Mesoscale nesting interface of the PALM model system 6.0

Abstract. In this paper, we present a newly developed mesoscale nesting interface for the PALM model system 6.0, which enables PALM to simulate the atmospheric boundary layer under spatially heterogeneous and non-stationary synoptic conditions. The implemented nesting interface, which is currently tailored to the mesoscale model COSMO, consists of two major parts: (i) the preprocessor INIFOR, which provides initial and time-dependent boundary conditions from mesoscale model output and (ii) PALM's internal routines for reading the provided forcing data and superimposing synthetic turbulence to accelerate the transition to a fully developed turbulent atmospheric boundary layer. We describe in detail the conversion between the sets of prognostic variables, transformations between model coordinate systems, as well as data interpolation onto PALM's grid, which are carried out by INIFOR. Furthermore, we describe PALM's internal usage of the provided forcing data, which besides the temporal interpolation of boundary conditions and removal of any residual divergence includes the generation of stability-dependent synthetic turbulence at the inflow boundaries in order to accelerate the transition from the turbulence-free mesoscale solution to a resolved turbulent flow. We demonstrate and evaluate the nesting interface by means of a semi-idealized benchmark case. We carried out a large-eddy simulation (LES) of an evolving convective boundary layer on a clear-sky spring day. Besides verifying that changes in the inflow conditions enter into and successively propagate through the PALM domain, we focus our analysis on the effectiveness of the synthetic turbulence generation. By analysing various turbulence statistics, we show that the inflow in the present case is fully adjusted after having propagated for about 1.5 eddy turn-over times downstream, which corresponds well to other state-of-the-art methods for turbulence generation. Furthermore, we observe that numerical artefacts in the form of under-resolved convective structures in the mesoscale model enter the PALM domain, biasing the location of the turbulent up- and downdrafts in the LES. With these findings presented, we aim to verify the mesoscale nesting approach implemented in PALM, point out specific shortcomings, and build a baseline for future improvements and developments.

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Mesoscale nesting interface of the PALM model system 6.0

Geoscientific Model Development ◽

10.5194/gmd-14-5435-2021 ◽

2021 ◽

Vol 14 (9) ◽

pp. 5435-5465

Author(s):

Eckhard Kadasch ◽

Matthias Sühring ◽

Tobias Gronemeier ◽

Siegfried Raasch

Keyword(s):

Boundary Layer ◽

Boundary Conditions ◽

Atmospheric Boundary Layer ◽

Mesoscale Model ◽

Model System ◽

Convective Boundary ◽

Convective Structures ◽

Eddy Simulation ◽

Synoptic Conditions ◽

Turbulence Generation

Abstract. In this paper, we present a newly developed mesoscale nesting interface for the PALM model system 6.0, which enables PALM to simulate the atmospheric boundary layer under spatially heterogeneous and non-stationary synoptic conditions. The implemented nesting interface, which is currently tailored to the mesoscale model COSMO, consists of two major parts: (i) the preprocessor INIFOR (initialization and forcing), which provides initial and time-dependent boundary conditions from mesoscale model output, and (ii) PALM's internal routines for reading the provided forcing data and superimposing synthetic turbulence to accelerate the transition to a fully developed turbulent atmospheric boundary layer. We describe in detail the conversion between the sets of prognostic variables, transformations between model coordinate systems, as well as data interpolation onto PALM's grid, which are carried out by INIFOR. Furthermore, we describe PALM's internal usage of the provided forcing data, which, besides the temporal interpolation of boundary conditions and removal of any residual divergence, includes the generation of stability-dependent synthetic turbulence at the inflow boundaries in order to accelerate the transition from the turbulence-free mesoscale solution to a resolved turbulent flow. We demonstrate and evaluate the nesting interface by means of a semi-idealized benchmark case. We carried out a large-eddy simulation (LES) of an evolving convective boundary layer on a clear-sky spring day. Besides verifying that changes in the inflow conditions enter into and successively propagate through the PALM domain, we focus our analysis on the effectiveness of the synthetic turbulence generation. By analysing various turbulence statistics, we show that the inflow in the present case is fully adjusted after having propagated for about two to three eddy-turnover times downstream, which corresponds well to other state-of-the-art methods for turbulence generation. Furthermore, we observe that numerical artefacts in the form of grid-scale convective structures in the mesoscale model enter the PALM domain, biasing the location of the turbulent up- and downdrafts in the LES. With these findings presented, we aim to verify the mesoscale nesting approach implemented in PALM, point out specific shortcomings, and build a baseline for future improvements and developments.

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The diurnal evolution of <sup>222</sup>Rn and its progeny in the atmospheric boundary layer during the Wangara experiment

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-8895-2007 ◽

2007 ◽

Vol 7 (3) ◽

pp. 8895-8931

Author(s):

J.-F. Vinuesa ◽

S. Basu ◽

S. Galmarini

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Turbulent Mixing ◽

Secular Equilibrium ◽

Convective Boundary ◽

Large Eddy Simulation Model ◽

Eddy Simulation ◽

Boundary Layer Processes ◽

Turbulent Characteristics ◽

Large Eddy

Abstract. The diurnal atmospheric boundary layer evolution of the 222Rn decaying family is studied by using a state-of-the-art large-eddy simulation model. In particular, a diurnal cycle observed during the Wangara experiment is successfully simulated together with the effect of diurnal varying turbulent characteristics on radioactive compounds in a secular equilibrium. This study allows us to clearly analyze and identify the boundary layer processes driving the 222Rn and its progeny concentration behaviors. The activity disequilibrium observed in the nocturnal boundary layer is due to the proximity of the radon source and the trapping of fresh 222Rn close to the surface induced by the weak vertical transport. During the morning transition, the secular equilibrium is fast restored by the vigorous turbulent mixing. The evolution of 222Rn and its progeny concentration in the unsteady growing convective boundary layer depends on the strength of entrainment events.

