Combined effect of the Weber-Bergeron-Findeisen mechanism and large eddies on microphysics of mixed-phase stratiform clouds

The process of glaciation in mixed-phase stratiform clouds was investigated by a novel Lagrangian-Eulerian model (LEM) in which thousands of adjoining Lagrangian parcels moved within a turbulent-like velocity field with statistical parameters typical of the Arctic boundary layer. We used detailed bin microphysics to describe the condensation/evaporation processes in each parcel, in which droplets, aerosols, and ice particles were described using size distributions of 500 mass bins. The model also calculated aerosol mass inside droplets and ice particles. Gravitational sedimentation of droplets and ice particles was also accounted for. Assuming that droplet freezing is the primary source of ice particles, the Arctic clouds observed in ISDAC were successfully simulated. The model showed that at a low ice particle concentration typical of ISDAC, large vortices (eddies) led to a quasi-stationary regime, in which mixed-phase St existed for a long time. The large eddies controlled the water partitioning in the mixed-phase clouds. Droplets formed and grew in updrafts, typically reaching the cloud top, and evaporated in downdrafts. Ice particles grew in updrafts and downdrafts. The Wegener-Bergeron-Findeisen (WBF) mechanism was efficient in downdrafts and some parts of updrafts, depending on ice concentration and vertical velocity. At low ice concentrations, the effect of ice on the phase partitioning was negligible. In this regime, liquid droplets were found near the cloud top, whereas ice particles precipitated through the cloud base. When ice concentration exceeded about 10 per liter, the WBF mechanism led to glaciation of almost the entire cloud, with the exception of narrow cloud regions associated with strong updrafts. At ice particle concentrations of a few tens per liter, the oscillatory regime took place due to the ice-liquid interaction. The microphysical structure of mixed-phase St forms as a combined effect of cloud dynamics (large eddies) and the WBF mechanism

Reynolds stress scaling in the near-wall region of wall-bounded flows

A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour.

The Influence of Variable Density on Turbulent Wall Jet: A Numerical Comparative Study

The first objective of this numerical research is to help understand the influence of variable density on the structure of turbulence, through the study of a wall jet, and to validate our results with those of the experimental study of A. Soudani. The source of density variation is the mixture between two different non-reactive fluids, with a fixed temperature and pressure. A mass weighted averaging for different variables is applied to the calculation, using ANSYS FLUENT 15.0 commercial software. The principal experience consists of injecting tangentially and alternatively near the wall a gas (air-helium) different from the external flow, through a slot of height 3mm between two plane walls. Such a process permits to provoke an important density difference. The study reaches the conclusion that turbulence is strong, with a slight increase of velocity near the wall and an evident diminution of skin friction, in the case of light fluid injection. The second aim is to estimate the Kolmogorov and large eddies’ scales to construct LES grid to access instant variables in experience.

Flow Characterization Near the Nozzle Exit of the Supersonic Steam Jet Injecting into the Stagnant Water

There has been a large amount of work being conducted on the thermo-dynamics of the Direct Contact Condensation (DCC), however, not much attention was given to the phenomena particularly active near the steam’s nozzle exit. A transparent rectangular upright duct of 4 ft high, was built with a supersonic nozzle positioned at the bottom of the channel to characterize flow behavior near the steam nozzle’s exit. Particle image velocimetry (PIV) was applied to draw information on the steam’s jet penetration into the water as well as the entrainment and mixing between the two phases under the steam’s inlet pressure ranging from 1.5 – 3.0 bars. PIV normalized contour measurements depicted not appreciable changes in the radial velocity of the jet. Whereas, in the core region of the jet, the change in the jet’s velocity was not much till Y/De ~ 4.3 and the vertical velocity of the jet decreased slowly till Y/De ~ 8. The jet’s normalized upward velocity attained an optimized value between Y/De ~ 8 and Y/De ~ 9.8. With varying pressures, 1.5 bars to 3.0 bars, the jet expanded radially in water. It was also found in the near nozzle exit region, the shear layer’s thickness remained within 0.2 – 0.5 De over the 1.5 – 3.0 bars pressure. Probability Density Function (PDF) analysis of Reynolds shear and normal stresses confirmed the existence of the velocity fluctuations across the shear layer, owing to the large eddies across the steam-water interface.

Sensitivity of Fine-Scale Structure in Tropical Cyclone Boundary Layer to Model Horizontal Resolution at Sub-Kilometer Grid Spacing

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone (TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7–10 h simulation of an idealized TC are examined using the Weather Research and Forecast (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex, the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future.

ENTRAINMENT IN A SIMULATED SUPERCELL THUNDERSTORM, PART I: THE EVOLUTION OF DIFFERENT ENTRAINMENT MECHANISMS AND THEIR DILUTIVE EFFECTS

The current study identifies and quantifies various mechanisms of entrainment, and their diluting effects, in the developing and mature stages of a simulated supercell thunderstorm. The two stages, differentiated by the lack or presence of a rotating updraft, are shown to entrain air by different, but related mechanisms that result from the strong vertical wind shear of the environment. The greatest entrainment rates in the developing stage result from the asymmetric overturning of large eddies near cloud top on the down-shear side. These rates are greater than those published in the literature for cumuli developing in environments lacking strong shear. Although the entrainment rate increases exponentially in time throughout the developing stage, successive cloud turrets help to replenish some of the lost buoyancy and condensate, allowing the nascent storm to develop further. During the mature stage, the greatest entrainment rates occur via “ribbons” of horizontal vorticity wrapping around the rotating updraft that ascend in time. The smaller width of the ribbons in comparison to the wider storm core limits their dilutive effects. Passive tracers placed in the low-level air ingested by the mature storm indicate that on average 20% of the core contains some undiluted air ingested from below the storm base, unaffected by any entrainment mechanism.

