HR: 17:30h
AN: A44A-07    [Abstracts]
TI: Observations of Homogeneous and Inhomogeneous Mixing in Warm Cumulus Clouds
AU: * Lehmann, K
EM: kalehmann@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0221, United States
AU: Siebert, H
EM: siebert@tropos.de
AF: Leibniz-Institute for Tropospheric Research, Permoser Str. 15, Leipzig, 04318, Germany
AU: Shaw, R A
EM: rashaw@mtu.edu
AF: Leibniz-Institute for Tropospheric Research, Permoser Str. 15, Leipzig, 04318, Germany
AB: The helicopter-borne instrument payload ACTOS was used to study the entrainment/mixing process in shallow warm cumulus clouds. Using ACTOS, high resolution measurements of the three-dimensional wind, temperature and humidity fields were made. In addition, cloud microphysical parameters such as the droplet number concentration and size were measured with a modified Fast-FSSP. The effect of entrained subsaturated air on the droplet number size distribution was analyzed using mixing diagrams which correlate droplet number concentration and droplet size. Both homogeneous and inhomogeneous mixing was observed to take place. The characteristic of the mixing process is compared to the Damköhler number. The Damköhler number is given by the ratio of the timescale for turbulent mixing and the reaction timescale, which is either the time for droplet evaporation, or the phase relaxation timescale. With ACTOS' instrumentation, the Damköhler number can be determined with a spatial resolution of about 15 m. In agreement with literature, low values of the Damköhler number correlate with the homogeneous mixing scenario, while higher values of the Damköhler number correlate with the inhomogeneous mixing scenario. It is shown that even within one cloud, different mixing scenarios can take place. The data suggest that homogeneous mixing is more likely to occur in the vicinity of the vigorous cloud core, while inhomogeneous mixing dominates in the outer, less turbulent part of the cloud. A case is presented in which the mixing led to the formation of drops that are larger than in the unmixed adiabatic core. This is of potential importance for precipitation formation in warm cumulus clouds.
DE: 0320 Cloud physics and chemistry
DE: 3300 ATMOSPHERIC PROCESSES
DE: 3307 Boundary layer processes
DE: 3311 Clouds and aerosols
DE: 3379 Turbulence (4490)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting