An algorithm to retrieve ice water content profiles in cirrus cloudsfrom the synergy of ground-based lidar and thermal infraredradiometer measurements

Abstract. The algorithm presented in this paper was developed to retrieve ice water content (IWC) profiles in cirrus clouds. It is based on optimal estimation theory and combines ground-based visible lidar and thermal infrared (TIR) radiometer measurements in a common retrieval framework to retrieve profiles of IWC together with a correction factor for the backscatter intensity of cirrus cloud particles. In a first step, we introduce a method to retrieve extinction and IWC profiles in cirrus clouds from the lidar measurements alone and demonstrate the shortcomings of this approach due to the backscatter-to-extinction ambiguity. In a second step, we show that TIR radiances constrain the backscattering of the ice crystals at the visible lidar wavelength by constraining the ice water path (IWP) and hence the IWC which is linked to the optical properties of the ice crystals via a realistic bulk ice microphysical model. The scattering phase function obtained from the microphysical model has a flat ending without backscattering peak. We show that retrievals with the lidar only algorithm using this phase function in backscattering direction to define the backscatter-to-extinction ratio of the ice crystals result in an overestimation of IWC which is inconsistent with the TIR radiometer measurements. Hence, a synergy algorithm was developed that combines the profiles of backscattering measured by the lidar and the measurements of TIR radiances in a common optimal estimation framework to retrieve together with the IWC profile a correction factor for the phase function of the bulk ice crystals in backscattering direction. We show that this approach allows to simultaneously converge towards the measurements of two independent instruments and that the first results of the retrieved lidar ratios for cirrus clouds agree with previous studies.