Interaction between turbulent mixing processes and cloud microphysical properties in stratiform boundary layer clouds
Final Report Abstract
Small-scale turbulence in clouds, e.g., velocity fluctuations down to the mm-scale play an essential role for the development of clouds, the cloud life cycle and finally for the onset of precipitation due to droplet growth by droplet collisions and coalescence. Turbulence at cloud edges causes mixing of drop-free and subsaturated air into the cloud - so-called entrainment - and subsequent mixing inside the cloud modifies the droplet sizes and number concentrations. To measure and understand these small-scale processes in more detail was the main goal of this project. We used the helicopter-borne measurement payload ACTOS (Airborne Cloud Turbulence Observation System) to perform combined and spatial closely collocated measurements of turbulence and cloud microphysical parameters. This was the first time a slow-flying helicopter was used for cloud measurements which results in a spatial resolution of the turbulence measurements in the order of a few centimeter never accomplished before with fast-flying research aircraft. The first cloud measurement flights with a helicopter in Koblenz/Wlnningen attracted some public Interest and were accompanied by the television (see "[W] wie Wissen" (ARD, Oct. 29, 2006) and "hitec" (3sat, Nov. 11,2006)). With these new observations, the mixing process in warm cumulus clouds could be investigated in a much more detailed way. The consequence of turbulent mixing In clouds for the cloud droplets (e.g., sizes and number concentration) is closely linked to the key question of this process: Which part of the process is faster, the turbulent mixing or the evaporation/condensation of droplets in a new environment (so-called homogeneous and inhomogeneous mixing)? It was found that the two completely different mixing scenarios can take place at the same time in different cloud regions which is a direct consequence of our second main result: Turbulence In clouds Is highly Intermittent on all scales which means that regions with strong velocity fluctuations and regions with calm turbulence can be found close together. Therefore, describing the turbulence state of an entire cloud with averaged parameters Is crude without considering the high spatial and temporal variability of velocity fluctuations. Motivated by these observations, a few concepts to describe the mixing process have been refined. This results should stimulate modelers to develop a more sophisticated parametrlzatlon of the collision process which takes into account the intermittent character of cloud turbulence.
Publications
- Shaw, R. A., K. Lehmann, E. W. Saw, and H. Siebert: 2006, 'Simultaneous Measurements of droplet clustering and turbulence in cumulus clouds'. In: 12th Conference on Cloud Ptiysics, 10 - 14 July, Madison, Wl, USA.
- Siebert, H., H. Franke, K. Lehmann, R. Maser, E. W. Saw, R. A. Shaw, D. Schell, and M. Wendisch: 2006, 'Probing Fine-Scale Dynamics and Microphysics of Clouds with Helicopter-Borne Measurements'. Bull. Am. Meteor. Soc. 87, 1727-1738.
- Siebert, H., K. Lehmann, M. Wendisch, and R. Shaw: 2006, 'The helicopter-borne ACTOS for small-scale cloud turbulence observations'. In: 12th Conference on Cloud Physics, 10 - 14 July, Madison, Wl, USA.
- Lehmann, K., H. Siebert, M. Wendisch, and R. Shaw: 2007, 'Evidence for inertial droplet clustering in weakly turbulent clouds'. Tellus 59B, 57-65.
- Collins, L. R., S. Ayyalasomayajula, S. Gerashchenko, K. Lehmann, R. A. Shaw, H. Siebert, and Z. Warhaft: 2008, 'Experimental measurements and numerical modeling of inertial particles In high-Reynolds-number turbulence'. In: Proceedings 15th International Conference on Clouds and Precipitation. Cancun, Mexico, July 7-11 2008.
- Lehmann, K., R. A. .Shaw, and H. Siebert: 2008, 'Observations of homogeneous and inhomogeneous mixing in cumulus clouds'. In: Proceedings 15th International Conference on Clouds and Precipitation. Cancun, Mexico, July 7-11 2008.
- Siebert, H. and R. A. Shaw: 2008, 'The small-scale structure of turbulence in marine stratocumulus'. In: Proceedings 15th International Conference on Clouds and Precipitation. Cancun, Mexico, July 7-11 2008.
- Lehmann, K.: 2007, 'Experimental Investigations of the Influence of Turbulent Mixing on Cloud Microphysical Processes'. Ph.D. thesis, University of Leipzig
- Siebert, H., K. Lehmann, and M. Wendisch: 2006, 'Observations of small scale turbulence and energy dissipation rates in the cloudy boundary layer.'. J. Atmos. Sei. 63,1451 - 1466.
- Siebert, H., K. Lehmann, and R. Shaw: 2007, 'On the use of a hot-wire anemometer for turbulence measurements in clouds'. J. Atmos. Oceanic Technol. 24, 980 - 993.
- Siebert, H., K. Lehmann, M. Wendisch, and R. Shaw: 2006, 'Small-scale Turbulence in Clouds'. In: 12th Conference on Cloud Physics. 10 - 14 July, Madison, Wl, USA.
