scholarly journals Large-Eddy Simulation of Air Parcels in Stratocumulus Clouds: Time Scales and Spatial Variability

2006 ◽  
Vol 63 (3) ◽  
pp. 952-967 ◽  
Author(s):  
Yefim L. Kogan
Keyword(s):  
Large Eddy Simulation ◽  
Spatial Variability ◽  
Time Scales ◽  
Residence Time ◽  
Turnover Time ◽  
Spatial Inhomogeneity ◽  
Cloud Base ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Stratocumulus Clouds

Abstract Large ensembles of air parcel trajectories driven by the (large-eddy simulation) LES-generated velocity fields from simulations of stratocumulus clouds were analyzed, focusing on statistics of air parcel in-cloud time scales, as well as their spatial variability. In the case of a drizzling stratocumulus cloud the in-cloud residence time is 2–5 times longer than the characteristic cloud eddy turnover time. About 70% of all air parcels cycle in the cloud more than 2 times and about 50% more than 3 times, thus indicating that air cycling is an essential feature of drizzling stratocumulus cloud dynamics. The extent of cycling is different in the case of nondrizzling stratocumulus cloud, where mean in-cloud time scales are on the order of eddy turnover time. Evidently air cycling in cloud depends on boundary layer stability and flow circulation; the latter is affected by cooling of evaporating drizzle and heating by solar radiation. Results show significant inhomogeneity of in-cloud time scales, which leads to inhomogeneity in cloud microphysical parameters. The potential effects of in-cloud residence time spatial inhomogeneity on cloud microstructure are obvious and significant. Older parcels will contain larger droplets and previously processed cloud condensation nuclei (CCN). Nonadiabatic mixing between old and new parcels provides new embryos for coagulation and accelerates drizzle formation. It is hypothesized that mixing of parcels with different histories, that is, with drop size distributions at different stages of their evolution, may contribute to the drop spectrum broadening. The results also suggest a possible positive feedback mechanism between drizzle and decoupling, namely, parcels with long time trajectories will favor enhanced drizzle growth, which, in turn, will lead to stronger evaporation below cloud base followed by a stronger increase in stability of the subcloud layer and stronger decoupling; all resulting in more air parcel cycling in cloud and more drizzle, which may eventually lead to stratocumulus cloud breakup.

scholarly journals Large Eddy Simulation of Microphysics and Influencing Factors in Shallow Convective Clouds

Atmosphere ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 485
Author(s):  
Zhuangzhuang Zhou ◽  
Chongzhi Yin ◽  
Chunsong Lu ◽  
Xingcan Jia ◽  
Fang Ye ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Influencing Factors ◽  
Vertical Velocity ◽  
Large Scale ◽  
Cloud Droplet ◽  
Relative Dispersion ◽  
Cloud Base ◽  
Convective Clouds ◽  
Eddy Simulation ◽  
Large Eddy

A flight of shallow convective clouds during the SCMS95 (Small Cumulus Microphysics Study 1995) observation project is simulated by the large eddy simulation (LES) version of the Weather Research and Forecasting Model (WRF-LES) with spectral bin microphysics (SBM). This study focuses on relative dispersion of cloud droplet size distributions, since its influencing factors are still unclear. After validation of the simulation by aircraft observations, the factors affecting relative dispersion are analyzed. It is found that the relationships between relative dispersion and vertical velocity, and between relative dispersion and adiabatic fraction are both negative. Furthermore, the negative relationships are relatively weak near the cloud base, strengthen with the increasing height first and then weaken again, which is related to the interplays among activation, condensation and evaporation for different vertical velocity and entrainment conditions. The results will be helpful to improve parameterizations related to relative dispersion (e.g., autoconversion and effective radius) in large-scale models.

Modeling a Stratocumulus-Topped PBL: Intercomparison among Different One-Dimensional Codes and with Large Eddy Simulation

1996 ◽  
Vol 77 (9) ◽  
pp. 2033-2042 ◽  
Author(s):  
P. Bechtold ◽  
S. K. Krueger ◽  
W. S. Lewellen ◽  
E. van Meijgaard ◽  
C.-H. Moeng ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Water Cycle ◽  
Global Scale ◽  
Model Complexity ◽  
One Dimensional ◽  
Eddy Simulation ◽  
Cloud Water Content ◽  
Large Eddy ◽  
Stratocumulus Clouds

Several one-dimensional (ID) cloud/turbulence ensemble modeling results of an idealized nighttime marine stratocumulus case are compared to large eddy simulation (LES). This type of model intercomparison was one of the objects of the first Global Energy and Water Cycle Experiment Cloud System Study boundary layer modeling workshop held at the National Center for Atmospheric Research on 16–18 August 1994. Presented are results obtained with different 1D models, ranging from bulk models (including only one or two vertical layers) to various types (first order to third order) of multilayer turbulence closure models. The ID results fall within the scatter of the LES results. It is shown that ID models can reasonably represent the main features (cloud water content, cloud fraction, and some turbulence statistics) of a well-mixed stratocumulus-topped boundary layer. Also addressed is the question of what model complexity is necessary and can be afforded for a reasonable representation of stratocumulus clouds in mesoscale or global-scale operational models. Bulk models seem to be more appropriate for climate studies, whereas a multilayer turbulence scheme is best suited in mesoscale models having at least 100- to 200-m vertical resolution inside the boundary layer.

scholarly journals A Year-Long Large-Eddy Simulation of the Weather over Cabauw: An Overview

