scholarly journals High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part II: Flow Structure and Heat Budgets

2006 ◽  
Vol 45 (1) ◽  
pp. 87-107 ◽  
Author(s):  
Andreas P. Weigel ◽  
Fotini K. Chow ◽  
Mathias W. Rotach ◽  
Robert L. Street ◽  
Ming Xue
Keyword(s):  
High Resolution ◽  
Flow Structure ◽  
Mixed Layer ◽  
Heat Budget ◽  
Measurement Data ◽  
Large Eddy Simulations ◽  
Driving Mechanism ◽  
Alpine Valley ◽  
Regional Prediction ◽  
Large Eddy

Abstract This paper analyzes the three-dimensional flow structure and the heat budget in a typical medium-sized and steep Alpine valley—the Riviera Valley in southern Switzerland. Aircraft measurements from the Mesoscale Alpine Programme (MAP)-Riviera field campaign reveal a very pronounced valley-wind system, including a strong curvature-induced secondary circulation in the southern valley entrance region. Accompanying radio soundings show that the growth of a well-mixed layer is suppressed, even under convective conditions. Our analyses are based on the MAP-Riviera measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Three sunny days of the measurement campaign are simulated. Using horizontal grid spacings of 350 and 150 m (with a vertical spacing as fine as 20 m), the model reproduces the observed flow features very well. The ARPS output data are then used to calculate the components of the heat budget of the valley atmosphere, first in profiles over the valley base and then as averages over almost the entire valley volume. The analysis shows that the suppressed growth of the well-mixed layer is due to the combined effect of cold-air advection in the along-valley direction and subsidence of warm air from the free atmosphere aloft. It is further influenced by the local cross-valley circulation. This had already been hypothesized on the basis of measurement data and is now confirmed through a numerical model. Averaged over the entire valley, subsidence turns out to be one of the main heating sources of the valley atmosphere and is of comparable magnitude to turbulent heat flux divergence. On the mornings of two out of the three simulation days, this subsidence is even identified as the only major heating source and thus appears to be an important driving mechanism for the onset of thermally driven upvalley winds.

High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part I: Methodology, Verification, and Sensitivity Experiments

2006 ◽  
Vol 45 (1) ◽  
pp. 63-86 ◽  
Author(s):  
Fotini Katopodes Chow ◽  
Andreas P. Weigel ◽  
Robert L. Street ◽  
Mathias W. Rotach ◽  
Ming Xue
Keyword(s):  
Land Use ◽  
Soil Moisture ◽  
High Resolution ◽  
Turbulence Models ◽  
Large Eddy Simulations ◽  
Swiss Alps ◽  
Advanced Regional Prediction System ◽  
Alpine Valley ◽  
Regional Prediction ◽  
Large Eddy

Abstract This paper investigates the steps necessary to achieve accurate simulations of flow over steep, mountainous terrain. Large-eddy simulations of flow in the Riviera Valley in the southern Swiss Alps are performed at horizontal resolutions as fine as 150 m using the Advanced Regional Prediction System. Comparisons are made with surface station and radiosonde measurements from the Mesoscale Alpine Programme (MAP)-Riviera project field campaign of 1999. Excellent agreement between simulations and observations is obtained, but only when high-resolution surface datasets are used and the nested grid configurations are carefully chosen. Simply increasing spatial resolution without incorporating improved surface data gives unsatisfactory results. The sensitivity of the results to initial soil moisture, land use data, grid resolution, topographic shading, and turbulence models is explored. Even with strong thermal forcing, the onset and magnitude of the upvalley winds are highly sensitive to surface processes in areas that are well outside the high-resolution domain. In particular, the soil moisture initialization on the 1-km grid is found to be crucial to the success of the finer-resolution predictions. High-resolution soil moisture and land use data on the 350-m-resolution grid also improve results. The use of topographic shading improves radiation curves during sunrise and sunset, but the effects on the overall flow are limited because of the strong lateral boundary forcing from the 1-km grid where terrain slopes are not well resolved. The influence of the turbulence closure is also limited because of strong lateral forcing and hence limited residence time of air inside the valley and because of the stable stratification, which limits turbulent stress to the lowest few hundred meters near the surface.

Exploring microphysical properties of marine boundary layer clouds through network analysis

2021 ◽  
Author(s):  
Lucile Ricard ◽  
Athanasios Nenes ◽  
Jakob Runge ◽  
Paraskevi Georgakaki
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Network Analysis ◽  
Marine Boundary Layer ◽  
Large Eddy Simulations ◽  
Dynamical Structure ◽  
Boundary Layer Clouds ◽  
Microphysical Properties ◽  
Spatial Coverage ◽  
Large Eddy

<p>Aerosol-cloud interactions remain the largest uncertainty in assessments of anthropogenic climate forcing, while the complexity of these interactions require methods that enable abstractions and simplifications that allow their improved treatment in climate models. Marine boundary layer clouds are an important component of the climate system as their large albedo and spatial coverage strongly affect the planetary radiative balance. High resolution simulations of clouds provide an unprecedented understanding of the structure and behavior of these clouds in the marine atmosphere, but the amount of data is often too large and complex to be useful in climate simulations. Data reduction and inference methods provide a way that to reduce the complexity and dimensionality of datasets generated from high-resolution Large Eddy Simulations.</p><p>In this study we use network analysis, (the δ-Maps method) to study the complex interaction between liquid water, droplet number and vertical velocity in Large Eddy Simulations of Marine Boundary Layer clouds. δ-Maps identifies domains that are spatially contiguous and possibly overlapping and characterizes their connections and temporal interactions. The objective is to better understand microphysical properties of marine boundary layer clouds, and how they are impacted by the variability in aerosols. Here we will capture the dynamical structure of the cloud fields predicted by the MIMICA Large Eddy Simulation (LES) model. The networks inferred from the different simulation fields are compared between them (intra-comparisons) using perturbations in initial conditions and aerosol, using a set of four metrics. The networks are then evaluated for their differences, quantifying how much variability is inherent in the LES simulations versus the robust changes induced by the aerosol fields. </p>

scholarly journals The Role of Wave-Induced Coriolis–Stokes Forcing on the Wind-Driven Mixed Layer

