Resolution and domain‐size sensitivity in implicit large‐eddy simulation of the stratocumulus‐topped boundary layer

Pedersen, Jesper Grønnegaard; Malinowski, Szymon P.; Grabowski, Wojciech W.

doi:10.1002/2015ms000572

Cited by 29 publications

(48 citation statements)

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“…[] over 10 h. Pedersen et al . [] conducted simulations over 6 h and noted that larger coherent structures could form in longer simulations. The simulations in this work give similar results in the first 6 h, after which domain D 1 diverges (supporting information Figure S2).…”

Section: Discussionmentioning

confidence: 99%

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Mesoscale organization, entrainment, and the properties of a closed‐cell stratocumulus cloud

Kazil

Yamaguchi

Feingold

2017

J Adv Model Earth Syst

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Closed‐cell mesoscale organization and its relationship to entrainment and the properties of a low, nonprecipitating stratocumulus cloud is investigated. Large eddy simulations were run over 10 periodic diurnal cycles during which mesoscale organization could fully develop and approach a quasi‐steady state on five domains sized from 2.4 km × 2.4 km to 38.4 km × 38.4 km. The four smaller domains hosted a single cell with an aspect ratio that increased with domain size. On the largest domain, mesoscale organization consisted of a cell population that evolved over the course of the diurnal cycle. It is found that with increasing cell aspect ratio, entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled. This causes an increase in cloud water path and cloud radiative effect up to a cell aspect ratio of 16. With further increase in cell aspect ratio, circulation on the cell scale becomes less effective in supplying moisture to the cloud and in producing turbulent kinetic energy (TKE). This mechanism can explain scale saturation in closed‐cell mesoscale organization. The simulations support a maximum stable aspect ratio of closed‐cell mesoscale organization between 32 and 64, consistent with the observational limit of ≈ 40. The simulations show furthermore that entrainment does not, in general, scale with buoyant production of TKE. Instead, entrainment correlates with the vertical component of TKE. This implies vertical motion as a driver of entrainment, and a convective velocity scale based on the vertical component of TKE rather than on buoyant production of TKE.

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Section: Discussionmentioning

confidence: 99%

“…Large eddy simulations of stratocumulus clouds converge with grid refinement toward reduced entrainment and higher cloud water path [ Stevens et al ., ; Pedersen et al ., ]. In addition to the grid spacing G 1 , we tested the grid spacing G 2 on the domain D 1 .…”

Section: Discussionmentioning

confidence: 99%

Mesoscale organization, entrainment, and the properties of a closed‐cell stratocumulus cloud

Kazil

Yamaguchi

Feingold

2017

J Adv Model Earth Syst

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“…For further details regarding the moist physics scheme, see Grabowski () and Grabowski (). The lateral boundaries of the computational domain are periodic; the lower boundary is impermeable with a partial slip condition characterized by a specified drag coefficient, and the upper boundary is impermeable with a free slip condition (Pedersen et al, ).…”

Section: Methodsmentioning

confidence: 99%

“…We consequently obtain SHF=5W/m 2 and LHF=20W/m 2 , in good agreement with the regional daily mean values calculated based on the above mentioned Objectively Analyzed Air‐Sea Fluxes data. The applied surface fluxes (i.e., the SHF/LHF and momentum flux) attenuate with height (as

\propto e^{- z false/ 50}

; e.g., Pedersen et al, ). The surface drag coefficient is set to 0.001, and the surface pressure is set to 1,017.5 hPa.…”

Section: Methodsmentioning

confidence: 99%

Influences of Subsidence and Free‐Tropospheric Conditions on the Nocturnal Growth of Nonclassical Marine Stratocumulus

