Academic literature on the topic 'Intense atmospheric vortices'

Intense vortical circulations, often organized in counterrotating vortex pairs, were detected in midcontinental cumulus congestus over southeast Wyoming in July 2003. The sampled clouds developed in dry environments and at cold temperatures, and were a few kilometers in depth and width. Observations were obtained with the Wyoming Cloud Radar from aboard the Wyoming King Air research aircraft. Dual-Doppler analyses of the data yielded high-resolution (30–45 m) depictions of the horizontal components of air motions across vigorously growing clouds. The vortices found in the horizontal cross sections are interpreted as components of the toroidal circulations in thermals when those are tilted because of the effect of ambient cross flow. This configuration also leads to a partial stabilization of the vertical trajectory of the updraft, by opposing the drag by the ambient wind. Additionally, dry air intrusions were seen to accompany these features when the vortices developed near the cloud outer boundaries; recirculation of hydrometeors occurred when the vortices were adjacent to in-cloud downdrafts. These features are also evident in the radar reflectivity patterns. In general, gradients of velocities and vorticity values in horizontal planes are comparable to those found in vertical planes.

Abstract A neutrally stratified turbulent airflow over a very young sea surface at a high-wind condition was investigated using large-eddy simulations. In such a state, the dominant drag at the sea surface occurs over breaking waves, and the relationship between the dominant drag and local instantaneous surface wind is highly stochastic and anisotropic. To model such a relationship, a bottom boundary stress parameterization was proposed for the very young sea surface resolving individual breakers. This parameterization was compared to the commonly used parameterization for isotropic surfaces. Over both the young sea and isotropic surfaces, the main near-surface turbulence structure was wall-attached, large-scale, quasi-streamwise vortices. Over the young sea surface, these vortices were more intense, and the near-surface mean velocity gradient was smaller. This is because the isotropic surface weakens the swirling motions of the vortices by spanwise drag. In contrast, the young sea surface exerts little spanwise drag and develops more intense vortices, resulting in greater turbulence and mixing. The vigorous turbulence decreases the mean velocity gradient in the roughness sublayer below the logarithmic layer. Thus, the enhancement of the air–sea momentum flux (drag coefficient) due to breaking waves is caused not only by the streamwise form drag over individual breakers but also by the enhanced vortices. Furthermore, contrary to an assumption used in existing wave boundary layer models, the wave effect may extend as high as 10–20 times the breaking wave height.

Abstract An idealized analytical model and numerical large-eddy simulations are used to explore fluid-dynamic mechanisms by which tornadoes may be intensified near the surface relative to conditions aloft. The analytical model generalizes a simple model of Barcilon and Fiedler and Rotunno for a steady supercritical end-wall vortex to more general vortex corner flows, angular momentum distributions, and time dependence. The model illustrates the role played by the corner flow swirl ratio in determining corner flow structure and intensification; predicts an intensification of near-surface swirl velocities relative to conditions aloft of Iυ ∼ 2 for supercritical end-wall vortices in agreement with earlier analytical, numerical, and laboratory results; and suggests how larger intensification factors might be achieved in some more general corner flows. Examples of the latter are presented using large-eddy simulations. By tuning the lateral inflow boundary conditions near the surface, quasi-steady vortices exhibiting nested inner and outer corner flows and Iυ ∼ 4 are produced. More significantly, these features can be produced without fine tuning, along with an additional doubling (or more) of the intensification, in a broad class of unsteady evolutions producing a dynamic corner flow collapse. These scenarios, triggered purely by changes in the far-field near-surface flow, provide an attractive mechanism for naturally achieving an intense near-surface vortex from a much larger-scale less-intense swirling flow. It is argued that, applied on different scales, this may sometimes play a role in tornadogenesis and/or tornado variability. This phenomenon of corner flow collapse is considered further in a companion paper.

A detailed set of data has been compiled on large fire whirlwinds occurring on prescribed burns conducted in Ontario. There appear to be two types of such whirlwinds: one occurs in pairs on the leeward side of the convection column and the other is created after the entire convection column begins to rotate. The second type occurs in association with very intense fires that may be described as fire storms. Fire whirlwind occurrence appears to be related principally to meteorological conditions in which wind speeds are less than 10 km/h, to the stability of the atmosphere up to 3000 m altitude, and to conditions where the amount of energy released from the fire is high. The roles of atmospheric stability, rate of energy release from the fire, and ignition pattern in the development of whirlwinds require further study.

