A novel method for ground-based and mobile remote sensing of profiles of cloud microphysical properties using scanning radiometry supplemented by Lidar
Zusammenfassung der Projektergebnisse
The ground-based remote sensing instrument package LIRAS (LIdar and RAdiation System for cloud profiling) and aircraft measurements using a hyperspectral imaging system (specMACS: spectral Munich Aerosol Cloud Scanner) were used to derive vertical profiles of microphysical parameters of convective clouds from cloud side observations. For both applications, profiles of the thermodynamic phase were retrieved by spectral information of the radiation measurements. Based on the differential absorption by ice and liquid water in the spectral range between 1550 and 1700 nm, we defined a phase index, which shows negative values for liquid particles, whereas ice particles are characterized by a positive phase index. It was shown by 3D radiative transfer simulations that the mixed phase zone is characterized by a significant gradient in the profile of the phase index. In this layer, phase transition from liquid water via the mixed phase to ice takes place, which is linked to the vertical evolution of convective clouds. The discrimination between shadowed and illuminated cloud regions is essential, because in shadowed regions the spectral signature is contaminated by diffuse radiation coming from unknown directions. We found that the cloud masking method developed for groundbased observations was not applicable for aircraft measurements. While we could use the spectral signature of the surface albedo in cloud shadow regions as indicator for groundbased measurements, we used the frequency distribution of color values of the observed cloud scene to identify the illuminated and shadowed cloud parts for aircraft observations. 3D radiative transfer simulations were performed to validate the approach. We deployed the phase retrieval during the HALO (High LAtitude and LOng range research aircraft) campaign ACRIDICON-CHUVA in 2014 over the Brazilian rainforest. The vertical allocation of the observed cloud element and the cloud distance was estimated from stereographic analysis of additional video camera data. Vertical profiles of the cloud phase were derived for different cloud scenes on three flight days under moderate and polluted aerosol conditions. It was found, that the height and thickness of the mixed phase layer can vary up to 900 m in upper and lower limit for one single cloud scene, mainly attributed to the different stages of cloud development in a scene. Comparing the glaciation heights for moderate aerosol conditions (6.8 ± 0.2 km) and polluted aerosol conditions (7.4 ± 0.4 km), we found an indication of an increase of glaciation height and a decrease of glaciation temperature for polluted aerosol conditions, but also a shift of the lower boundary of the mixed phase layer from 5.5 - 6.0 km (moderate case) to 6.0 - 6.5 km for polluted conditions, which agrees with theory. Furthermore, the retrieved profiles were compared with in situ measurements and satellite observations. Consistent results of mixed phase zone levels were found from specMACS and in situ measurements. In contrast, the glaciation temperature derived from cloud top measurements of MODIS by applying the so-called ensemble method (which assumes time–space–exchangeability for a cluster of clouds with different states of evolution) deviates by more than 1.5 km from in situ and cloud side measurements for the polluted case. In general the ensemble method can give an indication when phase transition arises for the first time. However, for estimation of the cloud phase profile at a later stage of the DCC evolution, in situ and also cloud side remote sensing might be the better observation strategy, when phase distribution is altered for example by up- and downdrafts. Wie Forscher Kumulus, Zirrus und Co von Innen erkunden, Abenteuer Wolkenforschung, NZZ, 10 June 2015
Projektbezogene Publikationen (Auswahl)
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Thermodynamic phase retrieval of convective clouds: impact of sensor viewing geometry and vertical distribution of cloud properties, Atmos. Meas. Tech., 6, 539–547
Jäkel, E., Walther, J., and Wendisch, M.
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Analysis of the Vertical Distribution of the Thermodynamic Phase in Tropical Deep-convective Clouds, in Light, Energy and the Environment, OSA Technical Digest (online) (Optical Society of America, 2016), paper HTu2F.1
E. Jäkel, M. Wendisch, F. Ewald, and T. Kölling
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Spectral optical layer properties of cirrus from collocated airborne measurements and simulations, Atmos. Chem. Phys., 16, 7681-7693
Finger, F., Werner, F., Klingebiel, M., Ehrlich, A., Jäkel, E., Voigt, M., Borrmann, S., Spichtinger, P., and Wendisch, M.
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The ACRIDICON-CHUVA campaign: Studying tropical deep convective clouds and precipitation over Amazonia using the new German research aircraft HALO, Bull. Am. Meteorol. Soc.
Wendisch, M., Pöschl, U., Andreae, M. O., Machado, L. A. T., Albrecht, R., Schlager, H., Rosenfeld, D., Martin, S. T., Abdelmonem, A., Afchine, A., Araujo, A., Artaxo, R., Aufmhoff, H., Barbosa, H. M. J., Borrmann, S., Braga, R., Buchholz, B., Cecchini, M. A., Costa, A., Curtius, J., Dollner, M., Dorf, M., Dreiling, V., Ebert, V., Ehrlich, A., Ewald, F., Fisch, G., Fix, A., Frank, F., Fütterer, D., Heckl, C., Heidelberg, F., Hüneke, T., Jäkel, E., ... and Zöger, M.
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Directional, horizontal inhomogeneities of cloud optical thickness fields retrieved from ground-based and airbornespectral imaging, Atmos. Chem. Phys., 17, 2359- 2372
Schäfer, M., Bierwirth, E., Ehrlich, A., Jäkel, E., Werner, F., and Wendisch, M.
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Vertical distribution of the phase state of particles in tropical deep-convective clouds as derived from cloud-side reflected solar radiation measurements. Atmos. Chem. Phys.
Jäkel, E., Wendisch, M., Krisna, T. C., Ewald, F., Kölling, T., Jurkat, T., Voigt, C., Cecchini, M. A., Machado, L. A. T., Afchine, A., Costa, A., Krämer, M., Andreae, M. O., Pöschl, U., Rosenfeld, D., and Yuan, T.
