Network approach to patterns in stratocumulus clouds

Abstract

Stratocumulus clouds (Sc) have a significant impact on the amount of sunlight reflected back to space, with important implications for Earth's climate. Representing Sc and their radiative impact is one of the largest challenges for global climate models. Sc fields self-organize into cellular patterns and thus lend themselves to analysis and quantification in terms of natural cellular networks. Based on large-eddy simulations of Sc fields, we present a first analysis of the geometric structure and self-organization of Sc patterns from this network perspective. Our network analysis shows that the Sc pattern is scale-invariant as a consequence of entropy maximization that is known as Lewis's Law (scaling parameter: 0.16) and is largely independent of the Sc regime (cloud-free vs. cloudy cell centers). Cells are, on average, hexagonal with a neighbor number variance of about 2, and larger cells tend to be surrounded by smaller cells, as described by an AboavWeaire parameter of 0.9. The network structure is neither completely random nor characteristic of natural convection. Instead, it emerges from Sc-specific versions of cell division and cell merging that are shaped by cell expansion. This is shown with a heuristic model of network dynamics that incorporates our physical understanding of cloud processes.louds reflect incoming sunlight back to space and thus play an important role in modulating energy flows in the climate system. The description of shallow clouds in current global circulation models remains a challenge, however (1-3); due to computational constraints, the small scales of cloud processes cannot explicitly be resolved, and their subgrid-scale variability needs to be diagnosed (parameterized) from grid-scale mean quantities. The representation of stratocumulus (Sc) clouds, in particular, is one of the largest uncertainties for future climate projections (4, 5).Sc clouds cover extensive parts of the subtropical oceans with an intricate tapestry of shape and structure. Satellite images reveal hexagonal cells that are reminiscent of patterns arising from Rayleigh-Bénard convection (6). Indeed, Sc fields can be considered a form of Rayleigh-Bénard convection in moist atmospheric air (7); atmospheric flow is driven by a temperature difference over the depth of the planetary boundary layer, and adiabatic cooling in upwelling regions leads to condensation and cloud formation (Fig. 1). In the absence of rain, Sc fields are arranged as approximately stationary cloudy cells separated by cloud-free rings of downwelling air (closed cells, Fig. 1A) (8). The formation of rain means that cloudy updraft regions develop into regions of negatively buoyant air (cold pools) as a result of sedimentation and evaporation of rain (Fig. 1B), (9-11). Cold pools correspond to horizontally divergent flow at the surface and are bounded by convergent rings of upwelling air that are caused by the impingement of neighboring cold pools. Cold pools thus form cloud-free cells surrounded by cloudy rings and organize into patterns of open...

Phase field crystal simulations of the kinetics of Ostwald ripening in two dimensions

“…universally observed in many systems such as twodimensional foam [58], epithelial cells [59], and stratocumulus clouds [60]. Eq.…”

Particle‐Based Tracking of Cold Pool Gust Fronts

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The Radiative Forcing of Aerosol–Cloud Interactions in Liquid Clouds: Wrestling and Embracing Uncertainty

“…While the atmospheric science community has spent many decades developing models of ever-increasing complexity, complementary efforts have invested in alternative, and often much simpler, heuristic models, employing empiricism (e.g., [29]), network approaches [14,44], simple computational frameworks [39,156], or energy budget considerations ([25, 137], see below).…”