Genesis Of Vortices Research Articles

Origin and Evolution of Synoptic-Scale Vortices Initiated at Low Level Downwind of the Hoggar Mountains

Abstract Numerous low-level vortices are initiated downwind of the Hoggar Mountains and progress toward the Atlantic coast on the northern path of African easterly waves (AEWs). These vortices occur mostly in July and August and more specifically when the northern position of the Saharan heat low (SHL) generates stronger and vertically expanded easterly winds over the Hoggar Mountains. At synoptic time scales, a composite analysis reveals that vortex initiation and westward motion are also statistically triggered by a reinforcement of these easterly winds by a wide and persistent high pressure anomaly developing around the Strait of Gibraltar and by a weak wave trough approaching from the east. The vortices are generated in the lee of the Hoggar, about 1000 km west of this approaching trough, and intensify rapidly. The evolution of the vortex perturbation is afterward comparable with the known evolution of the AEWs of the northern path and suggest a growth due to dry barotropic and baroclinic processes induced in particular by the strong cyclonic shear between the reinforced easterly winds and the monsoon flow. These results show that vortex genesis promoted by changes in orographic forcing due to the strengthening of easterly winds over the Hoggar Mountains is a source of intensification of the northern path of AEWs in July and August. These results also provide a possible mechanism to explain the role of the SHL and of particular midlatitude intraseasonal disturbances on the intensity of these waves.

Two types of Tibetan Plateau vortex genesis in June and the associated mechanisms

The Tibetan Plateau vortex (TPV) is a key system triggering rainfall over the Tibetan Plateau (TP) during the boreal summer. The TPV genesis mechanisms are complicated and its classification is a great challenge. This study attempts to elucidate these aspects. By introducing the standardized index of 24-h increment of equivalent potential temperature Delta_{24h} theta_{{text{e}}} at 500 hPa, all the TPV cases generated in June between 1980 and 2016 are classified as either positive or negative. Composite analysis is subsequently applied to the extremes of these two types, i.e., the first and last fifth-percentile cases with extremely negative and positive standardized Delta_{24h} theta_{{text{e}}}, respectively. Results indicate that 70% of them occur in relatively warmer and wetter environments, with diabatic heating dominating the positive type and the dynamic effect of large-scale circulation dominating the negative type. For the extremely positive cluster, the geopotential-height increment over the TP exhibits a negative/west–positive/east dipole, which enhances the southerly flow over the western TP, while forming surface water–vapor convergence. Consequently, strong condensation heating occurs near the sub-cloud level, resulting in the development of potential vorticity below and eventually TPV genesis. For the negative cluster, local shear lines at 500 hPa and upstream troughs at 250 hPa occur at the TPV genesis location. In conjunction with anomalous westerlies, positive potential vorticity is generated in situ due to zonal advection. The retardation caused by the Kunlun Mountains on the impinging westerly flow associated with side-boundary friction also contributes to TPV genesis southeast of the mountains.

Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: A tornadic cold front in amplifying northwesterly flow

AbstractThe structure and evolution of a cold front that produced a tornadic narrow cold‐frontal rainband (NCFR) over the UK on 20 November 2013 is explored using reanalysis data, high‐resolution model simulations and observations. Physical links are found to exist across a range of spatiotemporal scales, through which the evolving large‐scale flow field exerts an influence on the timing and location of miso‐scale vortex genesis, and therefore the potential for tornado genesis. The synoptic‐scale flow pattern exhibited amplification, consisting of upstream ridge building and downstream trough extension. A prominent jet streak and associated positive potential vorticity (PV) anomaly and tropopause fold moved rapidly southeastward on the rear flank of the extending upper‐level trough, the leading edge of these features eventually overspreading the surface cold front over the UK. Increasing 850 hPa frontogenesis occurred underneath the left exit of the jet streak and associated, intensifying, PV anomaly. A filament of dry, high‐PV air was extruded from the overlying tropopause fold within an intensifying front‐transverse circulation in this region. This dry filament eventually penetrated to low levels immediately behind the front, where it undercut the upper, rearward parts of the wide cold‐frontal precipitation band. Model fields and observations suggest that diabatic cooling, associated with sublimation of solid hydrometeors falling into the dry filament, led to the development of local downdraft and near‐surface divergence maxima and a prominent cold pool immediately behind the surface cold front over central England. Increases in horizontal convergence, updraft speed, and horizontal temperature gradients along the frontal boundary immediately ahead of the cold pool resulted in development of a locally well‐defined cold front and associated NCFR, where the surface front was formerly (and elsewhere continued to be) relatively weak. Tornadoes occurred in association with miso‐scale vortices that developed rapidly along an intensifying vertical vortex sheet at the NCFR.