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LES of a Spatially Developing Atmospheric Boundary Layer: Application of a Fringe Method for the Stratocumulus to Shallow Cumulus Cloud Transition

Monthly Weather Review ◽

10.1175/mwr-d-13-00400.1 ◽

2014 ◽

Vol 142 (9) ◽

pp. 3418-3424 ◽

Cited By ~ 8

Author(s):

M. Inoue ◽

G. Matheou ◽

J. Teixeira

Keyword(s):

Boundary Layer ◽

Boundary Conditions ◽

Atmospheric Boundary Layer ◽

Cumulus Cloud ◽

Eddy Simulation ◽

Shallow Cumulus ◽

Outflow Boundary Conditions ◽

Large Eddy ◽

Inflow Condition ◽

Outflow Boundary

An arrangement of a large-eddy simulation (LES) is described that facilitates a spatially developing thermally stratified atmospheric boundary layer (ABL). When the inflow and outflow boundary conditions are specified, the LES of stably stratified ABL turns out to be challenging because spurious reflections of waves at the boundary accumulate inside the domain. To tackle this problem, a fringe method with an auxiliary LES running concurrently is applied to enforce upstream/downstream boundary conditions. An artificial forcing term is applied within a fringe region located at the beginning of the main LES domain in order to ensure statistically stationary inflow boundary conditions. The auxiliary LES, which is horizontally homogeneous in a doubly periodic domain, is used to determine the inflow condition of the main LES domain. The present scheme is used to provide an Eulerian perspective of the stratocumulus to shallow cumulus cloud (Sc–Cu) transition, one of the key cloud regimes over the subtropical ocean. In this study, the transition is triggered by increasing the sea surface temperature (SST) and the LES runs until a statistically steady evolution of the Sc–Cu transition is achieved. The flow statistics are compared with those from a recycling-type method and it is found that the fringe method is more suitable for the current applications.

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The diurnal evolution of <sup>222</sup>Rn and its progeny in the atmospheric boundary layer during the Wangara experiment

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-5003-2007 ◽

2007 ◽

Vol 7 (18) ◽

pp. 5003-5019 ◽

Cited By ~ 18

Author(s):

J.-F. Vinuesa ◽

S. Basu ◽

S. Galmarini

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Turbulent Mixing ◽

Secular Equilibrium ◽

Convective Boundary ◽

Large Eddy Simulation Model ◽

Eddy Simulation ◽

Boundary Layer Processes ◽

Turbulent Characteristics ◽

Large Eddy

Abstract. The diurnal atmospheric boundary layer evolution of the 222Rn decaying family is studied using a state-of-the-art large-eddy simulation model. In particular, a diurnal cycle observed during the Wangara experiment is successfully simulated together with the effect of diurnal varying turbulent characteristics on radioactive compounds initially in a secular equilibrium. This study allows us to clearly analyze and identify the boundary layer processes driving the behaviour of 222Rn and its progeny concentrations. An activity disequilibrium is observed in the nocturnal boundary layer due to the proximity of the radon source and the trapping of fresh 222Rn close to the surface induced by the weak vertical transport. During the morning transition, the secular equilibrium is fast restored by the vigorous turbulent mixing. The evolution of 222Rn and its progeny concentrations in the unsteady growing convective boundary layer depends on the strength of entrainment events.

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A Nested Multi-Scale System Implemented in the Large-Eddy Simulation Model PALM model system 6.0

10.5194/gmd-2020-222 ◽

2020 ◽

Author(s):

Antti Hellsten ◽

Klaus Ketelsen ◽

Matthias Sühring ◽

Mikko Auvinen ◽

Björn Maronga ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Large Eddy Simulation ◽

Model System ◽

Computational Domain ◽

Convective Boundary ◽

Large Eddy Simulation Model ◽

Eddy Simulation ◽

Large Eddy ◽

Parent Domain

Abstract. Large-eddy simulation provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES-LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by simulating first a purely convective boundary layer flow system and then three different neutrally-stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

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A nested multi-scale system implemented in the large-eddy simulation model PALM model system 6.0

Geoscientific Model Development ◽

10.5194/gmd-14-3185-2021 ◽

2021 ◽

Vol 14 (6) ◽

pp. 3185-3214

Author(s):

Antti Hellsten ◽

Klaus Ketelsen ◽

Matthias Sühring ◽

Mikko Auvinen ◽

Björn Maronga ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Large Eddy Simulation ◽

Model System ◽

Computational Domain ◽

Convective Boundary ◽

Large Eddy Simulation Model ◽

Eddy Simulation ◽

Large Eddy ◽

Parent Domain

Abstract. Large-eddy simulation (LES) provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES–LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains, which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by first simulating a purely convective boundary layer flow system and then three different neutrally stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs, while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

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