An algorithm to retrieve ice water content profiles in cirrus clouds from the synergy of ground-based lidar and thermal infrared radiometer measurements

摘要

The algorithm presented in this paper was developed to retrieve ice watercontent (IWC) profiles in cirrus clouds. It is based on optimal estimationtheory and combines ground-based visible lidar and thermal infrared (TIR)radiometer measurements in a common retrieval framework in order to retrieveprofiles of IWC together with a correction factor for the backscatterintensity of cirrus cloud particles. As a first step, we introduce a methodto retrieve extinction and IWC profiles in cirrus clouds from the lidarmeasurements alone and demonstrate the shortcomings of this approach due tothe backscatter-to-extinction ambiguity. As a second step, we show that TIRradiances constrain the backscattering of the ice crystals at the visiblelidar 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 realisticbulk ice microphysical model. The scattering phase function obtained from themicrophysical model is flat around the backscatter direction (i.e., there isno backscatter peak). We show that using this flat backscattering phasefunction to define the backscatter-to-extinction ratio of the ice crystals inthe retrievals with the lidar-only algorithm results in an overestimation ofthe IWC, which is inconsistent with the TIR radiometer measurements. Hence, asynergy algorithm was developed that combines the attenuated backscatterprofiles measured by the lidar and the measurements of TIR radiances in acommon optimal estimation framework to retrieve the IWC profile together witha correction factor for the phase function of the bulk ice crystals in thebackscattering direction. We show that this approach yields consistent lidarand TIR results. The resulting lidar ratios for cirrus clouds are found to beconsistent with previous independent studies.