How Do Dust Devil-Like Vortices Depend on Model Resolution? A Grid Convergence Study Using Large-Eddy Simulation

Dust devils are organized convective vortices with pressure drops of hundreds of pascals that spirally lift surface material into the air. This material modifies the radiation budget by contributing to the atmospheric aerosol concentration. Quantification of this contribution requires good knowledge of the dust devil statistics and dynamics. The latter can also help to understand vortex genesis, evolution and decay, in general. Dust devil-like vortices are numerically investigated mainly by large-eddy simulation (LES). A critical parameter in these simulations is the grid spacing, which has a great influence on the dust devil statistics. So far, it is unknown which grid size is sufficient to capture dust devils accurately. We investigate the convergence of simulated convective vertical vortices that resemble dust devils by using the LES model PALM. We use the nesting capabilities of PALM to explore grid spacings from 10 to 0.625 m. Grid spacings of 1 m or less have never been used for the analysis of dust devil-like vortices that develop in a horizontal domain of more than 10 km^2. Our results demonstrate that a minimum resolution of 1.25 m is necessary to achieve a convergence for sample-averaged quantities like the core pressure drop. This grid spacing or smaller should be used for future quantifications of dust devil sediment fluxes. However, sample maxima of the investigated dust devil population and peak velocity values of the general flow show no convergence. If a qualitative description of the dust devil flow pattern is sufficient, we recommend a grid spacing of 2.5 m or smaller.

Numerical investigation of solar impacts on canyon vortices and its dynamical generation mechanism

With the booming concerns over urban heat island phenomenon, impacts of solar radiation on flow and temperature in street canyons have been revealed by some experimental and numerical investigations, where amplified single-vortex structures are found in shallow canyons in high-Re scenarios (Re ~ 107). Canyon surfaces are heated mainly by absorbing solar radiation, and street air is warmed primarily through thermal exchange with canyon surfaces. Most previous publications focus on simulations of flow regimes, pollutant dispersion and thermal comfort in canyon's microenvironments, while dynamical investigations on vortex genesis are vague and mostly emphasize analysis of dimensionless numbers. This research will theoretically explore the impacts of solar radiation on vortex genesis over idealized two-dimensional urban canyons by coupling k-ε turbulence model and P-1 radiation model. A possible dynamical mechanism of canyon-vortex generation will be introduced based on baroclinic vortex generation theory. Result shows that, the activity of wind shear (momentum field) and buoyancy imbalance (thermal field) are mutual, and their activation have a prerequisite of low vertical wind-speed environment. Signal of disturbance wind shear is critical for main vortex maintenance and the genesis of secondary or corner vortices. This mechanism has been proved both by our steady- and unsteady-RANS simulation.

Genesis, evolution, and apocalypse of Loop Current rings

We carry out assessments of the life cycle of Loop Current vortices, so-called rings, in the Gulf of Mexico by applying three objective (i.e., observer-independent) coherent Lagrangian vortex detection methods on velocities derived from satellite altimetry measurements of the sea-surface height (SSH). The methods reveal material vortices with boundaries that withstand stretching or diffusion or whose fluid elements rotate evenly. This involved a technology advance that enables framing vortex genesis and apocalypse robustly and with precision in a truly parameter-free fashion. We find that the stretching- and diffusion-withstanding assessments produce consistent results, which show large discrepancies with Eulerian assessments that identify vortices with regions instantaneously filled with streamlines of the SSH field. The even-rotation assessment, which is vorticity-based, is found to be quite unstable, suggesting life expectancies much shorter than those produced by all other assessments.

