Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part II: Implications for Mixed-Layer Modeling

A numerical study based on large eddy simulation is performed to investigate a bottom boundary layer under an oscillating tidal current. The focus is on the boundary layer response to an external stratification. The thermal field shows a mixed layer that is separated from the external stratified fluid by a thermocline. The mixed layer grows slowly in time with an oscillatory modulation by the tidal flow. Stratification strongly affects the mean velocity profiles, boundary layer thickness and turbulence levels in the outer region although the effect on the near-bottom unstratified fluid is relatively mild. The turbulence is asymmetric between the accelerating and decelerating stages. The asymmetry is more pronounced with increasing stratification. There is an overshoot of the mean velocity in the outer layer; this jet is linked to the phase asymmetry of the Reynolds shear stress gradient by using the simulation data to examine the mean momentum equation. Depending on the height above the bottom, there is a lag of the maximum turbulent kinetic energy, dissipation and production with respect to the peak external velocity and the value of the lag is found to be influenced by the stratification. Flow instabilities and turbulence in the bottom boundary layer excite internal gravity waves that propagate away into the ambient. Unlike the steady case, the phase lines of the internal waves change direction during the tidal cycle and also from near to far field. The frequency spectrum of the propagating wave field is analysed and found to span a narrow band of frequencies clustered around 45°.

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A Dual Mass Flux Framework for Boundary Layer Convection. Part I: Transport

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2635.1 ◽

2009 ◽

Vol 66 (6) ◽

pp. 1465-1487 ◽

Cited By ~ 93

Author(s):

Roel A. J. Neggers ◽

Martin Köhler ◽

Anton C. M. Beljaars

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Mixed Layer ◽

Mass Flux ◽

Model Complexity ◽

Cumulus Convection ◽

Eddy Simulation ◽

Shallow Cumulus ◽

Large Eddy ◽

Simulation Results

Abstract This study considers the question of what is the least complex bulk mass flux framework that can still conceptually reproduce the smoothly varying coupling between the shallow convective cloud layer and the subcloud mixed layer. To this end, the model complexity of the classic single bulk mass flux scheme is enhanced. Inspired by recent large-eddy simulation results, the authors argue that two relatively minor but key conceptual modifications are already sufficient to achieve this goal: (i) retaining a dry transporting updraft in the moist limit and (ii) applying continuous updraft area partitioning to this dual mass flux (DualM) framework. The dry updraft represents all internal mixed layer updrafts that terminate near the mixed layer top, whereas the moist updraft represents all updrafts that condense and rise out of the mixed layer as buoyant cumulus clouds. The continuous area partitioning between the dry and moist updraft is a function of moist convective inhibition above the mixed layer top. Updraft initialization is a function of the updraft area fraction and is therefore consistent with the updraft definition. It is argued that the model complexity thus enhanced is sufficient to allow reproduction of various phenomena involved in the cloud–subcloud coupling, namely (i) dry countergradient transport within the mixed layer that is independent of the moist updraft, (ii) soft triggering of moist convective flux throughout the boundary layer, and (iii) a smooth response to smoothly varying forcings, including the reproduction of gradual transitions to and from shallow cumulus convection. The DualM framework is evaluated by implementing in the Eddy Diffusivity Mass Flux (EDMF) boundary layer scheme of the ECMWF’s Integrated Forecasting System. Single column model experiments are evaluated against large-eddy simulation results for a range of different cases that span a broad parameter space of cloud–subcloud coupling intensities. The results illustrate that also in numerical practice the DualM framework can reproduce gradual transitions to and from shallow cumulus convection. Model behavior is further explored through experiments in which model complexity is purposely reduced, thus mimicking a single bulk updraft setup. This gives more insight into the new model-internal interactions and explains the obtained case results.

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Large eddy simulation of shock/boundary-layer interaction

AIAA Journal ◽

10.2514/3.15281 ◽

2002 ◽

Vol 40 ◽

pp. 1935-1944 ◽

Cited By ~ 2

Author(s):

E. Garnier ◽

P. Sagaut ◽

M. Deville

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Layer Interaction ◽

Eddy Simulation ◽

Shock Boundary Layer Interaction ◽

Large Eddy ◽

Boundary Layer Interaction

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Correction: Wall-Modeled Large Eddy Simulation of a Three-Dimensional Shock-Boundary Layer Interaction

2018 AIAA Aerospace Sciences Meeting ◽

10.2514/6.2018-1298.c1 ◽

2018 ◽

Author(s):

Nicholas Marco ◽

Jeffrey R. Komives

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Three Dimensional ◽

Layer Interaction ◽

Eddy Simulation ◽

Shock Boundary Layer Interaction ◽

Large Eddy ◽

Boundary Layer Interaction

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Nocturnal Boundary Layer Erosion Analysis in the Amazon Using Large-Eddy Simulation during GoAmazon Project 2014/5

Atmosphere ◽

10.3390/atmos12020240 ◽

2021 ◽

Vol 12 (2) ◽

pp. 240

Author(s):

Rayonil Carneiro ◽

Gilberto Fisch ◽

Theomar Neves ◽

Rosa Santos ◽

Carlos Santos ◽

...

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Rainy Season ◽

Nocturnal Boundary Layer ◽

Enso Event ◽

Dry Season ◽

Dry Period ◽

Additional Energy ◽

Eddy Simulation ◽

Large Eddy

This study investigated the erosion of the nocturnal boundary layer (NBL) over the central Amazon using a high-resolution model of large-eddy simulation (LES) named PArallel Les Model (PALM) and observational data from Green Ocean Amazon (GoAmazon) project 2014/5. This data set was collected during four intense observation periods (IOPs) in the dry and rainy seasons in the years 2014 (considered a typical year) and 2015, during which an El Niño–Southern Oscillation (ENSO) event predominated and provoked an intense dry season. The outputs from the PALM simulations represented reasonably well the NBL erosion, and the results showed that it has different characteristics between the seasons. During the rainy season, the IOPs exhibited slow surface heating and less intense convection, which resulted in a longer erosion period, typically about 3 h after sunrise (that occurs at 06:00 local time). In contrast, dry IOPs showed more intensive surface warming with stronger convection, resulting in faster NBL erosion, about 2 h after sunrise. A conceptual model was derived to investigate the complete erosion during sunrise hours when there is a very shallow mixed layer formed close to the surface and a stable layer above. The kinematic heat flux for heating this layer during the erosion period showed that for the rainy season, the energy emitted from the surface and the entrainment was not enough to fully heat the NBL layer and erode it. Approximately 30% of additional energy was used in the system, which could come from the release of energy from biomass. The dry period of 2014 showed stronger heating, but it was also not enough, requiring approximately 6% of additional energy. However, for the 2015 dry period, which was under the influence of the ENSO event, it was shown that the released surface fluxes were sufficient to fully heat the layer. The erosion time of the NBL probably influenced the development of the convective boundary layer (CBL), wherein greater vertical development was observed in the dry season IOPs (~1500 m), while the rainy season IOPs had a shallower layer (~1200 m).

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