scholarly journals Modeling microphysical effects of entrainment in clouds observed during EUCAARI-IMPACT field campaign

2013 ◽  
Vol 13 (1) ◽  
pp. 1489-1526 ◽  
Author(s):  
D. Jarecka ◽  
H. Pawlowska ◽  
W. W. Grabowski ◽  
A. A. Wyszogrodzki
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Marine Boundary Layer ◽  
Subgrid Scale ◽  
Field Campaign ◽  
Eddy Simulation ◽  
Warm Rain ◽  
Large Eddy ◽  
Double Moment ◽  
The Mean

Abstract. This paper discusses aircraft observations and large-eddy simulation (LES) of the 15 May 2008, North Sea boundary-layer clouds from the EUCAARI-IMPACT field campaign. These clouds were advected from the north-east by the prevailing lower-tropspheric winds, and featured stratocumulus-over-cumulus cloud formations. Almost-solid stratocumulus deck in the upper part of the relatively deep weakly decoupled marine boundary layer overlaid a field of small cumuli with a cloud fraction of ~10%. The two cloud formations featured distinct microphysical characteristics that were in general agreement with numerous past observations of strongly-diluted shallow cumuli on the one hand and solid marine boundary-layer stratocumulus on the other. Macrophysical and microphysical cloud properties were reproduced well by the double-moment warm-rain microphysics large-eddy simulation. A novel feature of the model is its capability to locally predict homogeneity of the subgrid-scale mixing between the cloud and its cloud-free environment. In the double-moment warm-rain microphysics scheme, the homogeneity is controlled by a single parameter α, that ranges from 0 to 1 and limiting values representing the homogeneous and the extremely inhomogeneous mixing scenarios, respectively. Parameter α depends on the characteristic time scales of the droplet evaporation and of the turbulent homogenization. In the model, these scales are derived locally based on the subgrid-scale turbulent kinetic energy, spatial scale of cloudy filaments, the mean cloud droplet radius, and the humidity of the cloud-free air entrained into the cloud. Simulated mixing is on average quite inhomogeneous, with the mean parameter α around 0.7 across the entire depth of the cloud field, but with local variations across almost the entire range, especially near the base and the top of the cloud field.

Homogeneity of the Subgrid-Scale Turbulent Mixing in Large-Eddy Simulation of Shallow Convection

2013 ◽  
Vol 70 (9) ◽  
pp. 2751-2767 ◽  
Author(s):  
Dorota Jarecka ◽  
Wojciech W. Grabowski ◽  
Hugh Morrison ◽  
Hanna Pawlowska
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Mixing ◽  
Droplet Radius ◽  
Subgrid Scale ◽  
Shallow Convection ◽  
Eddy Simulation ◽  
Droplet Concentration ◽  
Large Eddy ◽  
Double Moment ◽  
The Mean

Abstract This paper presents an approach to locally predict homogeneity of the subgrid-scale turbulent mixing in large-eddy simulation of shallow clouds applying double-moment warm-rain microphysics. The homogeneity of subgrid-scale mixing refers to the partitioning of the cloud water evaporation due to parameterized entrainment between changes of the mean droplet radius and changes of the mean droplet concentration. Homogeneous and extremely inhomogeneous mixing represent two limits of possible scenarios, where the droplet concentration and the mean droplet radius remains unchanged during the microphysical adjustment, respectively. To predict the subgrid-scale mixing scenario, the double-moment microphysics scheme is merged with the approach to delay droplet evaporation resulting from entrainment. Details of the new scheme and its application in the Barbados Oceanographic and Meteorological Experiment (BOMEX) shallow convection case are discussed. The simulated homogeneity of mixing varies significantly inside small convective clouds, from close to homogeneous to close to extremely inhomogeneous. The mean mixing characteristics become more homogeneous with height, reflecting increases of the mean droplet size and the mean turbulence intensity, both favoring homogeneous mixing. Model results are consistent with microphysical effects of entrainment and mixing deduced from field observations. Mixing close to homogeneous is predicted in volumes with the highest liquid water content (LWC) and strongest updraft at a given height, whereas mixing in strongly diluted volumes is typically close to extremely inhomogeneous. The simulated homogeneity of mixing has a small impact on mean microphysical characteristics. This result agrees with the previous study applying prescribed mixing scenarios and can be explained by the high humidity of the clear air involved in the subgrid-scale mixing.

