Roll convection during a cold air outbreak: A large eddy simulation with stationary model domain

Abstract. We present an analysis of the interaction between a topographically forced recirculation of the large-scale flow above an urbanized coastal valley and a local breeze-like circulation. We found that such an interaction can enhance the stagnation inside the valley under cold air pool conditions. Analysis of a large dataset of air quality measurements in Bergen, Norway, revealed that the most extreme cases of recurring winter-time air pollution episodes are usually accompanied by an increased wind speed above the valley. The 10 m ERA-Interim wind-speed distribution against local NO2 measurements had a maximum at 3 m s−1 in contrast to a monotonic decrease, as it would be expected from theory developed for flat, homogenous surfaces. We conducted a set of 16 Large Eddy Simulation (LES) experiments with the PALM model to account for the realistic orography of the mountains surrounding the city. The simulations were driven by the typical circulation above the valley during observed air pollution episodes, and a heterogeneous combination of constant temperatures over water and a constant negative sensible surface heat flux over land. The LES revealed a strong steering of the local circulation during cold air pool conditions by a land-breeze between the warm sea and the cold land. This breeze circulation is counteracted by a recirculation of the flow above the valley. For certain combinations of both, this leads to a maximum in the local stagnation. Furthermore, a relatively small local water body acted as a barrier for the dispersion of air pollutants along the valley bottom, dispersing them vertically and hence diluting them. These findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

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Large Eddy Simulation of lifted turbulent flame in cold air using Doubly Conditional Source-term Estimation

Combustion and Flame ◽

10.1016/j.combustflame.2019.07.025 ◽

2019 ◽

Vol 208 ◽

pp. 420-435

Author(s):

Mohammad Mortada ◽

Cecile Devaud

Keyword(s):

Large Eddy Simulation ◽

Source Term ◽

Turbulent Flame ◽

Cold Air ◽

Eddy Simulation ◽

Large Eddy ◽

Source Term Estimation

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Large-Eddy Simulation of Moist Convection during a Cold Air Outbreak over the Gulf Stream

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2755.1 ◽

2009 ◽

Vol 66 (5) ◽

pp. 1274-1293 ◽

Cited By ~ 16

Author(s):

Eric D. Skyllingstad ◽

James B. Edson

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Gulf Stream ◽

Heat Fluxes ◽

Water Dynamics ◽

Moist Convection ◽

Cold Air ◽

Stratus Clouds ◽

Eddy Simulation ◽

Large Eddy

Abstract Cold air outflow over the Gulf Stream is modeled using a cloud-resolving large-eddy simulation model with three classes of precipitation. Simulations are conducted in a quasi-Lagrangian framework using an idealized sounding and uniform geostrophic winds based on observations taken on 20 February 2007 as part of the World Climate Research Program Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE) project. Two cases are considered, one with an increasing sea surface temperature (SST) representing the crossing of the Gulf Stream front, and a second case with constant SST. Cloud systems develop in the model with strong convective plumes that spread into regions of stratus clouds at the top of the boundary layer. Simulated boundary layer growth is forced by a combination of evaporative cooling at the cloud top, upward radiative flux, and mechanical entrainment of the overlying warmer and drier air. Constant growth of the boundary layer acts to maintain a near-constant water vapor level in the boundary layer, promoting high latent and sensible heat fluxes. Frictional surface drag is distributed throughout the boundary layer by convection, causing increased shear at the cloud top, qualitatively agreeing with observed sounding profiles. Overall, the frontal case develops stronger precipitation and turbulence in comparison with the constant SST case. A near-uniform stratocumulus layer and stronger radiative cooling are produced in the constant SST case, whereas the frontal case generates open cumuliform clouds with reduced cloud coverage. Cloud evolution in the frontal case is similar to the transition from stratocumulus to shallow cumulus observed in the subtropics, as cumuliform clouds enhance cloud-top entrainment and evaporation of stratus clouds.

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urbanChemFoam 1.0: large-eddy simulation of non-stationary chemical transport of traffic emissions in an idealized street canyon

Geoscientific Model Development ◽

10.5194/gmd-14-4555-2021 ◽

2021 ◽

Vol 14 (7) ◽

pp. 4555-4572

Author(s):

Edward C. Chan ◽

Timothy M. Butler

Keyword(s):

Nitrogen Oxides ◽

Large Eddy Simulation ◽

Street Canyon ◽

Chemical Transport ◽

Traffic Emissions ◽

Background Concentrations ◽

Stationary Model ◽

Eddy Simulation ◽

Pollutant Concentrations ◽

Large Eddy

Abstract. This paper describes a large-eddy simulation based chemical transport model, developed under the OpenFOAM framework, implemented to simulate dispersion and chemical transformation of nitrogen oxides from traffic sources in an idealized street canyon. The dynamics of the model, in terms of mean velocity and turbulent fluctuation, are evaluated using available stationary measurements. A transient model run using a photostationary reaction mechanism for nitrogen oxides and ozone subsequently follows, where non-stationary conditions for meteorology, background concentrations, and traffic emissions are applied over a 24 h period, using regional model data and measurements obtained for the city of Berlin in July 2014. Diurnal variations of pollutant concentrations indicate dependence on emission levels, background concentrations, and solar state. Comparison of vertical and horizontal profiles with corresponding stationary model runs at select times show that while there are only slight differences in velocity magnitude, visible changes in primary and secondary flow structures can be observed. In addition, temporal variations in diurnal profile and cumulative species concentration result in significant deviations in computed pollutant concentrations between transient and stationary model runs.

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urbanChemFoam 1.0: Large-Eddy Simulation of Non-Stationary Chemical Transport of Traffic Emissions in an Idealized Street Canyon

10.5194/gmd-2020-432 ◽

2021 ◽

Author(s):

Edward C. Chan ◽

Timothy M. Butler

Keyword(s):

Nitrogen Oxides ◽

Large Eddy Simulation ◽

Street Canyon ◽

Chemical Transport ◽

Traffic Emissions ◽

Background Concentrations ◽

Stationary Model ◽

Eddy Simulation ◽

Pollutant Concentrations ◽

Large Eddy

Abstract. This paper describes a large-eddy simulation based chemical transport model, developed under the OpenFOAM framework, implemented to simulate dispersion and chemical transformation of nitrogen oxides from traffic sources in an idealized street canyon. The dynamics of the model, in terms of mean velocity and turbulent fluctuation, are evaluated using available stationary measurements. A transient model run using a photostationary reaction mechanism for nitrogen oxides and ozone subsequently follows, where non-stationary conditions for meteorology, background concentrations, and traffic emissions are applied over a 24-hour period, using regional model data and measurements obtained for the City of Berlin in July, 2014. Diurnal variations of pollutant concentrations indicate dependence on emission levels, background concentrations, and solar state. Comparison of vertical and horizontal profiles with corresponding stationary model runs at select times show that, while there are only slight differences in velocity magnitude, visible changes in primary and secondary flow structures can be observed. In addition, temporal variations in diurnal profile and cumulative species concentration result in significant deviations in computed pollutant concentrations between transient and stationary model runs.

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