Turbulent mixing in clouds latent heat and cloud microphysics effects

Turbulent mixing in clouds latent heat and cloud microphysics effects Szymon P. Malinowski1*, Mirosław Andrejczuk2, Wojciech W. Grabowski3, Piotr Korczyk4, Tomasz A. Kowalewski4 and Piotr K. Smolarkiewicz3 1 Warsaw University, Institute of Geophysics, Warsaw, Poland. Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 3 National Center for Atmospheric Research, Boulder, Colorado, USA. 4 Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland. 2 Stirring and Mixing in Turbulence: the Lagrangian Approach

MOTIVATION: Cloud droplets are particles which may grow or evaporate, depending on humidity in their closest vicinity. Growth/evapotation influences thermodynamics and microphisics of the flow. Turbulence in clouds is a two-phase reacting flow. Cloud droplets move with respect to air. This means that the transport processes of liquid water differs from other variables, like temperature, and humidity. Understanding of the above is important for our basic knowledge as well as for applications, like radiative transfer through clouds (climate), warm rain formation (weather and climate), parameterization of small-scale processes in models resolving larger scales.

Seminarium Fizyki Atmosfery

Temperature fluctuations at the cloud top.

The only experimental paper on the small-scale turbulence in free atmosphere by Siebert, Lehmann and Wendisch (2006) deal with two series of TKE measurements in BL clouds with the spatial resolution down to 15cm...at least two orders of magnitude too big to resolve scales of flow responsible for turbulence/microphysics interaction. Conclusion of the literature studies: FACT 1: Combined measurements aimed at investigations of interaction between turbulence, thermodynamics (phase change) and microphysics (cloud droplets) in small scales have never been documented. Even reliable data from the in-situ measurements of small-scale turbulence in clouds are not available. FACT2: Such measurements can hardly be performed in-situ due lacking measurement techniques. FACT 3: In theoretical studies turbulent velocities in small scales are assumed to be isotropic and are described by statistical distribution fitting laboratory/wind tunnel/atmospheric boundary layer experimental data or DNS.

In order to investigate possible differences between the small-scale turbulence in cloud undergoing mixing and the idealized turbulence assumed in theoretical works an attempt to simulate small-scale cloud-clear air mixing is undertaken (Andrejczuk et al., 2004, Andrejczuk et al., 2006). A small scale cloud turbulence is investigated by direct simulation of the microscale mixing (i.e., mixing occurring at sub-meter scales). Results from a series of idealized numerical simulations of decaying moist turbulence in a sample volume 64cm*64cm*64cm with the resolution up to 2563 gridpoints are presented.

Mixing diagram of cloudy and environmental air. Tv =T(1+ εqv+ qc) density temperature

Liquid water content after 11s of simulations.

Temporal evolution of turbulent kinetic energy (TKE) in the experiments with grid 2563 (upper panel) 1283 (middle panel) and 643 (lower panel). The evolution of TKE is governed by production due to evaporative cooling of droplets (dominating till 9th second) and dissipation.

Effect of variation of proportions of cloudy to clear air in a mixing event on production of TKE by evaporation of cloud droplets. Notice low LWC, characteristic for Sc clouds.

Effect of droplet sedimentation velocity on the evolution of mixing process illustrates importance of the cloud water transport across the cloud-clear air interface.

Distribution of horizontal (green u' and red v') and vertical (blue w') turbulent velocity fluctuations after 15s of simulations. Composite distribution of horizontal (red) and vertical (blue) turbulent velocity fluctuations from three series of measurements.

Experimental Numerical autocorrelations of longitudinal velocity components: solid red vertical, dashed - horizontal.

Ratio of the root mean square (RMS) of horizontal vertical turbulent velocity component to RMS of vertical component from numerical experiment (stars) series of laboratory and laboratory measurements (dots). Data from the numerical experiment show the temporal evolution of <u2>/<w2> (blue) and <v2>/<w2>. After first 10 seconds of simulations (spin-up time) results are similar to these measured in independent experiments and are equal to 0.5±0.1.

CONCLUSIONS: Mixing of cloud with clear air is a two-phase reacting flow. In the appropriate conditions (mixing with subsaturated air) evaporation of droplets substantially modifies smallest scales of turbulence due to: - production of TKE by buoyancy forces resulting from evaporation of droplets; -making small-scale turbulence highly anisotropic. For moderate and small values of initial TKE this influence is substantial or even dominating. Sedimentation of droplets is important as a transport mechanism of liquid water from cloudy to clear air filaments for low levels of initial TKE. This mechanism depends strongly on the droplet spectral distribution. It is likely, that small-scale anisotropy may also occur in mixing of volumes of slightly different density. This is pretty complicated and very different from homogeneous isotropic turbulence!!!!

References: Andrejczuk, M., W. W. Grabowski, S. P. Malinowski and P. K. Smolarkiewicz, 2004: Numerical simulationof cloud-clear air interfacial mixing. J. Atmos.Sci., 61, 1726-1739. Andrejczuk, M., W. W. Grabowski, S.P. Malinowski and P.K. Smolarkiewicz 2006: Numerical Simulation of Cloud-Clear Air Interfacial Mixing: Effects on cloud microphysics J. Atmos. Sci., in press. Banat, P. and S. P. Malinowski, 1999: Properties of the turbulent cloud-clear air interface observed in the laboratory experiment. Phys. Chem. Earth (B), 24, 741-745. Grabowski, W.W. and P. K. Smolarkiewicz, 2002: A multiscale model for meteorological research. Mon. Wea. Rev., 130, 939 956. Jaczewski, A and S.P. Malinowski, 2005: Spatial distribution of cloud droplets investigated in a turbulent cloud chamber, Q. J. Roy. Meteorol. Soc., 131, 2047-2062; Haman, K.E., S.P. Malinowski, M. Kurowski, H. Gerber and J-L. Brenguier, 2006: Small-scalemixing process at the top of marine Stratocumulus a case study. Q.J.Roy.Meteorol.Soc., accepted. Korczyk, P. M., S. P. Malinowski and T. A. Kowalewski, 2006: Mixing of cloud and clear air in centimeter scales observed in laboratory by means of particle image velocimetry. Atmos. Res. In press. Malinowski, S. P., I. Zawadzki and P. Banat, 1998: Laboratory observations of cloud clear air mixing in small scales. J. Atmos. Oceanic Technol., 15, 1060-1065. Siebert, H., K. Lehmann and M. Wendisch, 2006: Observations of Small Scale Turbulence and energy Dissipation Rates in Cloudy Boundary Layer. J. Atmos. Sci., in press.