2015 ◽  
Vol 143 (3) ◽  
pp. 828-844 ◽  
Author(s):  
Jerôme Schalkwijk ◽  
Harmen J. J. Jonker ◽  
A. Pier Siebesma ◽  
Fred C. Bosveld
Keyword(s):  
Time Series ◽  
Large Eddy Simulation ◽  
Time Scales ◽  
Large Scale ◽  
Measurement Techniques ◽  
Three Dimensional ◽  
Scale Model ◽  
Eddy Simulation ◽  
Wide Range ◽  
Large Eddy

Abstract Results are presented of two large-eddy simulation (LES) runs of the entire year 2012 centered at the Cabauw observational supersite in the Netherlands. The LES is coupled to a regional weather model that provides the large-scale information. The simulations provide three-dimensional continuous time series of LES-generated turbulence and clouds, which can be compared in detail to the extensive observational dataset of Cabauw. The LES dataset is available from the authors on request. This type of LES setup has a number of advantages. First, it can provide a more statistical approach to the study of turbulent atmospheric flow than the more common case studies, since a diverse but representative set of conditions is covered, including numerous transitions. This has advantages in the design and evaluation of parameterizations. Second, the setup can provide valuable information on the quality of the LES model when applied to such a wide range of conditions. Last, it also provides the possibility to emulate observation techniques. This might help detect limitations and potential problems of a variety of measurement techniques. The LES runs are validated through a comparison with observations from the observational supersite and with results from the “parent” large-scale model. The long time series that are generated, in combination with information on the spatial structure, provide a novel opportunity to study time scales ranging from seconds to seasons. This facilitates a study of the power spectrum of horizontal and vertical wind speed variance to identify the dominant variance-containing time scales.

scholarly journals On Large Eddy Simulation Based Conjugate Heat Transfer Procedure for Transient Natural Convection

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
M. Fadl ◽  
L. He
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Large Eddy Simulation ◽  
Time Scales ◽  
Time Scale ◽  
Gas Turbines ◽  
Conjugate Heat Transfer ◽  
Mesh Dependency ◽  
Eddy Simulation ◽  
Large Eddy

Natural convection is an important heat transfer mode for flexible operations of gas turbines and steam turbines. Its prediction presents considerable challenges. The strong interdependence between fluid and solid parts points to the need for coupled fluid–solid conjugate heat transfer (CHT) methods. The fundamental fluid–solid time scale disparity is further compounded by the long-time scales of practical turbine flexible operations. In addition, if a high-fidelity flow model (e.g., large eddy simulation (LES)) is adopted to resolve turbulence fluctuations, extra mesh dependency on solid domain mesh may arise. In this work, understanding of the extra solid mesh dependency in a directly coupled LES based CHT procedure is gained by an interface response analysis, leading to a simple and clear characterization of erroneously predicted unsteady interface temperatures and heat fluxes. A loosely coupled unsteady CHT procedure based on a multiscale methodology for solving problems with large time scale disparity is subsequently developed. A particular emphasis of this work is on efficient and accurate transient CHT solutions in conjunction with the turbulence eddy resolved modeling (LES) for natural convection. A two-scale flow decomposition associated with a corresponding time-step split is adopted. The resultant triple-timing formation of the flow equations can be solved efficiently for the fluid–solid coupled system with disparate time scales. The problem statement, analysis, and the solution methods will be presented with case studies to underline the issues of interest and to demonstrate the validity and effectiveness of the proposed methodology and implemented procedure.

scholarly journals Large-Eddy Simulation of the Stratocumulus-Capped Boundary Layer with Explicit Filtering and Reconstruction Turbulence Modeling

2018 ◽  
Vol 75 (2) ◽  
pp. 611-637 ◽  
Author(s):  
Xiaoming Shi ◽  
Hannah L. Hagen ◽  
Fotini Katopodes Chow ◽  
George H. Bryan ◽  
Robert L. Street
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Turbulence Modeling ◽  
Layer Structure ◽  
Turbulence Models ◽  
Eddy Simulation ◽  
Terra Incognita ◽  
Large Eddy ◽  
Stratocumulus Clouds ◽  
Subgrid Scales

Abstract Large-eddy simulation (LES) has been an essential tool in the development of theory and parameterizations for clouds, but when applied to stratocumulus clouds under sharp temperature inversions, many LES models produce an unrealistically thin cloud layer and a decoupled boundary layer structure. Here, explicit filtering and reconstruction are used for simulation of stratocumulus clouds observed during the first research flight (RF01) of the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II). The dynamic reconstruction model (DRM) is used within an explicit filtering and reconstruction framework, partitioning subfilter-scale motions into resolvable subfilter scales (RSFSs) and unresolvable subgrid scales (SGSs). The former are reconstructed, and the latter are modeled. Differing from traditional turbulence models, the reconstructed RSFS stress/fluxes can produce backscatter of turbulence kinetic energy (TKE) and, importantly, turbulence potential energy (TPE). The modeled backscatter reduces entrainment at the cloud top and, meanwhile, strengthens resolved turbulence through preserving TKE and TPE, resulting in a realistic boundary layer with an adequate amount of cloud water and vertically coupled turbulent eddies. Additional simulations are performed in the terra incognita, when the grid spacing of a simulation becomes comparable to the size of the most energetic eddies. With 20-m vertical and 1-km horizontal grid spacings, simulations using DRM provide a reasonable representation of bulk properties of the stratocumulus-capped boundary layer.

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