Pedersen

Grabowski

et al. 2018

J Adv Model Earth Syst

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Effects of free‐tropospheric thermodynamic properties and large‐scale subsidence on the nocturnal growth of marine stratocumulus clouds are investigated through large‐eddy simulation based on Flight 5 of the Physics of Stratocumulus Top research campaign. It was characterized by a weak inversion and moist troposphere. Sensitivity simulations are performed using variations in the subsidence and free‐tropospheric temperature and humidity. The results confirm that the cloud‐top entrainment instability parameter κ alone cannot unambiguously control the behaviors of marine stratocumulus due to the opposite response of liquid water path (LWP) to changes in the humidity and temperature jumps with the same variation in κ, thereby both jumps should be considered separately. However, sometimes it could be considered as a joint factor of the humidity and temperature jumps controlling LWP even though cloud‐top entrainment instability does not occur or κ < 0. An obvious role of subsidence is to push the inversion layer down nearly identically and at the same rate under different ambient conditions. Enhanced subsidence, as expected, diminishes the entrainment rate, cloud‐top height, cloud thickness, and LWP. However, it has a small influence on the inversion thickness and the dynamic instability across the inversion. An LWP budget analysis shows that the direct effect of subsidence is a small reduction in the LWP, but the indirect effect of subsidence is large due to contributions from other physical processes. Therefore, subsidence is an important physical process controlling the LWP budget, even though its direct contribution is significantly smaller than the largest contributions from both radiation and entrainment.

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“…The Lagrangian approach applied in the UWLCM is referred to as the traditional super-droplet method in the discussion below. Both the babyEULAG model and the UWLCM apply the implicit large-eddy simulation approach, that is, without modeling of the unresolved subgrid-scale transport (see references to other studies applying this method in Grabowski, 2014;Pedersen et al, 2016).…”

Section: Example Of Application: Two-dimensional Moist Thermal Simulamentioning

confidence: 99%

Lagrangian condensation microphysics with Twomey CCN activation

2018

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Abstract. We report the development of a novel Lagrangian microphysics methodology for simulations of warm icefree clouds. The approach applies the traditional Eulerian method for the momentum and continuous thermodynamic fields such as the temperature and water vapor mixing ratio, and uses Lagrangian "super-droplets" to represent condensed phase such as cloud droplets and drizzle or rain drops. In other applications of the Lagrangian warm-rain microphysics, the super-droplets outside clouds represent unactivated cloud condensation nuclei (CCN) that become activated upon entering a cloud and can further grow through diffusional and collisional processes. The original methodology allows for the detailed study of not only effects of CCN on cloud microphysics and dynamics, but also CCN processing by a cloud. However, when cloud processing is not of interest, a simpler and computationally more efficient approach can be used with super-droplets forming only when CCN is activated and no super-droplet existing outside a cloud. This is possible by applying the Twomey activation scheme where the local supersaturation dictates the concentration of cloud droplets that need to be present inside a cloudy volume, as typically used in Eulerian bin microphysics schemes. Since a cloud volume is a small fraction of the computational domain volume, the Twomey super-droplets provide significant computational advantage when compared to the original superdroplet methodology. Additional advantage comes from significantly longer time steps that can be used when modeling of CCN deliquescence is avoided. Moreover, other formulation of the droplet activation can be applied in case of low vertical resolution of the host model, for instance, linking the concentration of activated cloud droplets to the local updraft speed. This paper discusses the development and testing of the Twomey super-droplet methodology, focusing on the activation and diffusional growth. Details of the activation implementation, transport of super-droplets in the physical space, and the coupling between super-droplets and the Eulerian temperature and water vapor field are discussed in detail. Some of these are relevant to the original superdroplet methodology as well and to the ice phase modeling using the Lagrangian approach. As a computational example, the scheme is applied to an idealized moist thermal rising in a stratified environment, with the original superdroplet methodology providing a benchmark to which the new scheme is compared.

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Resolution and domain‐size sensitivity in implicit large‐eddy simulation of the stratocumulus‐topped boundary layer

Cited by 29 publications

References 58 publications

Mesoscale organization, entrainment, and the properties of a closed‐cell stratocumulus cloud

Mesoscale organization, entrainment, and the properties of a closed‐cell stratocumulus cloud

Influences of Subsidence and Free‐Tropospheric Conditions on the Nocturnal Growth of Nonclassical Marine Stratocumulus

Lagrangian condensation microphysics with Twomey CCN activation

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