Stratocumulus clouds (Sc) cover about one-quarter of the Earth’s surface and have a large impact on the Earth’s radiative balance. A series of numerical experiments where both physical and numerical model parameters are varied with respect to a reference setup is used to investigate the physics of a stratocumulus cloud and the performance of a large-eddy simulation (LES) model.
The study is based on the nocturnal Sc observed during the DYCOMS-II (RF01) campaign. A grid-converged simulation that agrees with the observations is only possible at very fine grid resolutions (grid spacing less than 2.5 m). However, the simulations show some robust features, such as the grid resolution independence of entrainment rate and mean profiles. A delicate balance of physical processes with some sensitivities amplified by numerical model features is observed. A strong feedback between cloud liquid, cloud-top radiative cooling, and turbulence leads to slow grid convergence of the turbulent fluxes. For a methodology that diagnoses cloud liquid from conserved variables, small errors in the total water amount result in large liquid water errors, which are amplified by the cloud-top radiative cooling leading to large variations of buoyancy forcing. In contrast, when the liquid–radiation–buoyancy feedback is not present in simulations without radiation, the turbulence structure of the boundary layer remains essentially identical for grid resolutions between 20 and 1.25 m. In a series of numerical experiments latent heat exchange is suppressed in the LES model, which inhibits evaporative cooling at the cloud top. Analysis of liquid water path (LWP) and cloud liquid water content shows that cloud-top evaporative cooling generates relatively shallow slits near the cloud top. Most of liquid water mass is concentrated near the cloud top, thus cloud-top slits of clear air have a large impact on the entire-column LWP. When evaporative cooling is suppressed in the LES, LWP exhibits cellular lumpy structure without elongated low-LWP regions. LES results show that the cloud-top evaporative cooling process significantly affects integral boundary layer quantities, such as the vertically integrated turbulent kinetic energy, mean liquid water path, and entrainment rate. In a pair of simulations driven only by cloud-top radiative cooling, evaporative cooling nearly doubles the entrainment rate.
Dr Georgios “George” Matheou research group
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