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Retrieval of vertical profiles of cloud droplet effective radius using solar reflectance from cloud sides
Retrieval of vertical profiles of cloud droplet effective radius using solar reflectance from cloud sides
Convective clouds play an essential role for Earth’s climate as well as for regional weather events since they have a large influence on the global radiation budget and the global water cycle. In particular, cloud albedo and the formation of precipitation are influenced by aerosol particles within clouds. In order to improve the understanding of processes from aerosol activation, over cloud droplet growth to changes in cloud properties, remote sensing techniques to monitor these microphysical processes throughout the cloud are becoming more and more important. While passive retrievals for spaceborne observations have become sophisticated and commonplace to infer cloud optical thickness and droplet size from cloud tops, cloud sides have remained largely uncharted territory for passive remote sensing. Faced with the small-scale structure of cloud sides, ‘classical’ passive remote sensing techniques, like Nakajima-King, are rendered inappropriate. The aim of this work is to test the theoretical and practical feasibility to gain new insights into the vertical evolution of cloud droplet effective radius by using reflected solar radiation from cloud sides. A central aspect of this study was the close analysis of the impact unknown cloud surface geometry has on effective radius retrievals. In order to handle spatially highly resolved measurements from cloud sides, this work therefore rethought the Nakajima-King approach in the context of a unknown cloud surface geometry. Using extensive Monte-Carlo calculations to explore 3D-effects at convective cloud sides, the sensitivity of reflected solar radiation to cloud droplet size was examined. Furthermore, a method was established to resolve ambiguous radiance regions and thus enhance this sensitivity. Influencing factors were identified and masked out like shadows, ground reflections and cloud ice phase. Based on these findings, a statistical approach was used to develop an effective radius retrieval. Putting the method into practice, the new hyperspectral cloud and sky imager specMACS (Spectrometer of the Munich Aerosol Cloud Scanner) was developed and thoroughly characterized in this work. Additionally, the instrument was applied to convective cloud sides from a ground-based perspective as well as on board the new German research aircraft HALO (High Altitude and LOng Range Research Aircraft). In order to validate this approach, the retrieval was compared to aircraft in situ measurements made during the ACRIDICON-CHUVA experiment conducted over the Brazilian rain forest. The present thesis demonstrates the feasibility to retrieve cloud particle size profiles from cloud sides and thus marks a further important step towards an operational application of this technique.
3D Radiative Transfer, Cloud side remote sensing, Hyperspectral Imaging
Ewald, Florian
2016
Englisch
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Ewald, Florian (2016): Retrieval of vertical profiles of cloud droplet effective radius using solar reflectance from cloud sides. Dissertation, LMU München: Fakultät für Physik
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Abstract

Convective clouds play an essential role for Earth’s climate as well as for regional weather events since they have a large influence on the global radiation budget and the global water cycle. In particular, cloud albedo and the formation of precipitation are influenced by aerosol particles within clouds. In order to improve the understanding of processes from aerosol activation, over cloud droplet growth to changes in cloud properties, remote sensing techniques to monitor these microphysical processes throughout the cloud are becoming more and more important. While passive retrievals for spaceborne observations have become sophisticated and commonplace to infer cloud optical thickness and droplet size from cloud tops, cloud sides have remained largely uncharted territory for passive remote sensing. Faced with the small-scale structure of cloud sides, ‘classical’ passive remote sensing techniques, like Nakajima-King, are rendered inappropriate. The aim of this work is to test the theoretical and practical feasibility to gain new insights into the vertical evolution of cloud droplet effective radius by using reflected solar radiation from cloud sides. A central aspect of this study was the close analysis of the impact unknown cloud surface geometry has on effective radius retrievals. In order to handle spatially highly resolved measurements from cloud sides, this work therefore rethought the Nakajima-King approach in the context of a unknown cloud surface geometry. Using extensive Monte-Carlo calculations to explore 3D-effects at convective cloud sides, the sensitivity of reflected solar radiation to cloud droplet size was examined. Furthermore, a method was established to resolve ambiguous radiance regions and thus enhance this sensitivity. Influencing factors were identified and masked out like shadows, ground reflections and cloud ice phase. Based on these findings, a statistical approach was used to develop an effective radius retrieval. Putting the method into practice, the new hyperspectral cloud and sky imager specMACS (Spectrometer of the Munich Aerosol Cloud Scanner) was developed and thoroughly characterized in this work. Additionally, the instrument was applied to convective cloud sides from a ground-based perspective as well as on board the new German research aircraft HALO (High Altitude and LOng Range Research Aircraft). In order to validate this approach, the retrieval was compared to aircraft in situ measurements made during the ACRIDICON-CHUVA experiment conducted over the Brazilian rain forest. The present thesis demonstrates the feasibility to retrieve cloud particle size profiles from cloud sides and thus marks a further important step towards an operational application of this technique.