Evaluation of Large-Eddy Simulations via Observations of Nocturnal Marine Stratocumulus

Abstract Data from the first research flight (RF01) of the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study are used to evaluate the fidelity with which large-eddy simulations (LESs) can represent the turbulent structure of stratocumulus-topped boundary layers. The initial data and forcings for this case placed it in an interesting part of parameter space, near the boundary where cloud-top mixing is thought to render the cloud layer unstable on the one hand, or tending toward a decoupled structure on the other hand. The basis of this evaluation consists of sixteen 4-h simulations from 10 modeling centers over grids whose vertical spacing was 5 m at the cloud-top interface and whose horizontal spacing was 35 m. Extensive sensitivity studies of both the configuration of the case and the numerical setup also enhanced the analysis. Overall it was found that (i) if efforts are made to reduce spurious mixing at cloud top, either by refining the vertical grid or limiting the effects of the subgrid model in this region, then the observed turbulent and thermodynamic structure of the layer can be reproduced with some fidelity; (ii) the base, or native configuration of most simulations greatly overestimated mixing at cloud top, tending toward a decoupled layer in which cloud liquid water path and turbulent intensities were grossly underestimated; (iii) the sensitivity of the simulations to the representation of mixing at cloud top is, to a certain extent, amplified by particulars of this case. Overall the results suggest that the use of LESs to map out the behavior of the stratocumulus-topped boundary layer in this interesting region of parameter space requires a more compelling representation of processes at cloud top. In the absence of significant leaps in the understanding of subgrid-scale (SGS) physics, such a representation can only be achieved by a significant refinement in resolution—a refinement that, while conceivable given existing resources, is probably still beyond the reach of most centers.

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Linear-eddy subgrid model for reacting large-eddy simulations - Heat release effects

10.2514/6.1997-368 ◽

1997 ◽

Cited By ~ 5

Author(s):

William Calhoon, Jr. ◽

Suresh Menon ◽

William Calhoon, Jr. ◽

Suresh Menon

Keyword(s):

Heat Release ◽

Large Eddy Simulations ◽

Subgrid Model ◽

Large Eddy

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Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

Monthly Weather Review ◽

10.1175/2008mwr2582.1 ◽

2009 ◽

Vol 137 (3) ◽

pp. 1083-1110 ◽

Cited By ~ 163

Author(s):

Andrew S. Ackerman ◽

Margreet C. vanZanten ◽

Bjorn Stevens ◽

Verica Savic-Jovcic ◽

Christopher S. Bretherton ◽

...

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Cloud Water ◽

Large Eddy Simulations ◽

Cloud Base ◽

Entrainment Rate ◽

Liquid Water Path ◽

Average Maximum ◽

Large Eddy ◽

The Mean

Abstract Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

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Large-Eddy Simulation of Roughened NACA65 Compressor Cascade

Volume 2B: Turbomachinery ◽

10.1115/gt2017-64889 ◽

2017 ◽

Cited By ~ 1

Author(s):

Jongwook Joo ◽

Gorazd Medic ◽

Om Sharma

Keyword(s):

Surface Roughness ◽

Large Eddy Simulations ◽

Surface Imaging ◽

Potential Solution ◽

Discrete Elements ◽

Compressor Cascade ◽

Eddy Simulation ◽

Large Eddy ◽

Rans Modeling ◽

The One

Large eddy simulations over a NACA65 compressor cascade with roughness were performed for multiple roughness heights. The experiments show flow separation as airfoil roughness is increased. In LES computations, surface roughness was represented by regularly arranged discrete elements using guidelines from Schlichting. Results from wall-resolved LES indicate that specifying an equivalent sandgrain roughness height larger than the one in experiments is required to reproduce the same effects observed in experiments. This highlights the persisting uncertainty with matching the experimental roughness geometry in LES computations, pointing towards surface imaging and digitization as a potential solution. Some initial analysis of flow physics has been conducted with the aim of guiding the RANS modeling for roughness.

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Quantifying how model assumptions affect aerosol-cloud interactions in large-eddy simulations of warm stratocumulus clouds

10.5194/egusphere-egu21-11169 ◽

2021 ◽

Author(s):

Matthias Schwarz ◽

Julien Savre ◽

Annica Ekman

Keyword(s):