Vertical and slanted sound propagation in the near-ground atmosphere : amplitude and phase fluctuations

Sound propagation along vertical and slanted paths through the near-ground atmosphere impacts detection and localization of low-altitude sound sources, such as small unmanned aerial vehicles, from ground-based microphone arrays. This article experimentally investigates the amplitude and phase fluctuations of acoustic signals propagating along such paths. The experiment involved nine microphones on three horizontal booms mounted at different heights to a 135-m meteorological tower at the National Wind Technology Center (Boulder, CO). A ground-based loudspeaker was placed at the base of the tower for vertical propagation or 56m from the base of the tower for slanted propagation. Phasor scatterplots qualitatively characterize the amplitude and phase fluctuations of the received signals during different meteorological regimes. The measurements are also compared to a theory describing the log-amplitude and phase variances based on the spectrum of shear and buoyancy driven turbulence near the ground. Generally, the theory correctly predicts the measured log-amplitude variances, which are affected primarily by small-scale, isotropic turbulent eddies. However, the theory overpredicts the measured phase variances, which are affected primarily by large-scale, anisotropic, buoyantly driven eddies. Ground blocking of these large eddies likely explains the overprediction.

Multifractals, free turbulence decay laws and the Scaling Gyroscopes Cascade model

<p>Turbulence being a dissipative system decays when being "free", i.e. without any force. The law of this decay has been intriguing for quite a while. Assuming that for vanishing viscosity, the whole spectrum is self-similar, as well as stationary for low wave numbers/large eddies  (E(k,t) ≈C<sub>S</sub> k<sup>S</sup>, k → 0) , it was shown [1] that the total energy of turbulence has a power-law decay: E(t) = ∫ E(k,t) dk ≈ t<sup>-a(s)</sup>: a(s) =2(s+1)/(s+3) . This was particularly thought to be relevant for s=4, C<sub>4</sub> being proportional to the Loitsianski integral, assumed to be time-invariant [2]. However, it was shown with the help of the eddy-damped quasi-normal Markovian (EDQNM) [3] that there is an energy backscatter term transferring energy from energy-containing eddies by nonlocal triads interactions to large eddies, which behaves like T<sub>NL</sub>≈ k<sup>4</sup> and therefore prevents the invariance of the Loitsianski integral. This implies that the theoretical exponent a(s) = 2(s+1)/(s+3)  is only valid for s<4 and that a(s) =a(4)=-(10-2γ)/7 for s≥ 4 with C<sub>4</sub>(t) ≈ t <sup>γ</sup>, γ>0. The turbulence decay is therefore slower than previously expected for s ≥ 4 due to the backscatter term that progressively stores energy in large eddies. <br>EDQNM provides the estimate γ ≈ 0.16. However, a strong limitation of EDQNM and similar models (e.g. Direct Interaction Approximation, Test Field Model) is that these models are not able to represent intermittency, which is a fundamental phenomenon of turbulence [4] and this could bring into questions the previous results. We, therefore, investigate this question with the Scaling Gyroscopes Cascade (SGC) model [5], which is based on nonlocal interactions and display multifractal intermittency [6]. We first theoretically argue that SGC confirms the existence of the backscatter term, but the turbulence decay is no longer smooth but occurs by puffs and we provide numerical evidence of this.</p><p>Keywords: Loitsianski integral; intermittency; infrared spectrum; SGC model; energy decay</p><p>[1]M. Lesieur and D. Schertzer, ‘‘Amortissement auto-similaire d’une turbulence a‘ grand nombre de Reynolds,’’ J. Mec. 17, 609 1978 .</p><p>[2]Davidson, P. A. (2000). Was Loitsyansky correct? A review of the arguments. <em>Journal of Turbulence</em>, <em>1</em>(1), 006-006.</p><p>[3]Frisch, U., Lesieur, M.,Schertzer, D. (1980). Comments on the quasi- normal Markovian approximation for fully-developed turbulence. Jour- nal of Fluid Mechanics, 97(1), 181-192.</p><p>[4]Morf, R. H., Orszag, S. A., Frisch, U. (1980). Spontaneous singularity in three-dimensional inviscid, incompressible ow. Physical Review Letters, 44(9), 572.</p><p>[5]Chigirinskaya, Y., Schertzer, D.,  Lovejoy, S. (1997). Scaling gyroscopes cascade: universal multifractal features of 2-D and 3-D turbulence. <em>Fractals and Chaos in Chemical Engineering. World Scientific, Singapore</em>, 371-384.</p><p>[6]Chigirinskaya, Y.,  Schertzer, D. (1997). Cascade of scaling gyroscopes: Lie structure, universal multifractals and self-organized criticality in turbulence. In <em>Stochastic Models in Geosystems</em> (pp. 57-81). Springer, New York, NY.</p>

Evaluation of the influence of the windbreak trees on the change of wind shear in weakly complex terrains

The turbulent nature of the wind above the earth’s surface depends on both the topology of the terrain and the presence of natural obstacles along the way such as low grasses and shrubs, as well as medium-tall trees. When the wind passes through the indicated obstacles, detachment is observed i.e. formation of large eddies, which are carried away by the main flow, after which they dissipate. The size of the vortices, as well as the period of dissipation, depends on the wind speed, as well as the type of obstacle. The presence of windbreak trees significantly changes the wind shear over the surface, and hence the energy potential of the wind in the vicinity of trees. In present work, the influence of the tree belt on the wind shear at the adopted prevailing wind direction is investigated. The degree of deformation of the speed profile after the obstacle in weakly complex terrain is shown. Relevant prescriptions for the location of wind turbines in the vicinity of windbreak trees are presented in view of minimum shading and maximum energy output.