PNNL

The Science 

Stratocumulus clouds cover a large fraction of Earth’s surface and strongly influence regional weather and the global energy balance. Entrainment, the mixing process at the top of the stratocumulus-topped boundary layer, plays a critical role in cloud lifetime, precipitation, and radiative properties. Despite its importance, entrainment remains poorly understood, largely due to the difficulty of observing fine-scale structures near the cloud top, which can be only meters thick. We therefore propose a laboratory facility with controllable conditions and use numerical simulations to demonstrate how cloud-top entrainment can be studied experimentally.

The Impact 

This study highlights the effectiveness of numerical simulations in designing, interpreting, and extending laboratory experiments. Additionally, a tall convection-cloud chamber with adjustable wall temperature and moisture has been conceptualized to study droplet collision–coalescence, a critical process in the transition from cloud to drizzle. This work enhances the value of such a facility by demonstrating how it can be used to investigate cloud-top entrainment, another key process that influences cloud properties and Earth’s water and energy balance.

Summary 

Stratocumulus-topped boundary layers play an important role in regulating regional weather and Earth’s energy balance. Entrainment at the cloud top strongly influences cloud lifetime, precipitation, and radiative effects, yet it remains poorly understood due to limited resolution in field observations and atmospheric numerical simulations.

A recently proposed tall convection-cloud chamber, with flexible control of sidewall temperatures, provides a unique opportunity for studying cloud-top entrainment under controlled laboratory conditions. In this work, we use large-eddy simulations coupled with a bin microphysics model to demonstrate that this configuration can reproduce key features of the entrainment interfacial layer observed in stratocumulus clouds. Our results show that a statistically steady cloud layer forms when the lower sidewalls are cooled and the bottom surface is warmed, while a stable temperature inversion at the cloud top is achieved by maintaining warmer upper sidewalls and a warmer top boundary. The resulting turbulent kinetic energy profiles and budgets resemble those observed in convective boundary layers, and inhomogeneous mixing near the cloud top is captured. These results increase the scientific value of constructing a tall convection-cloud chamber.

Contact 

Shaima Nasiri, Atmospheric System Research Program, shaima.nasiri@science.doe.gov  

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

Aaron Wang, corresponding author, Pacific Northwest National Laboratory, aaron.wang@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. Computing resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.