scholarly journals Large-Eddy Simulations of the Impact of Ground-Based Glaciogenic Seeding on Shallow Orographic Convection: A Case Study

2017 ◽  
Vol 56 (1) ◽  
pp. 69-84 ◽  
Author(s):  
Xia Chu ◽  
Bart Geerts ◽  
Lulin Xue ◽  
Roy Rasmussen
Keyword(s):  
Liquid Water ◽  
Supercooled Liquid ◽  
Liquid Water Content ◽  
Diffusional Growth ◽  
Large Eddy Simulation Model ◽  
Surface Precipitation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Glaciogenic Seeding ◽  
The Impact

AbstractThis study uses the WRF large-eddy simulation model at 100-m resolution to examine the impact of ground-based glaciogenic seeding on shallow (~2 km deep), cold-based convection producing light snow showers over the Sierra Madre in southern Wyoming on 13 February 2012, as part of the AgI Seeding Cloud Impact Investigation (ASCII). Detailed observations confirm that simulation faithfully captures the orographic flow, convection, and natural snow production, especially on the upwind side. A comparison between treated and control simulations indicates that glaciogenic seeding effectively converts cloud water in convective updrafts to ice and snow in this case, resulting in increased surface precipitation. This comparison further shows that seeding enhances liquid water depletion by vapor deposition, and enhances buoyancy, updraft strength, and cloud-top height. This suggests that the dynamic seeding concept applies, notwithstanding the clouds’ low natural supercooled liquid water content. But the simulated cloud-top-height changes are benign (typically <100 m). This, combined with the fact that most natural and enhanced snow growth occurs in a temperature range in which the Bergeron diffusional growth process is effective, suggests that the modeled snowfall enhancement is largely due to static (microphysical) processes rather than dynamic ones.

scholarly journals Evaluation of Orographic Cloud Seeding Using a Bin Microphysics Scheme: Three-Dimensional Simulation of Real Cases

2020 ◽  
Vol 59 (9) ◽  
pp. 1537-1555
Author(s):  
István Geresdi ◽  
Lulin Xue ◽  
Noémi Sarkadi ◽  
Roy Rasmussen
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Cloud Condensation Nuclei ◽  
Three Dimensional ◽  
Liquid Water Content ◽  
Weather Modification ◽  
Diffusional Growth ◽  
Surface Precipitation ◽  
Orographic Clouds ◽  
The Impact

AbstractThe University of Pécs and NCAR Bin (UPNB) microphysical scheme was implemented into the mesoscale Weather Research and Forecast (WRF) Model that was used to study the impact of silver iodide (AgI) seeding on precipitation formation in winter orographic clouds. Four different experimental units were chosen from the Wyoming Weather Modification Pilot Project to simulate the seeding effect. The results of the numerical experiments show the following: (i) Comparisons with the soundings, snow gauges, and microwave radiometer data indicate that the three-dimensional simulations with detailed microphysics reasonably represent both the dynamics and the microphysics of real clouds. (ii) The dispersion of the AgI particles from the simulated ground-based seeding was effective because of turbulent mixing. (iii) In the investigated cases (surface temperature is less than 0°C), surface precipitation and precipitation efficiency show low susceptibility to the concentrations of cloud condensation nuclei and natural ice nucleating particles. (iv) If the available liquid water content promotes the enhancement of the number of snowflakes by diffusional growth, the surface precipitation can be increased by more than 5%. A novel parameter relevant to orographic clouds, horizontally integrated liquid water path (LWP), was evaluated to find the relation between seeding efficiency and liquid water content. The impact of seeding is negligible if the horizontal LWP is less than 0.1 mm and is apparent if the horizontal LWP is larger than 1 mm, as based on the cases investigated in this study.

scholarly journals Comparison of Observed and Simulated Drop Size Distributions from Large-Eddy Simulations with Bin Microphysics

2019 ◽  
Vol 147 (2) ◽  
pp. 477-493
Author(s):  
Mikael K. Witte ◽  
Patrick Y. Chuang ◽  
Orlando Ayala ◽  
Lian-Ping Wang ◽  
Graham Feingold
Keyword(s):  
Liquid Water ◽  
Drop Size ◽  
Potential Temperature ◽  
Liquid Water Content ◽  
Liquid Water Path ◽  
Cloud Drop ◽  
Large Eddy Simulation Model ◽  
Surface Precipitation ◽  
Large Eddy ◽  
Bin Microphysics