Influence of Cosmic Ray Invasions and Aerosol Plasma on Powerful Atmospheric Vortices

The Earth’s atmosphere is affected by various ionizing sources. The maximum ionization of atmospheric particles by cosmic rays corresponds to the altitude of formation of tropospheric clouds. In the high-latitude troposphere for the region of the geomagnetic polar cap, in the winter period, the excitation of local cyclonic structures are observed which are accompanied with ice storms, with invasions into middle and subtropical latitudes. The time of excitation of such cyclones is about a day that is comparable with the time of excitation of tornadoes, which are generated at low latitudes. Localization of polar cyclones is not accidental. The region of the polar cap is connected with geomagnetic field lines extended into the tail of the Earth’s magnetosphere. This area is open for the penetration of cosmic rays. The ionization of aerosols in the stratosphere and the upper troposphere by precipitating particles of cosmic rays enhances the vortex activity of the atmosphere. The important role of the aerosol impurity is manifested in the generation of plasma vortices and in the accumulation of energy and mass in the atmosphere by vortices during condensation of moisture. Due to the cascade character of the ionization process, the influence of cosmic radiation turns out to be non-linear and increases with increasing pollution of the atmosphere. Aperiodic electrostatic perturbations, which play a remarkable role in the genesis of vortices, are stochastically excited in plasma inhomogeneities. During the interaction of plasma vortices and Rossby vortices, a large-scale vortex structure is formed and grows.

Solar plant with short diffuser concept: Further improvement of numerical model by included influence of guide vane topology on shape and stability of gravitational vortex

An alternative renewable energy concept, i.e. the concept of a solar power plant with short diffuser (SPD), was numerically investigated by more advanced computational fluid dynamics (CFD) model. Developed model is characterized by a more sophisticated and streamlined guide vane topology. The main novelty of this work is conducted optimization of the guide vane topology, for a specific novel application related to the alternative renewable energy concept (SPD). Optimization involved determining the required guide vane topology using minimal number of geometry influencing parameters. The objective was to result in vortex genesis and stabilization with respect to the desired circumferential velocity and to minimize the required pressure potential that is necessary for stable operation of the SDP plant. Provided numerical investigation was necessary, and for sure a step forward towards consideration of the experimental plant (which will assume introduction of the turbines). It needs to be taken into account that we deal with complex flow structure that requires gradual numerical investigation, in order to be able to get detail insight in the various influences and processes that can strongly affect SPD operating parameters. The guide vane topology was altered to develop an SPD capable of establishing and maintaining a stable gravitational vortex in pressure ranges which resemble atmospheric vortex phenomena (feasible for development of a compact system, and with maximal velocities in chimney throat regions ranging below 20 m s−1). The study outlines nine cases, each representing the altered guide vane design, where the best case is determined and compared with the available experimental data from other research groups. The comparison indicates that the numerical model, although quite simple, is accurate and robust in predicting the distribution of local velocity and pressure profiles and fit for implementation on wind turbines in order to determine the influence of the installed turbines on the vortex shape and stability in a future study. An important finding is that the swirl ratio can be manipulated by altering the guide vane shape, and it is independent of the Reynolds number (which will be important during the design phase since it can be used as a control strategy for vortex genesis and as a prevention of unintentional genesis regarding additional multiple vortices). The gained numerical results revealed specific operating conditions that will ensure a safer environment around the SPD and that will enable a carbon free electricity production.

An Idealized Numerical Simulation Investigation of the Effects of Surface Drag on the Development of Near-Surface Vertical Vorticity in Supercell Thunderstorms

AbstractIdealized simulations are used to investigate the contributions of frictionally generated horizontal vorticity to the development of near-surface vertical vorticity in supercell storms. Of interest is the relative importance of barotropic vorticity (vorticity present in the prestorm environment), baroclinic vorticity (vorticity that is principally generated by horizontal buoyancy gradients), and viscous vorticity (vorticity that originates from the subgrid-scale turbulence parameterization, wherein the effects of surface drag reside), all of which can be advected, tilted, and stretched. Equations for the three partial vorticities are integrated in parallel with the model. The partial vorticity calculations are complemented by analyses of circulation following material circuits, which are often able to be carried out further in time because they are less susceptible to explosive error growth.Near-surface mesocyclones that develop prior to cold-pool formation (this only happens when the environmental vorticity is crosswise near the surface) are dominated by only barotropic vertical vorticity when the lower boundary is free slip, but both barotropic and viscous vertical vorticity when surface drag is included. Baroclinic vertical vorticity grows large once a cold pool is established, regardless of the lower boundary condition and, in fact, dominates at the time the vortices are most intense in all but one simulation (a simulation dominated early by a barotropic mode of vortex genesis that may not be relevant to real convective storms).

