A dual perspective on cloud-aerosol-radiation interaction using novel polarimetric and hyper-spectral measurements

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Interaction of clouds and aerosols is a wide field of open questions highly relevant to the atmospheric scientific community. IPCC (2014) recognizes the need for additional scientific efforts to disentangle the variety of interacting processes and their role in a changing climate. This proposal aims at developing and testing a novel concept for remote sensing of the aerosol-cloud interaction. Aerosols have direct effects on the radiation budget and they cause indirect effects by alteration of cloud microphysics and subsequent effects on cloud dynamics, precipitation, cloud lifetime, water cycle, and even changes on regional and global circulation patterns. Aerosol itself is affected by clouds: Aerosol properties and spatial distribution are affected by swelling of particles as well as by dynamical and chemical processes in the cloud. The concentration of activated cloud condensation nuclei (CCNC) plays a crucial role. CCNC can be directly determined by in-situ measurements with limited coverage. However, they cannot be measured quantitatively by conventional optical remote sensing methods since their size range is far below the wavelengths of visible radiation. For that reason an alternative concept was proposed: Cloud side measurements, measuring spectral solar radiance reflected at cloud sides, aim at deriving profiles of cloud particle size and phase. It was hypothesized that these observations allow to study the impact of aerosol on vertical development of cloud microphysics, and even to derive CCNC. CCNC may also be derived directly from aerosol optical property measurements. Correlations between them and CCNC have been identified, however, with significant uncertainy. This proposal aims at bringing together these two approaches to test the involved hypotheses. Fast scanning spectral observations will be key to address these issues, since clouds change rapidly in time. For the purpose of this project, a hyper-spectral imager will be equipped with polarization capability. With the same instrumentation derivation of microphysical properties of convective clouds as well as aerosol optical properties in the cloudless sky in between is aimed at. The work is structured into two PhD projects. Highlights 1) Test of two hypotheses which are key for the development of aircraft and proposed satellite remote sensing missions: Can CCN concentration be derived from remote sensing of aerosol properties and cloud microphysics profiles? 2) Rapid hyper-spectral scanning allows for microphysics retrievals of changing clouds; it will be tested if this information allows deriving changes of microphysical properties with distance from the cloud edge. 3) Retrieval of aerosol from polarized hyper-spectral radiance observations in presence of clouds will be expored.