2005 ◽  
Vol 35 (4) ◽  
pp. 444-457 ◽  
Author(s):  
Jeff A. Polton ◽  
David M. Lewis ◽  
Stephen E. Belcher
Keyword(s):  
Analytical Solution ◽  
Observational Data ◽  
Mixed Layer ◽  
Analytical Solutions ◽  
Eddy Viscosity ◽  
Large Eddy Simulations ◽  
Current Profile ◽  
Large Eddy ◽  
The Mean ◽  
Mean Current

Abstract The interaction between the Coriolis force and the Stokes drift associated with ocean surface waves leads to a vertical transport of momentum, which can be expressed as a force on the mean momentum equation in the direction along wave crests. How this Coriolis–Stokes forcing affects the mean current profile in a wind-driven mixed layer is investigated using simple models, results from large-eddy simulations, and observational data. The effects of the Coriolis–Stokes forcing on the mean current profile are examined by reappraising analytical solutions to the Ekman model that include the Coriolis–Stokes forcing. Turbulent momentum transfer is modeled using an eddy-viscosity model, first with a constant viscosity and second with a linearly varying eddy viscosity. Although the Coriolis–Stokes forcing penetrates only a small fraction of the depth of the wind-driven layer for parameter values typical of the ocean, the analytical solutions show how the current profile is substantially changed through the whole depth of the wind-driven layer. It is shown how, for this oceanic regime, the Coriolis–Stokes forcing supports a fraction of the applied wind stress, changing the boundary condition on the wind-driven component of the flow and hence changing the current profile through all depths. The analytical solution with the linearly varying eddy viscosity is shown to reproduce reasonably well the effects of the Coriolis–Stokes forcing on the current profile computed from large-eddy simulations, which resolve the three-dimensional overturning motions associated with the turbulent Langmuir circulations in the wind-driven layer. Last, the analytical solution with the Coriolis–Stokes forcing is shown to agree reasonably well with current profiles from previously published observational data and certainly agrees better than the standard Ekman model. This finding provides evidence that the Coriolis–Stokes forcing is an important mechanism in controlling the dynamics of the upper ocean.

Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

2009 ◽  
Vol 48 (6) ◽  
pp. 1161-1180 ◽  
Author(s):  
Francis L. Ludwig ◽  
Fotini Katopodes Chow ◽  
Robert L. Street
Keyword(s):  
Dynamic Models ◽  
Turbulence Models ◽  
Coarse Grid ◽  
Large Eddy Simulations ◽  
Near Surface ◽  
Velocity Statistics ◽  
Neutral Boundary Layer ◽  
Regional Prediction ◽  
Large Eddy ◽  
Flow Features

Abstract This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

scholarly journals Heat Balance and Eddies in the Caribbean Upwelling System

2013 ◽  
Vol 43 (5) ◽  
pp. 1004-1014 ◽  
Author(s):  
Julien Jouanno ◽  
Julio Sheinbaum
Keyword(s):  
High Resolution ◽  
Mixed Layer ◽  
Heat Budget ◽  
Vertical Mixing ◽  
Warm Pool ◽  
Upwelling System ◽  
Mixed Layer Heat Budget ◽  
The Caribbean ◽  
Surrounding Areas ◽  
Surface Jets

Abstract The upper-ocean heat budget of the Caribbean upwelling system is investigated during the onset of the Atlantic warm pool (June–September) using high-resolution observations of sea surface temperature and a high-resolution (°) regional model. Vertical mixing is found to be the major cooling contribution to the mixed layer heat budget in the nearshore and offshore Colombia Basin. Numerical results show that intense mesoscale eddies in the Colombia Basin significantly shape the turbulent cooling and may participate in the maintenance of cooler temperature in this region compared to surrounding areas. Indeed, increased mixing at the base of the mixed layer occurs below energetic surface jets that form on the downstream side of the eddies. These jets generally flow offshore and may arise from the deformation of the surface mesoscale field. It is shown that significant contribution of horizontal advection to the mixed layer heat budget is limited to a radius of 300 km around the Guajira and Margarita upwelling zones.

scholarly journals Accumulation and Subduction of Buoyant Material at Submesoscale Fronts

2018 ◽  
Vol 48 (6) ◽  
pp. 1233-1241 ◽  
Author(s):  
John R. Taylor
Keyword(s):  
Mixed Layer ◽  
Baroclinic Instability ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Surface Cooling ◽  
Additional Source ◽  
Tracer Concentration ◽  
Vertical Diffusivity ◽  
Large Eddy ◽  
Submesoscale Currents

AbstractThe influence of submesoscale currents on the distribution and subduction of passive, buoyant tracers in the mixed layer is examined using large-eddy simulations. Submesoscale eddies are generated through an ageostrophic baroclinic instability associated with a background horizontal buoyancy gradient. The simulations also include various levels of surface cooling, which provides an additional source of three-dimensional turbulence. Submesoscales compete against turbulent convection and restratify the mixed layer while generating strong turbulence along a submesoscale front. Buoyant tracers accumulate at the surface along the submesoscale front where they are subducted down into the water column. The presence of submesoscales strongly modifies the vertical tracer flux, even in the presence of strong convective forcing. The correlation between high tracer concentration and strong downwelling enhances the vertical diffusivity for buoyant tracers.

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