Sub-Millimeter and Thermal Infrared Wavelength Retrieval of Ice Cloud Properties Using an Optimal Estimation Algorithm

Ice clouds play an important role in the Earth’s climate system, particularly through their influence on the global energy budget. Ice clouds have profound impacts on the energy budget, such as cooling to the earth-atmosphere system due to their reflection of incoming solar radiation, and warming due to absorption of infrared radiation from lower atmosphere and the surface. These opposing contributions, coupled with the complex and variable structure of iceclouds and their significant global coverage, pose large uncertainties in understanding the current climate and potential future changes. Cloud properties, such as ice mass, are prognostic variables used to parameterize cloud radiative effects in many General Circulation Models (GCMs). Thus, accurate knowledge of ice cloud properties is essential in reducing model uncertainties. Ice water path (IWP), or the integration of ice mass over a vertical column, varies by as much as order of magnitude in climatologies computed from different models. Inconsistent assumptions in modelsabout ice particle properties, such as the effective diameter (De), contribute to these discrepancies. Therefore, it is necessary to develop methodologies for accurate remote sensing of ice cloud properties, particularly IWP and De, to improve the performance of GCMs.

Advantages of sub-millimeter (sub-mm) wave instruments for characterizing of ice cloud properties, in contrast to traditional visible and infrared techniques, have been the focus of recent studies. In our previous work we performed sensitivity studies to explore the applicability of combining sub-mm and thermal infrared radiometric measurements to retrieve IWP and De. In this presentation, we deliver the continuation of previous work, with a focus on the retrieval methodology. An optimal estimation retrieval scheme is selected to infer cloud two ice cloud properties simultaneously: IWP and De, using sub-mm (874 GHz) and thermal infrared (TIR) wavelengths. This method is based on the atmospheric radiative transfer simulator (ARTS) and is intended for applications to IceCube sub-mm and MODerate resolution Imaging Spectroradiometer (MODIS) infrared radiances. Additionally, this study conducts thorough information content and error analyses to understand the error propagation and performance of retrievals under different cloud/atmosphere states.