Tropical Cyclogenesis due to ITCZ Breakdown: Idealized Numerical Experiments and a Case Study of the Event in July 1988

Abstract The mechanism of tropical cyclogenesis due to the breakdown of the intertropical convergence zone (ITCZ breakdown) and the structure of associated vortices are studied by numerical experiments using a nonhydrostatic mesoscale model. First, an idealized numerical experiment, in which a simple initial state without external disturbances is assumed, is performed without cumulus parameterization. A zonally uniform forcing of high sea surface temperature (SST) is imposed to generate an ITCZ-like structure. This “ITCZ” starts to undulate and eventually breaks down to form several tropical cyclones (TCs). These TCs merge and end up with a single TC. The energy budget analysis shows that barotropic instability of the low-level flow associated with the ITCZ is responsible for the genesis of vortices, and TC-scale buoyancy production soon takes over to contribute to the intensification of TCs. Conversion from the cumulus-scale kinetic energy into the TC-scale kinetic energy is found to be insignificant during ITCZ breakdown. Additional experiments show that the presence of the warm SST belt and an inclusion of the β effect are not essential for the occurrence of ITCZ breakdown. A numerical simulation of ITCZ breakdown over the Pacific Ocean in July 1988 shows that the mechanism of the tropical cyclogenesis is similar to that in the idealized numerical experiments from the viewpoint of the energy budget. Therefore, horizontal shear instability of the low-level flow and TC-scale buoyancy production are generally more essential than mergers of cumulus-scale vortices for the tropical cyclogenesis due to ITCZ breakdown.

The Influence of Environmental Low-Level Shear and Cold Pools on Tornadogenesis: Insights from Idealized Simulations

Abstract Idealized, dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near-surface vortices in supercell-like “pseudostorms.” A cyclonically rotating updraft is produced by a stationary, cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity. Once a nearly steady state is achieved, a heat sink, which emulates the effects of latent cooling associated with precipitation, is activated on the northeastern flank of the updraft at low levels. Cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near-surface vertical vorticity via the “baroclinic mechanism,” as has been diagnosed or inferred in actual supercells that have been simulated and observed. An intense cyclonic vortex forms in the simulations in which the environmental low-level wind shear is strong and the heat sink is of intermediate strength relative to the other heat sinks tested. Intermediate heat sinks result in the development (baroclinically) of substantial near-surface circulation, yet the cold pools are not excessively strong. Moreover, the strong environmental low-level shear lowers the base of the midlevel mesocyclone, which promotes strong dynamic lifting of near-surface air that previously resided in the heat sink. The superpositioning of the dynamic lifting and circulation-rich, near-surface air having only weak negative buoyancy facilitates near-surface vorticity stretching and vortex genesis. An intense cyclonic vortex fails to form in simulations in which the heat sink is excessively strong or weak or if the low-level environmental shear is weak.

Impacts of Lapse Rates on Low-Level Rotation in Idealized Storms

Abstract Adiabatic lapse rates appear to be a common feature in the lower troposphere on tornado days. This article reviews physical reasons why lapse rates may influence surface vortex intensification and reports on numerical simulations designed to study the key processes. In the idealized numerical model, an initial mesocyclone-like vortex and nonvarying convection-like heat source are used in different environmental stability profiles. The scales of interest in these simulations typify those of a parent supercell, and the developing circulations constitute direct responses to the imposed heating. Downward parcel displacements are needed for surface vortex development in environments with no preexisting surface vorticity. In the simulations, under neutral stratification there is strong heating-induced subsidence anchored near the storm edge, whereas under stable stratification there are instead gravity waves that propagate away to the far field. In addition, under weak or neutral low-level stratification there is very little resistance to downward parcel displacements. In the simulations, these two effects combine to bring high angular momentum air from aloft downward to the surface under neutral lapse rates; this in turn leads to surface vortex genesis, even without precipitation processes. When the lower troposphere is stable, surface vortex intensification is only simulated when there is already preexisting vertical vorticity at the ground. When the initial vortex is elevated (vertical vorticity falls off to zero above the ground), surface vortex intensification is only simulated under neutral low-level stability. The results are interpreted within the controlled experimental framework, after which the possible ramifications to processes in real storms are discussed.