Number Concentration ◽

Large Eddy Simulations ◽

Radiation Balance ◽

Marine Stratocumulus ◽

Aerosol Cloud ◽

Reference Simulation ◽

Model Aerosol ◽

Large Eddy ◽

Stratocumulus Clouds ◽

Climate Interactions

Subtropical low-level marine stratocumulus clouds effectively reflect downwelling shortwave radiation while having a small effect on outgoing longwave radiation. Hence, they impose a strong negative net radiative effect on the Earth&#8217;s radiation balance. The optical and microphysical properties of these clouds are susceptible to anthropogenic changes in aerosol abundance. Although these aerosol-cloud-climate interactions (ACI) are generally explicitly treated in state-of-the-art Earth System Models (ESMs), they are accountable for large uncertainties in current climate projections.Here, we present preliminary work where we exploit Large-Eddy-Simulations (LES) of warm stratocumulus clouds to identify and constrain processes and model assumptions that affect the response of cloud droplet number concentration (Nd) to changes in aerosol number concentration (Na). Our results are based on simulations with the MISU-MIT Cloud-Aerosol (MIMICA, Savre et al., 2014) LES, which has two-moment bulk microphysics (Seifert and Beheng, 2001) and a two-moment aerosol scheme (Ekman et al., 2006). The reference simulation is based on observations made during the Dynamics and Chemistry of Marine Stratocumulus Field Study (DYCOMS-II, Stevens et al., 2003) which were used extensively during previous LES studies (e.g., Ackerman et al., 2009).Starting from the reference simulation, we conduct sensitivity experiments to examine how the susceptibility (&#946;=dln(Nd)/dln(Na)) changes depending on different model setups. We run the model with fixed and interactive aerosol concentrations, with and without saturation adjustment, with different aerosol populations, and with different model parameter choices. Our early results suggest that &#946; is sensitive to these choices and can vary roughly between 0.6 to 0.9 depending on the setup. The overall purpose of our study is to guide future model developments and evaluations concerning aerosol-cloud-climate interactions. &#160;&#160;ReferencesAckerman, A. S., vanZanten, M. C., Stevens, B., Savic-Jovcic, V., Bretherton, C. S., Chlond, A., et al. (2009). Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer. Monthly Weather Review, 137(3), 1083&#8211;1110. https://doi.org/10.1175/2008MWR2582.1Ekman, A. M. L., Wang, C., Str&#246;m, J., & Krejci, R. (2006). Explicit Simulation of Aerosol Physics in a Cloud-Resolving Model: Aerosol Transport and Processing in the Free Troposphere. Journal of the Atmospheric Sciences, 63(2), 682&#8211;696. https://doi.org/10.1175/JAS3645.1Savre, J., Ekman, A. M. L., & Svensson, G. (2014). Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers. Journal of Advances in Modeling Earth Systems, 6(3), 630&#8211;649. https://doi.org/10.1002/2013MS000292Stevens, B., Lenschow, D. H., Vali, G., Gerber, H., Bandy, A., Blomquist, B., et al. (2003). Dynamics and Chemistry of Marine Stratocumulus&#8212;DYCOMS-II. Bulletin of the American Meteorological Society, 84(5), 579&#8211;594. https://doi.org/10.1175/BAMS-84-5-579

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Scalar Subgrid Model with Flow Structure for Large-Eddy Simulations of Scalar Variances

Fluid Mechanics and Its Applications - Advances in Turbulence VII ◽

10.1007/978-94-011-5118-4_140 ◽

1998 ◽

pp. 565-566 ◽

Cited By ~ 1

Author(s):

P. Flohr ◽

J. C. Vassilicos

Keyword(s):

Flow Structure ◽

Large Eddy Simulations ◽

Subgrid Model ◽

Large Eddy

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Radiative Impacts of Free-Tropospheric Clouds on the Properties of Marine Stratocumulus

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0287.1 ◽

2013 ◽

Vol 70 (10) ◽

pp. 3102-3118 ◽

Cited By ~ 46

Author(s):

Matthew W. Christensen ◽

Gustavo G. Carrió ◽

Graeme L. Stephens ◽

William R. Cotton

Keyword(s):

Atmospheric Modeling ◽

Cloud Base ◽

Radiative Cooling ◽

Liquid Water Path ◽

Marine Stratocumulus ◽

Large Eddy ◽

Stratocumulus Clouds ◽

Cloud Optical Thickness ◽

Vertical Development ◽

Regional Atmospheric Modeling

Abstract Observations from multiple satellites and large-eddy simulations (LESs) from the Regional Atmospheric Modeling System (RAMS) are used to determine the extent to which free-tropospheric clouds (FTCs) affect the properties of stratocumulus. Overlying FTCs decrease the cloud-top radiative cooling in stratocumulus by an amount that depends on the upper-cloud base altitude, cloud optical thickness, and abundance of moisture between the cloud layers. On average, FTCs increase the downward longwave radiative flux above stratocumulus clouds (at 3.5 km) by approximately 30 W m−2. As a consequence, this forcing translates to a relative decrease in stratocumulus cooling rates by about 20%. Overall, the reduced cloud-top radiative cooling decreases the turbulent mixing, vertical development, and precipitation rate in stratocumulus clouds at night. During the day these effects are greatly reduced because the overlying clouds shade the stratocumulus from strong solar radiation, thus reducing the net radiative effect by the upper cloud. Differences in liquid water path are also observed in stratocumulus; however, the response is tied to the diurnal cycle and the time scale of interaction between the FTCs and the stratocumulus. Radiative effects by FTCs tend to be largest in the midlatitudes where the clouds overlying stratocumulus tend to be more frequent, lower, and thicker on average. In conclusion, changes in net radiation and moisture brought about by FTCs can significantly affect the dynamics of marine stratocumulus and these processes should be considered when evaluating cloud feedbacks in the climate system.