Abstract Two case studies of marine stratocumulus (one nocturnal and drizzling, the other daytime and nonprecipitating) are simulated by the UCLA large-eddy simulation model with bin microphysics for comparison with aircraft in situ observations. A high-bin-resolution variant of the microphysics is implemented for closer comparison with cloud drop size distribution (DSD) observations and a turbulent collision–coalescence kernel to evaluate the role of turbulence on drizzle formation. Simulations agree well with observational constraints, reproducing observed thermodynamic profiles (i.e., liquid water potential temperature and total moisture mixing ratio) as well as liquid water path. Cloud drop number concentration and liquid water content profiles also agree well insofar as the thermodynamic profiles match observations, but there are significant differences in DSD shape among simulations that cause discrepancies in higher-order moments such as sedimentation flux, especially as a function of bin resolution. Counterintuitively, high-bin-resolution simulations produce broader DSDs than standard resolution for both cases. Examination of several metrics of DSD width and percentile drop sizes shows that various discrepancies of model output with respect to the observations can be attributed to specific microphysical processes: condensation spuriously creates DSDs that are too wide as measured by standard deviation, which leads to collisional production of too many large drops. The turbulent kernel has the greatest impact on the low-bin-resolution simulation of the drizzling case, which exhibits greater surface precipitation accumulation and broader DSDs than the control (quiescent kernel) simulations. Turbulence effects on precipitation formation cannot be definitively evaluated using bin microphysics until the artificial condensation broadening issue has been addressed.

scholarly journals On the Fidelity of Large-Eddy Simulation of Shallow Precipitating Cumulus Convection

2011 ◽  
Vol 139 (9) ◽  
pp. 2918-2939 ◽  
Author(s):  
Georgios Matheou ◽  
Daniel Chung ◽  
Louise Nuijens ◽  
Bjorn Stevens ◽  
Joao Teixeira
Keyword(s):  
Large Eddy Simulation ◽  
Liquid Water ◽  
Cloud Cover ◽  
Scale Model ◽  
Trade Wind ◽  
Numerical Dissipation ◽  
Surface Precipitation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Impact

The present study considers the impact of various choices pertaining to the numerical solution of the governing equations on large-eddy simulation (LES) prediction and the association of these choices with flow physics. These include the effect of dissipative versus nondissipative advection discretizations, different implementations of the constant-coefficient Smagorinsky subgrid-scale model, and grid resolution. Simulations corresponding to the trade wind precipitating shallow cumulus composite case of the Rain in Cumulus over the Ocean (RICO) field experiment were carried out. Global boundary layer quantities such as cloud cover, liquid water path, surface precipitation rate, power spectra, and the overall convection structure were used to compare the effects of different discretization implementations. The different discretization implementations were found to exert a significant impact on the LES prediction even for the cases where the process of precipitation was not included. Increasing numerical dissipation decreases cloud cover and surface precipitation rates. For nonprecipitating cases, grid convergence is achieved for grid spacings of 20 m. Cloud cover was found to be particularly sensitive, exhibiting variations between different resolution runs even when the mean liquid water profile had converged.

scholarly journals A Case Study of Cloud Radar Observations and Large-Eddy Simulations of a Shallow Stratiform Orographic Cloud, and the Impact of Glaciogenic Seeding

2017 ◽  
Vol 56 (5) ◽  
pp. 1285-1304 ◽  
Author(s):  
Xia Chu ◽  
Bart Geerts ◽  
Lulin Xue ◽  
Binod Pokharel
Keyword(s):  
Liquid Water ◽  
Supercooled Liquid ◽  
Cold Season ◽  
Cloud Base ◽  
Target Area ◽  
Blowing Snow ◽  
Large Eddy ◽  
Orographic Clouds ◽  
Glaciogenic Seeding ◽  
The Impact

AbstractThe impact of glaciogenic seeding on precipitation remains uncertain, mainly because of the noisy nature of precipitation. Operational seeding programs often target cold-season orographic clouds because of their abundance of supercooled liquid water. Such clouds are complicated because of common natural seeding from above (seeder–feeder effect) or from below (blowing snow). Here, observations, mainly from a profiling airborne Doppler radar, and numerical simulations are used to examine the impact of glaciogenic seeding on a very shallow (<1 km), largely blocked cloud that is not naturally seeded from aloft or from below. This cloud has limited but persistent supercooled liquid water, a cloud-base (top) temperature of −12°C (−16°C), and produces only very light snowfall naturally. A Weather Research and Forecasting Model large-eddy simulation at 100-m resolution captures the observed upstream stability and wind profiles and reproduces the essential characteristics of the orographic flow, cloud, and precipitation. Both observations and simulations indicate that seeding locally increases radar (or computed) reflectivity in the target area, even after removal of the natural trend between these two periods in a nearby control region. A model sensitivity run suggests that seeding effectively glaciates the mostly liquid cloud and substantially increases snowfall within the seeding plume. This is due to a dramatic increase in the number of ice particles and not to their size. The increased ice particle concentration facilitates snow growth by vapor deposition in a cloud the temperature range of which is conducive to the Bergeron process.

