A Study of the Diurnal Variation of Stratocumulus Using A Multiple Mixed Layer Model

Abstract

A multiple mixed layer model of the cloud-topped boundary layer is developed to investigate the diurnal variation of stratocumulus. A simple representation of the microphysical properties of the cloud layer is included which enables high resolution, interactive radiation calculations to be made. A constraint on the buoyancy flux profile is introduced which permits the previously well-mixed layer to separate into two independently driven layers, thereby avoiding many of the unrealistic aspects of single-layer models and enabling the model to reproduce features seen in observational studies. The diagnosis of this decoupling, which generally occurs during the morning, is discussed and is shown to have a seasonal and latitudinal sensitivity due to its dependence on cloud layer short-wave absorption.Comparisons of results with those obtained when separation is not permitted show significant differences. In particular, the cloud layer displays a much enhanced diurnal variation in thickness when separation is allowed, with the minimum occurring in the afternoon. These results are shown to produce much better agreement with observations. The likely consequences of separation for boundary layer evolution are discussed. The surface energy balance can be quite strongly affected, suggesting that it is important to resolve this diurnal variation, especially in areas where stratocumulus is a dominant climatological feature.The sensitivity of the results to model assumptions are investigated and the limitations of the model are also assessed.

Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

“…In the nocturnal cases, the z is larger (indicating stronger decoupling) in the POC than the surrounding OVC region. In the one daytime flight (RF13), the OVC is more decoupled, consistent with observational and LES studies that find that solar insolation strongly decouples the daytime overcast boundary layer (Turton and Nicholls, 1987;Bretherton et al, 2004;Caldwell and Bretherton, 2009). Although Wood et al (2011a) POC was well mixed ( z = 86 m), we find that this feature is not shared across the other POC cases.…”

Mean Structure and Diurnal Cycle of Southeast Atlantic Boundary Layer Clouds: Insights from Satellite Observations and Multiscale Modeling Framework Simulations

“…3. 10a, 12a) and in agreement with a typical marine boundary layer (Turton and Nicholls 1987). A closer look at the 24-h harmonic amplitude reveals the details of the model overestimation (Figs.…”

Marine low cloud sensitivity to an idealized climate change: The CGILS LES intercomparison

“…Each LES still has a suite of microphysical, subgrid turbulence, surface flux and radiation parameterizations and schemes for advecting scalars and velocity that can have significant discretization errors in regions with sharp property gradients such as the capping inversion atop a typical marine stratocumulus cloud layer. Past GASS LES intercomparisons have shown that for stratocumulus under a strong inversion, the cloud thickness is sensitive to grid resolution, advection, and subgrid turbulence schemes [e.g., Bretherton et al, 1999;Stevens et al, 1995;Cheng et al, 2010], and for all precipitating boundary-layer cloud types, the cloud properties are sensitive to microphysical parameterizations [e.g., Ackerman et al, 2009;vanZanten et al, 2011]. Thus another important goal within CGILS is to assess whether the clouds simulated by different LESs all respond in a similar way to a given climate perturbation, and if so, what this might reveal about key mechanisms of subtropical low cloud feedback on climate change.…”