A case study of the Borneo Vortex genesis and its interactions with the global circulation

[1] During the East Asian winter or the northeast monsoon (November–March), cold surges associated with cold air outbreaks from the Siberian high propagate equatorward and interact with the near-equatorial trough in the southern South China Sea. Usually, in the later phase of the monsoon, the so-called Borneo vortex develops over the sea and is the main driver for the formation of deep convection and heavy rainfall in East Malaysia. We present a case study of a cold-surge-induced event during January 2010. We diagnose a substantial export of potential energy from the Borneo vortex strengthening the subtropical jet. At 200 hPa, the velocity potential maximum related to the established Hadley circulation is shifted eastward from 140°E toward the dateline from presurge to postsurge periods. This modifies the general circulation patterns in the Southern Hemisphere with possible consequences for trace gas distributions. In addition, we explore implications for vertical transport by deep convection occurring in association with the Borneo vortex by diagnosing potential vorticity changes in the vicinity of the vortex.

Doppler radar observations of non‐occluding, cyclic vortex genesis within a long‐lived tornadic storm over southern England

AbstractOn 3 November 2009 a long‐lived tornadic storm developed close to the apex of a small frontal wave moving eastwards over southern England. Ten tornadoes and several instances of non‐tornadic wind damage occurred along a narrow path extending for over 120 km. The storm passed within 5 km of two of the radars in the Met Office's network, providing an opportunity for detailed observation. Initial development was in the form of an intense segment of line convection at the northern flank of the frontal wave and associated deepening surface mesolow (diameter 60–70 km). The line evolved rapidly; the southern portion bowed out to form a transient ‘S’‐shaped echo before dissipating. The northern section persisted, leaving a hook‐shaped storm which became quasi‐steady in overall echo shape.Radars revealed the cyclic development of five discrete low‐level mesovortices (diameter 1–4 km) between 1130 and 1330 UTC, along a zone of intense cyclonic vertical vorticity at the southeastern edge of the storm. These vortices intensified rapidly (typically from initial detection to maximum observed differential velocity in ∼10 min), some attaining low‐level differential velocities exceeding 40 m s−1. However, significant differential velocities (>20 m s−1) were not observed above 2.5 km above ground level, and no persistent mid‐level mesocyclone was present. The cyclic behaviour was unusual in that vortices were ‘non‐occluding’, moving southeastwards relative to the storm (to the right of the storm motion vector), away from the core. This is in contrast to the more commonly observed evolution of vortex occlusion (by storm downdraughts) and subsequent leftward and rearward storm‐relative movement during decay. Copyright © 2011 British Crown copyright, the Met Office. Published by JohnWiley & Sons Ltd.

Vortex genesis over the Bay of Bengal in spring and its role in the onset of the Asian Summer Monsoon