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Relationship between drizzle rate, liquid water path and droplet concentration at the scale of a stratocumulus cloud system

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-4641-2008 ◽

2008 ◽

Vol 8 (16) ◽

pp. 4641-4654 ◽

Cited By ~ 46

Author(s):

O. Geoffroy ◽

J.-L. Brenguier ◽

I. Sandu

Keyword(s):

Liquid Water ◽

Field Experiments ◽

Liquid Water Path ◽

Marine Stratocumulus ◽

Data Set ◽

Water Path ◽

Droplet Concentration ◽

Stratocumulus Clouds ◽

Shallow Clouds ◽

The One

Abstract. The recent ACE-2, EPIC and DYCOMS-II field experiments showed that the drizzle precipitation rate of marine stratocumulus scales with the cloud geometrical thickness or liquid water path, and the droplet concentration, when averaged over a domain typical of a GCM grid. This feature is replicated here with large-eddy-simulations using state-of-the-art bulk parameterizations of precipitation formation in stratocumulus clouds. The set of numerical simulations shows scaling relationships similar to the ones derived from the field experiments, especially the one derived from the DYCOMS-II data set. This result suggests that the empirical relationships were not fortuitous and that they reflect the mean effect of cloud physical processes. Such relationships might be more suited to GCM parameterizations of precipitation from shallow clouds than bulk parameterizations of autoconversion, that were initially developed for cloud resolving models.

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Stochastic backscatter in large-eddy simulations of boundary layers

Journal of Fluid Mechanics ◽

10.1017/s0022112092002271 ◽

1992 ◽

Vol 242 ◽

pp. 51-78 ◽

Cited By ~ 376

Author(s):

P. J. Mason ◽

D. J. Thomson

Keyword(s):

Large Scale ◽

Flow Simulation ◽

Surface Region ◽

Large Eddy Simulations ◽

Flow Profile ◽

Mean Velocity ◽

Near Surface ◽

Eddy Simulation ◽

Subgrid Model ◽

Large Eddy

The ability of a large-eddy simulation to represent the large-scale motions in the interior of a turbulent flow is well established. However, concerns remain for the behaviour close to rigid surfaces where, with the exception of low-Reynolds-number flows, the large-eddy description must be matched to some description of the flow in which all except the larger-scale ‘inactive’ motions are averaged. The performance of large-eddy simulations in this near-surface region is investigated and it is pointed out that in previous simulations the mean velocity profile in the matching region has not had a logarithmic form. A number of new simulations are conducted with the Smagorinsky (1963) subgrid model. These also show departures from the logarithmic profile and suggest that it may not be possible to eliminate the error by adjustments of the subgrid lengthscale. An obvious defect of the Smagorinsky model is its failure to represent stochastic subgrid stress variations. It is shown that inclusion of these variations leads to a marked improvement in the near-wall flow simulation. The constant of proportionality between the magnitude of the fluctuations in stress and the Smagorinsky stresses has been empirically determined to give an accurate logarithmic flow profile. This value provides an energy backscatter rate slightly larger than the dissipation rate and equal to idealized theoretical predictions (Chasnov 1991).

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Persistent Contrails and Contrail Cirrus. Part I: Large-Eddy Simulations from Inception to Demise

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0316.1 ◽

2014 ◽

Vol 71 (12) ◽

pp. 4399-4419 ◽

Cited By ~ 16

Author(s):

D. C. Lewellen ◽

O. Meza ◽

W. W. Huebsch

Keyword(s):

Parameter Space ◽

Companion Paper ◽

Large Eddy Simulations ◽

Ice Crystals ◽

Ice Crystal ◽

Adequate Treatment ◽

Large Eddy ◽

Shape Effects ◽

Aircraft Wake ◽

Ambient Turbulence

Abstract Large-eddy simulations with size-resolved microphysics are used to model persistent aircraft contrails and contrail-induced cirrus from a few wing spans behind the aircraft until their demise after many hours. Schemes for dynamic local ice binning and updating coupled radiation dynamically as needed in individual columns were developed for numerical efficiency, along with a scheme for maintaining realistic ambient turbulence over long times. These capabilities are used to study some of the critical dynamics involved in contrail evolution and to explore the simulation features required for adequate treatment of different components. A “quasi 3D” approach is identified as a useful approximation of the full dynamics, reducing the computation to allow a larger parameter space to be studied. Ice crystal number loss involving competition between different crystal sizes is found to be significant for both young contrails and aging contrail cirrus. As a consequence, the sensitivity to the initial number of ice crystals in the contrail above a threshold is found to decrease significantly over time, and uncertainties in the ice deposition coefficient and Kelvin effect for ice crystals assume an increased importance. Atmospheric turbulence is found to strongly influence contrail properties and lifetime in some regimes. Water from fuel consumption is found to significantly reduce aircraft-wake-induced ice crystal loss in colder contrails. Ice crystal shape effects, coupled radiation, and precipitation dynamics are also considered. An extensive set of simulations exploring a large parameter space with this model are analyzed in a companion paper.

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