Vortex identification in atmospheric data remains a challenge. One reason is the general presence of shear throughout the atmosphere that interferes with traditional vortex identification methods based on geopotential height or vorticity. Alternatively, kinematic methods can avoid some of the drawbacks of the traditional methods since they compare the rotational and deformational flow parts. In this work, we apply the kinematic vorticity number method ( W k -method) to atmospheric datasets ranging from the synoptic to the convective scales. The W k -method is tested for winter storm Kyrill, a high-impact extratropical cyclone that affected Germany in January 2007. This case is especially challenging for vortex identification methods since it produced a complex wind occurrence associated with a derecho along a narrow cold-frontal rain band and an area of high winds close to the low pressure center. The W k -method is able to identify vortices in differently-resolved datasets and at different height levels in a consistent manner. Additionally, it is able to determine and visualize the storm characteristics. As a result, we discovered that the total positive circulation of the vortices associated with Kyrill remains of similar order across different data sets though the vorticity magnitude of the most intense vortices increases with increasing resolution.

Abstract Tornadoes produced by quasi-linear convective systems (QLCSs) in low instability environments present distinctive challenges for forecasters. This study analyzes a population of 56 vortices (all cyclonic) in a full-physics, case study simulation to examine vortex characteristics and their relationships to the pre-line environment. Peak surface vortex intensity correlates with peak vortex depth, peak surface wind speed, and vortex pathlength. The strongest vortices are the deepest and longest lived, implying that they would be most detectable. The modeled surface vortices are primarily associated with gust front cusps and bow echoes, line breaks, and supercell-like features. Strong vortices frequently have sustained, superposed surface vorticity and near-ground updrafts for several minutes. Although weak vortices lack this superposition, they often exhibit impressive midlevel vorticity and midlevel updrafts. The environments of the weak and strong vortices are similar with small, yet statistically significant, differences in several thermodynamic and kinematic fields. The profiles near strong vortices have more low-level CAPE, steeper lapse rates, and stronger deep-layer vertical wind shear. However, the small magnitudes of the differences imply that forecasters might struggle to discriminate well between nontornadic and tornadic environments in high-shear, low-CAPE events. Despite the similarities, the profiles produce distinct reflectivity, updraft, and vertical vorticity distributions in idealized cloud model simulations. The most intense updrafts and vortices in the idealized runs occur when the environmental profiles from the strong vortex cases are combined with a QLCS orientation more normal to the lower-tropospheric vertical wind shear.

2007/2008
Mesocicloni e tornado sono intensi vortici atmosferici, molto rilevanti nella quotidiana attività umana essendo associati a fenomeni atmosferici tra i più violenti. Nello specifico, i mesocicloni sono associati ad intense precipitazioni mentre durante un fenomeno tornadico la massima velocità orizzontale può raggiungere i 120 m/s. Sebbene alcuni schemi fenomenologici per la previsione di intensi vortici verticali in atmosfera siano presenti (ad esempio gli schemi CAPE-SHEAR), l'efficacia di tali schemi rimane molto limitata come rimane lacunosa una chiara spiegazione della (termo)dinamica che porta alla formazione di tali vortici. Questo lavoro di tesi si propone di fornire una migliore comprensione della formazione di questi fenomeni in atmosfera fornendo una base teorica capace di capire e valutare i limiti e le zone di validità dei comuni schemi di previsione tornadica. Dopo due capitoli, uno di introduzione ed uno di revisione della letteratura oggi presente (capitolo 1 e capitolo 2), la tesi sviluppa, partendo dalle equazioni fondamentali della fluidodinamica (Navier-Stokes), un set di equazioni semplificato ma adatto a descrivere fenomeni atmosferici intensi come i tornado e i mesocicloni (capitolo 3). Sucessivamente i parametri di previsione CAPE and SHEAR sono introdotti in tali equazioni tramite integrazione verticale (capitolo 4). Un evento tornadico è sempre caraterrizzato dalla formazione di uno spot di vorticità molto intenso e localizzato nello spazio: questo ha suggerito l'idea di associare la tornadogenesis ad un problema di stabilità non lineare nello spazio di Fourier (capitolo 5). La ``nonlinear resonant wave interaction theory'' ha fornito la matematica di base per lo studio di questo tipo di instabilità permettendo la costruzione di uno schema predittivo dei fenomeni tornadici in cui compaiono CAPE e SHEAR (capitolo 6). Il modello teorico proposto ci permette di affermare che 1) La tornadogenesis è associata solo a certi valori (bande) del diagramma CAPE-SHEAR. 2) I schemi usuali CAPE e SHEAR devono essere corretti tenendo conto delle caratteristiche del mesociclone in cui il tornado si forma: questo punto spiegherebbe la debolezza dei comuni schemi di previsione SHEAR-CAPE. 3) Il modello teorico propone un vincolo ai valori di ``Bulk Richardson number (BRN)'' adatti per la tornadogenesis; questo si ricollegherebbe prepotentemente a lavori numerici precedenti in cui si \`e visto come il BRN regoli l'evoluzione di una supercella a singola cella temporalesca od ad eventi di tipo multicellula. Quindi, questo lavoro propone un ruolo del BRN anche nella formazione di eventi tornadici associati a mesocicloni. I risultati sopra proposti sono confermati da una indagine statistica eseguita sul database meteorologico degli eventi tornadici in Italia e negli USA. La tesi presenta altre due parti a corollario e sviluppo della precedente parte principale. Uno studio numerico sulla efficacia delle moderne simulazioni di mesocicloni è proposta nel capitolo 7; risulta chiara dal lavoro proposto come una delle maggiori difficoltà sia una efficace descrizione della dissipazione e della evoluzione di campi di velocità molto variabili nello spazio e nel tempo. Questo problema è fondamentale nelle moderne simulazione atmosferiche a mesoscala nel momento in cui si voglia raggiungere una risoluzione orizzontale inferiore ai 200m. Nella ultima parte un approccio alla formazione di vortici intensi in atmosfera in termini di massimizzazione del campo di helicità è proposta. Questo tipo di approccio permette di vedere come la formazione di vortici verticali sia dovuta ad un meccanismo di prevenzione della dissipazione turbolenta di energia associata a moti a grande scala in energia associata a moti a piccola scala caotici e turbolenti. Questo spiegherebbe la presenza cos\`i frequente di vortici verticali in atmosfera. Il principio di massimizzazione della helicità risulta essere generale e applicabile anche alla studio della formazione di anti-mesocicloni e di eventi tornadici (capitolo 8) e, in linea di principio, ad in qualsiasi sistema fluido.

