Simulations Reveal Details of Mixing of a Cloud with Dry Air

The Science

When environmental air becomes trapped in a cloud, a process known as entrainment can affect cloud radiative properties, precipitation formation, and ultimately the entire climate system. However, understanding how clouds mix with surrounding air at small scales remains a large challenge. If droplets evaporate quickly at the cloud edges before turbulence thoroughly mixes cloudy air with dry air, entrainment tends to decrease the droplet number concentration. If turbulence mixes the dry air and cloud droplets rapidly, the entrainment primarily reduces droplet size. Laboratory experiments have shown that evaporation is generally faster, but the results may appear as if turbulent mixing dominates when comparing different entrainment events. This study uses numerical simulations to confirm the laboratory experiments, analyze additional experimental variables, and reveal three-dimensional spatial details that local sensors in the laboratory cannot capture.

The Impact

Better understanding how clouds mix with surrounding dry air is crucial for accurate weather and climate predictions. This research shows how advanced computer simulations of cloud processes can be used to reproduce, interpret, and extend the entrainment experiment in a convection cloud chamber. The laboratory-observed entrainment signatures are confirmed by numerical simulations. Additionally, simulations show that the signature of evaporation being faster than turbulent mixing is weakened in a polluted cloud or/and with a coarser grid spacing. Lastly, the simulations provide the spatial distribution of cloud properties for examining how the entrained dry air mixes with the cloud in more detail. These results guide further laboratory experiments and help scientists refine cloud representations in weather and climate models.

Summary

Large-eddy simulations and size-resolved microphysics can resolve time scales for turbulent mixing and evaporation. This makes them well-suited tools for reproducing, extending, and interpreting the entrainment experiment in the convection-cloud chamber. The results from these simulations confirm: (i) the inhomogeneous mixing signature (i.e., evaporation of cloud droplets is faster than turbulent mixing) for an individual entrainment event, and (ii) the appearance of homogeneous mixing (i.e., turbulent mixing is faster) in an ensemble of entrainment episodes. This study also demonstrated that the inhomogeneous mixing signature is more pronounced in a polluted cloud, but coarser grid spacing in simulations could reduce the accuracy of this signature. Lastly, simulations facilitate an easy analysis of wall flux adjusted for varying entrainment intensities, which explains the homogeneous mixing signature. This mechanism is challenging to confirm in cloud chamber experiments, as there is no direct method to measure wall flux. 

Contacts

Aaron Wang, corresponding author, Pacific Northwest National Laboratory, aaron.wang@pnnl.gov

Mikhail Ovchinnikov, principal investigator, Pacific Northwest National Laboratory, mikhail.ovchinnikov@pnnl.gov

Funding

Funding for this work was provided by the Department of Energy (DOE), Office of Science Biological and Environmental Research program as part of the Atmospheric System Research program’s “A Community Laboratory Facility for Exploring and Sensing of Aerosol-Cloud-Drizzle Processes: The Aerosol-Cloud-Drizzle Convection Chamber” project. Computing resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.