Subgrid-scale modelling for the large-eddy simulation of high-Reynolds-number boundary layers

1997 ◽  
Vol 336 ◽  
pp. 151-182 ◽  
Author(s):  
BRANKO KOSOVIĆ
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Large Eddy Simulation ◽  
Boundary Layers ◽  
Nonlinear Model ◽  
High Reynolds Number ◽  
Subgrid Scale ◽  
Eddy Simulation ◽  
Large Eddy ◽  
High Reynolds

It has been recognized that the subgrid-scale (SGS) parameterization represents a critical component of a successful large-eddy simulation (LES). Commonly used linear SGS models produce erroneous mean velocity profiles in LES of high-Reynolds-number boundary layer flows. Although recently proposed approaches to solving this problem have resulted in significant improvements, questions about the true nature of the SGS problem in shear-driven high-Reynolds-number flows remain open.We argue that the SGS models must capture inertial transfer effects including backscatter of energy as well as its redistribution among the normal SGS stress components. These effects are the consequence of nonlinear interactions and anisotropy. In our modelling procedure we adopt a phenomenological approach whereby the SGS stresses are related to the resolved velocity gradients. We show that since the SGS stress tensor is not frame indifferent a more general nonlinear model can be applied to the SGS parameterization. We develop a nonlinear SGS model capable of reproducing the effects of SGS anisotropy characteristic for shear-driven boundary layers. The results obtained using the nonlinear model for the LES of a neutral shear-driven atmospheric boundary layer show a significant improvement in prediction of the non-dimensional shear and low-order statistics compared to the linear Smagorinsky-type models. These results also demonstrate a profound effect of the SGS model on the flow structures.

Evaluation of the WRF PBL Parameterizations for Marine Boundary Layer Clouds: Cumulus and Stratocumulus

2013 ◽  
Vol 141 (7) ◽  
pp. 2265-2271 ◽  
Author(s):  
Hsin-Yuan Huang ◽  
Alex Hall ◽  
Joao Teixeira
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Marine Boundary Layer ◽  
Layer Structure ◽  
Model Performance ◽  
Simulation Models ◽  
Eddy Diffusivity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Boundary Layer Cloud

Abstract The performance of five boundary layer parameterizations in the Weather Research and Forecasting Model is examined for marine boundary layer cloud regions running in single-column mode. Most parameterizations show a poor agreement of the vertical boundary layer structure when compared with large-eddy simulation models. These comparisons against large-eddy simulation show that a parameterization based on the eddy-diffusivity/mass-flux approach provides a better performance. The results also illustrate the key role of boundary layer parameterizations in model performance.

Large eddy simulation of a stratified boundary layer under an oscillatory current

2009 ◽  
Vol 643 ◽  
pp. 233-266 ◽  
Author(s):  
BISHAKHDATTA GAYEN ◽  
SUTANU SARKAR ◽  
JOHN R. TAYLOR
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Mixed Layer ◽  
Numerical Study ◽  
Bottom Boundary Layer ◽  
Mean Velocity ◽  
Bottom Boundary ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean

A numerical study based on large eddy simulation is performed to investigate a bottom boundary layer under an oscillating tidal current. The focus is on the boundary layer response to an external stratification. The thermal field shows a mixed layer that is separated from the external stratified fluid by a thermocline. The mixed layer grows slowly in time with an oscillatory modulation by the tidal flow. Stratification strongly affects the mean velocity profiles, boundary layer thickness and turbulence levels in the outer region although the effect on the near-bottom unstratified fluid is relatively mild. The turbulence is asymmetric between the accelerating and decelerating stages. The asymmetry is more pronounced with increasing stratification. There is an overshoot of the mean velocity in the outer layer; this jet is linked to the phase asymmetry of the Reynolds shear stress gradient by using the simulation data to examine the mean momentum equation. Depending on the height above the bottom, there is a lag of the maximum turbulent kinetic energy, dissipation and production with respect to the peak external velocity and the value of the lag is found to be influenced by the stratification. Flow instabilities and turbulence in the bottom boundary layer excite internal gravity waves that propagate away into the ambient. Unlike the steady case, the phase lines of the internal waves change direction during the tidal cycle and also from near to far field. The frequency spectrum of the propagating wave field is analysed and found to span a narrow band of frequencies clustered around 45°.

scholarly journals Sampling error in aircraft flux measurements based on a high-resolution large eddy simulation of the marine boundary layer

2021 ◽  
Vol 14 (3) ◽  
pp. 1959-1976
Author(s):  
Grant W. Petty
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Random Error ◽  
Marine Boundary Layer ◽  
Sampling Error ◽  
Track Length ◽  
Length Scales ◽  
Eddy Simulation ◽  
Large Eddy

Abstract. A high-resolution (1.25 m) large eddy simulation (LES) of the nocturnal cloud-topped marine boundary layer is used to evaluate random error as a function of continuous track length L for virtual aircraft measurements of turbulent fluxes of sensible heat, latent heat, and horizontal momentum. Results are compared with the widely used formula of Lenschow and Stankov (1986). In support of these comparisons, the relevant integral length scales and correlations are evaluated and documented. It is shown that for heights up to approximately 100 m (z/zi=0.12), the length scales are accurately predicted by empirical expressions of the form If=Azb. The Lenschow and Stankov expression is found to be remarkably accurate at predicting the random error for shorter (7–10 km) flight tracks, but the empirically determined errors decay more rapidly with L than the L-1/2 relationship predicted from theory. Consistent with earlier findings, required track lengths to obtain useful precision increase sharply with altitude. In addition, an examination is undertaken of the role of uncertainties in empirically determined integral length scales and correlations in flux uncertainties as well as of the flux errors associated with crosswind and along-wind flight tracks. It is found that for 7.2 km flight tracks, flux errors are improved by factor of approximately 1.5 to 2 for most variables by making measurements in the crosswind direction.

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