Turbulence Structure in a Stratocumulus Cloud

Georgios Matheou

doi:10.3390/atmos9100392

Abstract

The growth of computing power combined with advances in modeling methods can yield high-fidelity simulations establishing numerical simulation as a key tool for discovery in the atmospheric sciences. A fine-scale large-eddy simulation (LES) utilizing 1.25 m grid resolution and 5.12 × 5.12 km 2 horizontal domain is used to investigate the turbulence and liquid water structure in a stratocumulus cloud. The simulations capture the observed cloud morphology, including elongated regions of low liquid water path, cloud holes, and pockets of clear air within the cloud. The cloud can be partitioned into two broad layers with respect to the maximum mean liquid. The lower layer resembles convective turbulent structure with classical inertial range scaling of the velocity and scalar energy spectra. The top and shallower layer is directly influenced by the cloud top radiative cooling and the entrainment process. Near the cloud top, the liquid water spectra become shallower and transition to a k − 1 power law for scales smaller than about 1 km .

Highlights

Stratocumulus clouds (Sc) cover about one-quarter of the Earth’s surface and have a large impact on the Earth’s radiative balance because they strongly reflect incoming solar radiation but they have a small effect on outgoing longwave radiation, e.g., [1,2,3,4]
Our understanding of the cloud-scale macrophysical and microphysical processes in stratocumulus topped boundary layers primarily derives from several observational campaigns, e.g., [8,9,10,11]
All flow statistics are computed from a single time instance of the simulation at t = 2 h

Summary

Introduction

Stratocumulus clouds (Sc) cover about one-quarter of the Earth’s surface and have a large impact on the Earth’s radiative balance because they strongly reflect incoming solar radiation but they have a small effect on outgoing longwave radiation, e.g., [1,2,3,4]. In spite of the seeming simplicity of a “lumpy” low-cloud deck, it is challenging to quantify the properties of the cloud-topped atmospheric boundary layer given the large-scale meteorological conditions. A delicate balance of processes leads to difficulties in the formulation of accurate parameterizations for weather and climate models, e.g., [5,6,7]. Our understanding of the cloud-scale macrophysical and microphysical processes in stratocumulus topped boundary layers primarily derives from several observational campaigns, e.g., [8,9,10,11]. The growth of computing power combined with advances in modeling methods is yielding high-fidelity simulations establishing numerical simulation as a key tool for discovery in the atmospheric sciences

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