scholarly journals A Stochastic Approach for Parameterizing Unresolved Scales in a System with Memory

2009 ◽  
Vol 3 (3) ◽  
pp. 393-405 ◽  
Author(s):  
Aijun Du ◽  
Jinqiao Duan
Keyword(s):  
Differential Equation ◽  
Partial Differential Equation ◽  
Large Eddy Simulation ◽  
Simulation Model ◽  
Partial Differential ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Impact ◽  
With Memory

Complex systems display variability over a broad range of spatial and temporal scales. Some scales are unresolved due to computational limitations. The impact of these unresolved scales on the resolved scales needs to be parameterized or taken into account. One stochastic parameterization scheme is devised to take the effects of unresolved scales into account, in the context of solving a nonlinear partial differential equation with memory (a time-integral term), via large eddy simulations. The obtained large eddy simulation model is a stochastic partial differential equation. Numerical experiments are performed to compare the solutions of the original system and of the stochastic large eddy simulation model.

scholarly journals Performance of turbulence and cavitation models in prediction of incipient and developed cavitation

2016 ◽  
Vol 18 (4) ◽  
pp. 333-350 ◽  
Author(s):  
Phoevos Koukouvinis ◽  
Homa Naseri ◽  
Manolis Gavaises
Keyword(s):  
Large Eddy Simulation ◽  
Simulation Model ◽  
Reynolds Stress ◽  
Eddy Viscosity ◽  
Navier Stokes ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Reynolds Averaged Navier Stokes ◽  
The Impact

The aim of this article is to assess the impact of turbulence and cavitation models on the prediction of diesel injector nozzle flow. Two nozzles are examined, an enlarged one, operating at incipient cavitation, and an industrial injector tip, operating at developed cavitation. The turbulence model employed includes the re-normalization group k–ε, realizable k–ε and k–ω shear stress transport Reynolds-averaged Navier–Stokes models; linear pressure–strain Reynolds stress model and the wall adapting local eddy viscosity large eddy simulation model. The results indicate that all Reynolds-averaged Navier–Stokes and the Reynolds stress turbulence models have failed to predict cavitation inception due to their limitation to resolve adequately the low pressure existing inside vortex cores, which is responsible for cavitation development in this particular flow configuration. Moreover, Reynolds-averaged Navier–Stokes models failed to predict unsteady cavitation phenomena in the industrial injector. However, the wall adapting local eddy viscosity large eddy simulation model was able to predict incipient and developed cavitation, while also capturing the shear layer instability, vortex shedding and cavitating vortex formation. Furthermore, the performance of two cavitation methodologies is discussed within the large eddy simulation framework. In particular, a barotropic model and a mixture model based on the asymptotic Rayleigh–Plesset equation of bubble dynamics have been tested. The results indicate that although the solved equations and phase change formulation are different in these models, the predicted cavitation and flow field were very similar at incipient cavitation conditions. At developed cavitation conditions, standard cavitation models may predict unrealistically high liquid tension, so modifications may be essential. It is also concluded that accurate turbulence representation is crucial for cavitation in nozzle flows.

scholarly journals Impact of Gravity Waves on Marine Stratocumulus Variability

2012 ◽  
Vol 69 (12) ◽  
pp. 3633-3651 ◽  
Author(s):  
Qingfang Jiang ◽  
Shouping Wang
Keyword(s):  
Gravity Waves ◽  
Vertical Motion ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Large Eddy Simulation Model ◽  
Cloud Albedo ◽  
Eddy Simulation ◽  
Aerosol Number Concentration ◽  
Wave Induced ◽  
The Impact

