Abstract
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 in order to retrieve profiles of IWC together with a correction factor for the backscatter intensity of cirrus cloud particles. As 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. As 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 is flat around the backscatter direction (i.e., there is no backscatter peak). We show that using this flat backscattering phase function to define the backscatter-to-extinction ratio of the ice crystals in the retrievals with the lidar-only algorithm results in an overestimation of the IWC, which is inconsistent with the TIR radiometer measurements. Hence, a synergy algorithm was developed that combines the attenuated backscatter profiles measured by the lidar and the measurements of TIR radiances in a common optimal estimation framework to retrieve the IWC profile together with a correction factor for the phase function of the bulk ice crystals in the backscattering direction. We show that this approach yields consistent lidar and TIR results. The resulting lidar ratios for cirrus clouds are found to be consistent with previous independent studies.
Highlights
The importance of clouds for the climate system has been extensively discussed during the last decades (Stephens, 2005)
The algorithm is based on optimal estimation theory and combines the visible lidar and thermal infrared (TIR) radiometer measurements in a common retrieval framework to retrieve profiles of ice water content (IWC) together with a correction factor for the backscatter intensity of bulk ice cloud particles
Due to the backscatter-to-extinction ambiguity arising from the combination of scattering and absorption processes in the atmosphere, assumptions are required for the backscatter-to-extinction ratio, and the retrieval results strongly depend on these assumptions
Summary
The importance of clouds for the climate system has been extensively discussed during the last decades (Stephens, 2005). The important but complex impact of cirrus clouds has long been recognized (Liou, 1986) but is still not well quantified. The net radiative effect of cirrus clouds is generally positive but can be negative as well (Zhang et al, 1999). It is determined on the one hand by the macrophysical cloud properties, e.g., altitude, geometrical thickness, temperature, and the difference between the temperature of the cloud and the surface (e.g., Stephens and Webster, 1981).
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
