Helicopter-borne measurements of small-scale processes in shallow cumulus convection over Barbados

• Antragsteller: Dr. Holger Siebert; Professor Dr. Manfred Wendisch
• Fachliche Zuordnung: Physik und Chemie der Atmosphäre
• Förderung: Förderung von 2010 bis 2015
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 164011789

• Erstellungsjahr: 2014

Zusammenfassung der Projektergebnisse

Two intensive field campaigns called CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over Barbados) were performed in November 2010 and April 2011 near Barbados deploying the helicopter-borne measurement payloads ACTOS (Airborne Cloud Turbulence Observation System) and SMART-Helios (Spectral Modular Airborne Radiation measurement system). This unique combination of ACTOS and SMART-Helios allowed collocated measurements of microphysical and dynamical properties combined with radiative properties of trade wind clouds. The overall goal of this project was to obtain a better understanding of the fine-scale structure of aerosol and cloud microphysical parameters, cloud radiative properties, and cloud turbulence and their respective feedbacks. The trade wind region is characterized by extensive fields of shallow cumulus clouds, which are mostly fluffy and rarely reach higher than 1.5 to 2 km. However, they may produce drizzle and precipitation in a rather short time. Due to the relatively constant meteorological conditions, this area is favorable for studying many aspects of shallow cumulus convection under nearly reproducible conditions. During both campaigns, time periods with relatively low and high particle number concentrations were observed, with values as low as 100 cm3 (typical pristine conditions) and above 500 cm3 (typical polluted conditions). The notations pristine and polluted should be considered in the context of the generally clean maritime conditions. Increased particle number concentrations in the cloud condensation nuclei size range were observed when air masses originated from Africa. During the first campaign, these aerosol particles consisted of a mixture of dust and aerosol particles from biomass burning, while during the second campaign the aerosols originating from Africa were mainly dust. Additionally, measurements in April were made in clouds influenced by biomass burning and anthropogenic aerosol from the Caribbean. Altogether, a wide range of different situations in terms of aerosol load was observed during the two campaigns which allows to systematically investigate the influence of aerosol on the cloud microstructure and cloud radiation properties under quite similar meteorological conditions. As a first result, it could be quantitatively shown that the influence of the aerosol number concentration and the shape of the corresponding size distribution on cloud microphysical properties dominate over the influence of the observed updraft velocities. Based on accompanying numerical studies it could be shown that the influence of the updraft velocities increases with increasing number of aerosol particles in the Aitken mode range (≤80 nm). However, for this observation it could also be shown that the chemical composition of the aerosol particles has only a minor effect on aerosol activation. Furthermore, due to the close collocation of all measurements this study allowed for a convincing observational proof of the first indirect radiative effect of aerosol particles on cloud and radiation properties (Twomey effect). This effect is usually difficult to observe but of high importance when discussing an anthropogenic influence on cloud fields and, therefore, on the Earth’s energy budget. Another important result was the frequently observed bursts of ultrafine particles, which were most likely freshly nucleated in the vicinity of clouds. The horizontal extend of such events is typically smaller than 100 m which is difficult to observe with research aircraft equipped with commercially available particle counters with low temporal resolution. As one possible trigger for this new particle formation an increased irradiance at cloud edges is discussed which favors photochemical reactions. Our combination of radiation measurements and high-resolution particle measurements at the cloud edge corroborates this hypothesis. Finally, CARRIBA data was analyzed in order to illustrate the fine-scale structure of cloud edges depending on the different stages of their lifetime. Subsiding shells around the cumuli with increased local turbulence due to strong vertical wind shear were frequently observed. These shells enhance the turbulent exchange of cloud air with sub-saturated environmental air and it was shown that the width of these shells increases on the expense of the cloud interior with increasing cloud age. In addition to several newspaper articles (http://idw-online.de/de/news416168), the Barbados activities got attention in a TV series about global warming (arte-TV „Klima konkret: Von Kyoto nach Grindelwald“).

Projektbezogene Publikationen (Auswahl)

• 2010: Collocated measurements of boundarylayer cloud microphysical and radiative properties - A feasibility study. J. Geophys. Res., 115, D24214
  Henrich, F., H. Siebert, E. Jäkel, R. A. Shaw, and M. Wendisch
  
• 2013: Atmospheric Radiation Measurements. In Wendisch, M., and J.-L. Brenguier (Eds.), Airborne Measurements for Environmental Research: Methods and Instruments., Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim, Germany. ISBN: 978-3-527-40996-9
  Wendisch, M., P. Pilewskie, B. Bohn, A. Bucholtz, S. Crewell, C. Harlow, E. Jkel, K. S. Schmidt, R. Shetter, J. Taylor, D. D. Turner, and M. Zger
  
• 2013: Measurement of Aircraft State, Thermodynamic and Dynamic Variables. in: Airborne Measurements for Environmental Research: Methods and Instruments. Eds. M. Wendisch and J.-L. Brenguier, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. ISBN: 978-3-527-40996-9
  Bange, J., M. Esposito, D. H. Lenschow, P. R. A. Brown, V. Dreiling, A. Giez, L. Mahrt, S. Malinowski, A. R. Rodi, R. A. Shaw, H. Siebert, H. Smit, and M. Zöger
  
• 2013: New airborne retrieval approach for trade wind cumulus properties under overlying cirrus. J. Geophys. Res., 118, 3634–3649
  Werner, F., H. Siebert, P. Pilewskie, T. Schmeissner, R. A. Shaw, and M. Wendisch
  
• 2013: Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry. Atmospheric Measurement Techniques, 6, 1855–1868
  Schäfer, M., E. Bierwirth, A. Ehrlich, F. Heyner, and M. Wendisch
  
• 2013: The fine-scale structure of the trade wind cumuli over Barbados - an introduction to the CARRIBA project. Atmospheric Chemistry and Physics, 13, 10061–10077
  Siebert, H., M. Beals, J. Bethke, E. Bierwirth, T. Conrath, K. Dieckmann, F. Ditas, A. Ehrlich, D. Farrell, S. Hartmann, M. A. Izaguirre, J. Katzwinkel, L. Nuijens, G. Roberts, M. Schäfer, R. A. Shaw, T. Schmeissner, I. Serikov, B. Stevens, F. Stratmann, B. Wehner, M. Wendisch, F. Werner, and H. Wex
  
• 2014: Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus. J. Geophys. Res., 119, 1534–1545
  Werner, F., F. Ditas, H. Siebert, M. Simmel, B. Wehner, P. Pilewskie, T. Schmeissner, R. A. Shaw, S. Hartmann, H. Wex, G. C. Roberts, and M. Wendisch