Turbulence influence on cloud droplet growth: A computational investigation of the Pi Chamber using large eddy simulation
Abstract
Experiments in the Michigan Tech Pi Chamber have demonstrated the importance of supersaturation fluctuations in broadening of cloud droplet size distributions and cloud cleansing processes. Can the observed processes in the laboratory be faithfully represented in a numerical simulation? That would allow an approach not only for validating the simulation approach, but also for scaling results to an atmospheric context. Here we explore the aerosol-cloud interaction in turbulent environment through large eddy simulations (LES) of the full volume of the Pi Chamber, and compare results with the experimental point measurements.
The System for Atmospheric Modeling is a LES code with demonstrated capability to reproduce micro- and mesoscale atmospheric phenomena, and is complemented with spectral bin microphysics from the Hebrew University Cloud Model. The combined system is utilized to explore the physics of the flow and cloud properties in the Pi Chamber simulated with uniform grid size of 3.125 cm and time step of 0.02 second for a physical time of 2 hours. Unlike the open environment in the atmosphere, the Pi Chamber has top and lateral wall momentum and scalar fluxes, apart from the bottom fluxes. Hence, boundary conditions in SAM have been modified to accommodate these additional constraints. The chamber is initialized with an unstable temperature and saturated vapor gradient, and allowed to evolve temporally. On reaching a steady state, a mono-dispersed CCN distribution is introduced continuously to reach a dynamic balance between the activated and removed droplets (droplet removal is by gravitational sedimentation). The turbulence-microphysics interactions are evaluated at the core of the chamber to limit the boundary effects. Simulated turbulent kinetic energy and energy dissipation rate match the measurements within the experimental errors. Further, LES also quantitatively matches an observed slow oscillations of large-scale circulation in Rayleigh-Bénard convection in the Pi Chamber. The microphysics, with collisions turned off, is capable of reproducing the broadening of the cloud droplet size distribution resulting from turbulence-microphysics interactions. When droplet collisions are included, the change in the size distribution is negligible, confirming prior estimates.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A21K2855T
- Keywords:
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- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSESDE: 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSESDE: 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSESDE: 3314 Convective processes;
- ATMOSPHERIC PROCESSES