Entrainment laws and a bulk mixed layer model of rotating convection derived from large-eddy simulations

2003 ◽  
Vol 30 (18) ◽  
Author(s):  
Dailin Wang
Keyword(s):  
Mixed Layer ◽  
Large Eddy Simulations ◽  
Layer Model ◽  
Rotating Convection ◽  
Large Eddy ◽  
Mixed Layer Model

scholarly journals Subcloud-Layer Feedbacks Driven by the Mass Flux of Shallow Cumulus Convection over Land

2014 ◽  
Vol 71 (3) ◽  
pp. 881-895 ◽  
Author(s):  
Bart J. H. van Stratum ◽  
Jordi Vilá-Guerau de Arellano ◽  
Chiel C. van Heerwaarden ◽  
Huug G. Ouwersloot
Keyword(s):  
Boundary Layers ◽  
Mixed Layer ◽  
Mass Flux ◽  
Large Eddy Simulations ◽  
Cloud Layer ◽  
Layer Model ◽  
Flux Transport ◽  
Shallow Cumulus ◽  
Large Eddy ◽  
Mixed Layer Model

Abstract The processes and feedbacks associated with the mass flux of shallow cumulus clouds over land are studied by analyzing the results from large-eddy simulations and a mixed-layer model. The primary focus is to study the development of the (well mixed) subcloud layer and understand the four primary feedbacks between the subcloud-layer dynamics and cumulus mass flux. Guided by numerical experiments in large-eddy simulations that show the transition from clear to cloudy boundary layers at midlatitudes over land, the feedbacks introduced by shallow cumuli are first conceptually described. To study the complex interplay between the subcloud and cloud layer, a mixed-layer model is proposed and validated with large-eddy simulations for the Atmospheric Radiation Measurement Southern Great Plains case. The mixed-layer model is shown to identify and reproduce the most relevant feedbacks in the transition from clear to cloudy boundary layers: a reduced mixed-layer growth and drying of the subcloud layer by enhanced entrainment and mass flux transport of moisture to the cloud layer. To complete the study, the strength of the different feedbacks is further quantified by an analysis of the individual contributions to the tendency of the relative humidity at the top of the mixed layer.

Convective Boundary Layers Driven by Nonstationary Surface Heat Fluxes

2011 ◽  
Vol 68 (4) ◽  
pp. 727-738 ◽  
Author(s):  
Robert van Driel ◽  
Harm J. J. Jonker
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Layer Model ◽  
Convective Boundary ◽  
Surface Heat Fluxes ◽  
Large Eddy ◽  
Convective Boundary Layers ◽  
Mixed Layer Model

In this study the response of dry convective boundary layers to nonstationary surface heat fluxes is systematically investigated. This is relevant not only during sunset and sunrise but also, for example, when clouds modulate incoming solar radiation. Because the time scale of the associated change in surface heat fluxes may differ from case to case, the authors consider the generic situation of oscillatory surface heat fluxes with different frequencies and amplitudes and study the response of the boundary layer in terms of transfer functions. To this end both a mixed layer model (MLM) and a large-eddy simulation (LES) model are used; the latter is used to evaluate the predictive quality of the mixed layer model. The mixed layer model performs generally quite well for slow changes in the surface heat flux and provides analytical understanding of the transfer characteristics of the boundary layer such as amplitude and phase lag. For rapidly changing surface fluxes (i.e., changes within a time frame comparable to the large eddy turnover time), it proves important to account for the time it takes for the information to travel from the surface to higher levels of the boundary layer such as the inversion zone. As a follow-up to a 1997 study by Sorbjan, who showed that the conventional convective velocity scale is inadequate as a scaling quantity during the decay phase, this paper addresses the issue of defining, in (generic) transitional situations, a velocity scale that is solely based on the surface heat flux and its history.

scholarly journals Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

2006 ◽  
Vol 19 (20) ◽  
pp. 5227-5252 ◽  
Author(s):  
Serena Illig ◽  
Boris Dewitte
Keyword(s):  
Mixed Layer ◽  
El Niño ◽  
Time Scale ◽  
El Nino ◽  
Coupled Model ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Layer Model ◽  
Intermediate Coupled Model ◽  
Mixed Layer Model

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

scholarly journals Dryline Bulge Evolution in a Two-Dimensional Mixed-Layer Model

2004 ◽  
Vol 61 (21) ◽  
pp. 2528-2543 ◽  
Author(s):  
Glenn M. Auslander ◽  
Peter R. Bannon
Keyword(s):  
Heat Flux ◽  
Mixed Layer ◽  
Surface Heat Flux ◽  
Mixed Layer Depth ◽  
Surface Heat ◽  
Two Dimensional ◽  
Layer Model ◽  
Layer Depth ◽  
Inversion Strength ◽  
Mixed Layer Model

Abstract This study examines the diurnal response of a mixed-layer model of the dryline system to localized anomalies of surface heat flux, topography, mixed-layer depth, and inversion strength. The two-dimensional, mixed-layer model is used to simulate the dynamics of a cool, moist layer east of the dryline capped by an inversion under synoptically quiescent conditions. The modeled domain simulates the sloping topography of the U.S. Great Plains. The importance of this study can be related to dryline bulges that are areas with enhanced convergence that may trigger convection in suitable environmental conditions. All anomalies are represented by a Gaussian function in the horizontal whose amplitude, size, and orientation can be altered. A positive, surface-heat-flux anomaly produces increased mixing that creates a bulge toward the east, while a negative anomaly produces a westward bulge. Anomalies in topography show a similar trend in bulge direction with a peak giving an eastward bulge, and a valley giving a westward bulge. Anomalies in the initial mixed-layer depth yield an eastward bulge in the presence of a minimum and a westward bulge for a maximum. An anomaly in the initial inversion strength results in a westward bulge when the inversion is stronger, and an eastward bulge when the inversion is weak. The bulges observed in this study at 1800 LT ranged from 400 to 600 km along the dryline and from 25 to 80 km across the dryline. When the heating ceases at night, the entrainment and eastward movement of the line stops, and the line surges westward. This westward surge at night has little dependence on the type of anomaly applied. Whether a westward or eastward bulge was present at 1800 LT, the surge travels an equal distance toward the west. However, the inclusion of weak nocturnal friction reduces the westward surge by 100 to 200 km due to mechanical mixing of the very shallow leading edge of the surge. All model runs exhibit peaks in the mixed-layer depth along the dryline at 1800 LT caused by enhanced boundary layer convergence and entrainment of elevated mixed-layer air into the mixed layer. These peaks appear along the section of the dryline that is least parallel to the southerly flow. They vary in amplitude from 4 to 9 km depending on the amplitude of the anomaly. However, the surface-heat-flux anomalies generally result in peaks at the higher end of this interval. It is hypothesized that the formation of these peaks may be the trigger for deep convection along the dryline in the late afternoon.

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