Abstract The impact of gravity waves on marine stratocumulus is investigated using a large-eddy simulation model initialized with sounding profiles composited from the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-Rex) aircraft measurements and forced by convergence or divergence that mimics mesoscale diurnal, semidiurnal, and quarter-diurnal waves. These simulations suggest that wave-induced vertical motion can dramatically modify the cloud albedo and morphology through nonlinear cloud–aerosol–precipitation–circulation–turbulence feedback. In general, wave-induced ascent tends to increase the liquid water path (LWP) and the cloud albedo. With a proper aerosol number concentration, the increase in the LWP leads to enhanced precipitation, which triggers or strengthens mesoscale circulations in the boundary layer and accelerates cloud cellularization. Precipitation also tends to create a decoupling structure by weakening the turbulence in the subcloud layer. Wave-induced descent decreases the cloud albedo by dissipating clouds and forcing a transition from overcast to scattered clouds or from closed to open cells. The overall effect of gravity waves on the cloud variability and morphology depends on the cloud property, aerosol concentration, and wave characteristics. In several simulations, a transition from closed to open cells occurs under the influence of gravity waves, implying that some of the pockets of clouds (POCs) observed over open oceans may be related to gravity wave activities.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame With Coupled Experiments and Simulations

2021 ◽  
Author(s):  
Jihang Li ◽  
Hyunguk Kwon ◽  
Drue Seksinsky ◽  
Daniel Doleiden ◽  
Jacqueline O’Connor ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Equivalence Ratio ◽  
Vortex Breakdown ◽  
Eddy Simulation ◽  
Breakdown Region ◽  
Large Eddy ◽  
Rate Oscillations ◽  
Combustion Oscillation ◽  
The Stability ◽  
The Impact

Abstract Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. While the use of pilot flames is common in land-based gas turbine combustors, the mechanism by which they suppress instability is still unclear. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot gases as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. Further, the pilot flame efficacy increases with pilot flame equivalence ratio until it matches the main flame equivalence ratio; at pilot equivalence ratios greater than the main equivalence ratio, the pilot flame efficacy does not change significantly with pilot equivalence ratio. To understand these results, we use large-eddy simulation to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame. The simulation, using a flamelet/progress variable-based chemistry tabulation approach and standard eddy viscosity/diffusivity turbulence closure models, provides detailed information that is inaccessible through experimental measurements.

scholarly journals Large eddy simulation model for wind-driven sea circulation in coastal areas

2013 ◽  
Vol 20 (6) ◽  
pp. 1095-1112 ◽  
Author(s):  
A. Petronio ◽  
F. Roman ◽  
C. Nasello ◽  
V. Armenio
Keyword(s):  
Large Eddy Simulation ◽  
Simulation Model ◽  
Water Column ◽  
Eddy Viscosity ◽  
Vertical Mixing ◽  
Water Circulation ◽  
Bottom Surface ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Eddy

Abstract. In the present paper a state-of-the-art large eddy simulation model (LES-COAST), suited for the analysis of water circulation and mixing in closed or semi-closed areas, is presented and applied to the study of the hydrodynamic characteristics of the Muggia bay, the industrial harbor of the city of Trieste, Italy. The model solves the non-hydrostatic, unsteady Navier–Stokes equations, under the Boussinesq approximation for temperature and salinity buoyancy effects, using a novel, two-eddy viscosity Smagorinsky model for the closure of the subgrid-scale momentum fluxes. The model employs: a simple and effective technique to take into account wind-stress inhomogeneity related to the blocking effect of emerged structures, which, in turn, can drive local-scale, short-term pollutant dispersion; a new nesting procedure to reconstruct instantaneous, turbulent velocity components, temperature and salinity at the open boundaries of the domain using data coming from large-scale circulation models (LCM). Validation tests have shown that the model reproduces field measurement satisfactorily. The analysis of water circulation and mixing in the Muggia bay has been carried out under three typical breeze conditions. Water circulation has been shown to behave as in typical semi-closed basins, with an upper layer moving along the wind direction (apart from the anti-cyclonic veering associated with the Coriolis force) and a bottom layer, thicker and slower than the upper one, moving along the opposite direction. The study has shown that water vertical mixing in the bay is inhibited by a large level of stable stratification, mainly associated with vertical variation in salinity and, to a minor extent, with temperature variation along the water column. More intense mixing, quantified by sub-critical values of the gradient Richardson number, is present in near-coastal regions where upwelling/downwelling phenomena occur. The analysis of instantaneous fields has detected the presence of large cross-sectional eddies spanning the whole water column and contributing to vertical mixing, associated with the presence of sub-surface horizontal turbulent structures. Analysis of water renewal within the bay shows that, under the typical breeze regimes considered in the study, the residence time of water in the bay is of the order of a few days. Finally, vertical eddy viscosity has been calculated and shown to vary by a couple of orders of magnitude along the water column, with larger values near the bottom surface where density stratification is smaller.

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