Physical processes associated with onset of the 1998 Asian summer monsoon were examined in detail using multi-source datasets. We demonstrated that strong ocean-atmosphere-land interaction in the northern Indian Ocean and tropical Asian area during spring is a fundamental factor that induces the genesis and development of a monsoon onset vortex over the Bay of Bengal (BOB), with the vortex in turn triggering onset of the Asian summer monsoon. In spring, strong surface sensible heating over India and the Indochina Peninsula is transferred to the atmosphere, forming prominent in situ cyclonic circulation, with anticyclonic circulations over the Arabian Sea and northern BOB where the ocean receives abundant solar radiation. The corresponding surface winds along the North Indian Ocean coastal areas cause the ocean to produce the in situ offshore currents and upwelling, resulting in sea surface temperature (SST) cooling. With precipitation on the Indochina Peninsula increasing from late April to early May, the offshore current disappears in the eastern BOB or develops into an onshore current, leading to SST increasing. A southwest-northeast oriented spring BOB warm pool with SST >31°C forms in a band from the southeastern Arabian Sea to the eastern BOB. In early May, the Somali cross-equatorial flow forms due to the meridional SST gradient between the two hemispheres, and surface sensible heat over the African land surface. The Somali flow overlaps in phase with the anticyclone over the northern Arabian Sea in the course of its inertial fluctuation along the equator. The convergent cold northerlies on the eastern side of the anticyclone cause the westerly in the inertial trough to increase rapidly, so that enhanced sensible heat is released from the sea surface into the atmosphere. The cyclonic vorticity forced by such sensible heating is superimposed on the inertial trough, leading to its further increase in vorticity strength. Since atmospheric inertial motion is destroyed, the flow deviates from the inertial track in an intensified cyclonic curvature, and then turns northward toward the warm pool in the northern BOB. It therefore converges with the easterly flow on the south side of the anticyclone over the northern BOB, forming a cyclonic circulation center east of Sri Lanka. Co-located with the cyclonic circulation is a generation of atmospheric potential energy, due to lower tropospheric heating by the warm ocean. Eventually the BOB monsoon onset vortex (MOV) is generated east of Sri Lanka. As the MOV migrates northward to the warm pool it develops quickly such that the zonal oriented subtropical high is split over the eastern BOB. Thus, the tropical southwesterly on the southern and eastern sides of the MOV merges into the subtropical westerly in the north, leading to active convection over the eastern BOB and western Indochina Peninsula and onset of the Asian summer monsoon.

Planetary vortices and Jupiter's vertical structure

Measurements of the vertical structure of Jupiter's circulation have recently been made near the equator by the Galileo spacecraft probe. In other regions, planetary vortices exist selectively for a limited range of generic (exponential) vertical forms and can be used to probe the atmospheric structure theoretically. A study of vortex genesis with a three‐dimensional numerical model produces reasonably realistic simulations of the Great Red Spot for both the generic and galilean forms, provided that Jupiter's winds do not extend much beyond a 500 km depth. However, the actual depth of the winds remains uncertain.

A new mathematical model applied to tornado genesis

One of the most important topics in fluid mechanics research is the vortex genesis and the vortex stability. Tornadoes are directly related to these studies: a tornado can be represented by an infinite vortex line in a viscous fluid interacting with a plane boundary surface at right angles to the line. The purpose of this paper is to derive an analytical model which can describe the genesis of these vortex lines in agreement with meteorological mechanisms. This model is built upon a bifurcation process. We consider an axisymmetric non-rotating updraft satisfying an adherence condition at the ground. We show that this flow is unstable and bifurcates to a stationary tornado flow. The startup of a tornado is then modeled.

Coherent vortices in a turbulent and rotating fluid

The emergence of coherent, intense and quasi-steady vortices in a rotating fluid under the action of turbulence is a phenomenon which is not yet well accounted for. A first deterministic approach by Maxworthy, Hopfinger and Redekopp (1985) related it to the occurence of mixed density fluid intrusions in a linearly stratified fluid. In the first part we present visualizations of the vortex genesis verifying qualitatively Maxworthy et al.'s ideas. In the second part, a linear instability model is proposed, very similar to the thermal convection instability in a rotating fluid. For marginal stability conditions, the model shows the occurrence of a regular spatial distribution of steady vortices. This new approach does not contradict the visualizations presented in the first part.

The Stability and Genesis of Rossby Vortices

Abstract The stability and genesis of the vortices associated with long solitary divergent Rossby waves-the Rossby vortices–are studied numerically using the single-layer (SL) model with Jovian parameters. Vortex behavior depends on location and on balances among the translation, twisting, steepening, dispersion and advection processes. Advection is the main preserver of vortices. The solutions provide an explanation for the origin, uniqueness and longevity of the Great Red Spot (GRS). In midlatitudes, stable anticyclones exist in a variety of sizes and balances: from the large planetary-geostrophic (PG) and medium intermediate-geostrophic (IG) vortices that propagate westward, to the small quasi-geostrophic (QG) vortices that migrate equatorward. These vortices all merge during encounters. Geostrophic vortices in the f0-plane system adjust toward symmetry by rotating; those on the sphere adjust by rotating and propagating. Stable cyclones exist mainly at the QG scale or on the f0-plane. In low latitudes ...