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. With the rapidly growing number of automated single-wavelength backscatterlidars (ceilometers), their potential benefit for aerosol remote sensingreceived considerable scientific attention. When studying the accuracy ofretrieved particle backscatter coefficients, it must be considered that mostof the ceilometers are influenced by water vapor absorption in the spectralrange around 910 nm. In the literature methodologies have been proposed to correct for thiseffect; however, a validation was not yet performed. Inthe framework of the ceilometer intercomparison campaign CeiLinEx2015 inLindenberg, Germany, hosted by the German Weather Service, it was possible totackle this open issue. Ceilometers from Lufft (CHM15k and CHM15kx, operatingat 1064 nm), from Vaisala (CL51 and CL31) and from Campbell Scientific(CS135), all operating at a wavelength of approximately 910 nm, weredeployed together with a multi-wavelength research lidar (RALPH) that servedas a reference. In this paper the validation of the water vapor correction isperformed by comparing ceilometer backscatter signals with measurements ofthe reference system extrapolated to the water vapor regime. One inherentproblem of the validation is the spectral extrapolation of particle opticalproperties. For this purpose AERONET measurements and inversions of RALPHsignals were used. Another issue is that the vertical range where validationis possible is limited to the upper part of the mixing layer due to incompleteoverlap and the generally low signal-to-noise ratio and signal artifactsabove that layer. Our intercomparisons show that the water vapor correctionleads to quite a good agreement between the extrapolated reference signal andthe measurements in the case of CL51 ceilometers at one or more wavelengthsin the specified range of the laser diode's emission. This ambiguity is dueto the similar effective water vapor transmission at several wavelengths. Inthe case of CL31 and CS135 ceilometers the validation was not alwayssuccessful. That suggests that error sources beyond the water vaporabsorption might be dominant. For future applications we recommend monitoringthe emitted wavelength and providing “dark” measurements on a regularbasis.

Aerosol backscatter profiles from ceilometers: validation of water vapor correction in the framework of CeiLinEx2015