Probability distributions plotted to date from large volumes of high quality atmospheric observations invariably have ‘long tails’: relatively rare but intense events make significant contributions to the mean. Atmospheric fields are intermittent. Gaussian distributions, which are assumed to accompany second moment statistics and power spectra, are not seen. An inherently stochastic approach, that of statistical multifractals, was developed as generalized scale invariance by Schertzer and Lovejoy (1985, 1987, 1991); it incorporates intermittency and anisotropy in a way Kolmogorov theory does not. Generalized scale invariance demands in application to the atmosphere large volumes of high quality data, obtained in simple and representative coordinate systems in a way that is as extensive as possible in both space and time. In theory, these could be obtained for the whole globe by satellites from orbit, but in practice their high velocities and low spatial resolution have to date restricted them to an insufficient range of scales, particularly if averaging over scale height-like depths in the vertical is to be avoided; analysis has been successfully performed on cloud images, however (Lovejoy et al. 2001). Some suitable data were obtained as an accidental by-product of the systematic exploration of the rapid (1–4% per day) ozone loss in the Antarctic and Arctic lower stratospheric vortices during winter and spring by the high-flying ER-2 research aircraft in the late 1980s through to 2000. Data initially at 1Hz and later at 5Hz allowed horizontal resolution of wind speed, temperature, and pressure at approximately 200 m and later at 40 m, with ozone available at 1 Hz, over the long, direct flight tracks necessitated by the distances involved between the airfield and the vortex. Some later flights also had data from other chemical instruments, such as nitrous oxide, N2O, reactive nitrogen, NOy, and chlorine monoxide, ClO, which could sustain at least an analysis for H1, the most robust of the three scaling exponents. Better than four decades of horizontal scale were available for 1Hz and 5Hz data. Since then, a lesser volume of adequate data has been obtained away from the polar regions by the WB57F.

In the present paper, we are presenting a new mathematical vortex model, which is capable of simulating single and multi-celled steady, incompressible, intense vortices. The solution is obtained using the MATLAB and Maple 14 software. The new methodology is shown to fairly correlate the actual data of naturally occurring and industrial vortices.

Intense research on the thermoacoustic stability of premixed gas turbine combustors in the past two decades has led to an improved understanding of instabilities of longitudinal modes in the sub-kHz range and predictive tools for thermoacoustic stability analysis have also been developed. Circumferential modes in annular combustors have been studied in the past as well, even though to a much lower extent due to the high experimental effort. Combined experimental-numerical methods for the low-frequency regime (i.e. acoustically compact flames) are widely used. However, such experimental and numerical approaches with predictive capability have to be developed to also address the high-frequency (HF) regime. An experimental study of HF thermoacoustic coupling is presented in this paper. A fully premixed swirl-stabilized flame at atmospheric condition in a cylindrical combustion chamber is investigated. The test rig is equipped with several dynamic pressure transducers to identify and reconstruct the acoustic field in the combustion chamber. Planar information about the flame front location is obtained from Mie-scattering and the flow field is measured with particle image velocimetry (PIV). In the tests the first transverse mode of the combustion chamber exhibits instability for a particular operating condition, which leads to sustained limit-cycle pulsations. Mie-scattering images reveal periodic vortex shedding at the outlet of the burner. PIV results provide quantitative information on the strength of these coherent shear layer vortices.