Vortex Cloud Research Articles

Aerodynamic sources of acoustic resonance in a duct with baffles

Experimental and numerical investigations of the generation of resonant sound by flow in a duct containing two sets of baffles and the “feedback” of the sound on the vortex shedding process are reported. The experiments are conducted in a wind tunnel and the numerical simulations are used to predict the sources of resonant sound in the flow. The resonant sound field, which is principally longitudinal, is calculated by the finite element method and a discrete-vortex model is used to predict the observed separated flow. Analysis of the passage of a single point vortex past a baffle indicates that the amount of acoustic energy generated is a function of the phase of the acoustic cycle at which the vortex passes the baffle. A more elaborate model simulates the growth of vortex clouds through the clustering of elemental vortices shed from an upstream baffle, tracks the passage of these vortex clouds past a downstream baffle, predicts the generation of acoustic energy using Howe's theory of aerodynamic sound, and accounts for the feedback of sound on the vortex shedding. Comparison is made between the predicted time-dependent structures and the observed flow structures using smoke visualization. The vortex cloud model predicts the flow conditions under which net acoustic energy is generated by the flow and therefore when resonance can be sustained; the results are consistent with the occurrence of peaks in the observed resonant sound pressure levels.

Satellite climatology of ‘polar lows’ and broadscale climatic associations for the Southern Hemisphere

AbstractA climatology of polar air cloud vortices (so‐called ‘polar lows’) is derived for the Southern Hemisphere using sets of medium resolution (5–4 km) DMSP (Defense Meteorological Satellite Program) imagery. The climatology is for the winter season (June through to September) and covers the 7–year period 1977–1983. Both comma cloud and spiraliform polar air signature types are considered, as identified in recent satellite studies for the Northern Hemisphere. The comma clouds dominate over a wide range of ocean latitudes, whereas the spiraliform systems occur less frequently (about 1:10 for the hemisphere), and show maximum frequencies in sea ice latitudes around mid‐winter (about 1:3).Within‐season variations in the locations of maximum occurrence of polar air systems are consistent with the large‐scale changes in the longwaves associated with the semi‐annual oscillation of surface pressures. Interannual variations in polar low occurrence for the seven winters appear connected, at least in part, with changes in the amplitude of sea‐level pressure (SLP) wavenumber one. When the seasonal cycle is amplified, as in the year of a ‘warm’ ENSO (El Nino/Southern Oscillation) event (e.g. 1982), large numbers of polar lows are observed south‐east of Australia and around New Zealand. This contrasts with the situation for year (−1) of a warm event (e.g. 1981), when the annual cycle of the trough is suppressed. At that time, reduced frequencies of cold air outbreaks in the New Zealand area are accompanied by fewer polar lows.On a seasonally averaged basis, and in most winters, there is a positive relationship between the regional extent of the Antarctic sea ice, the longitudes of preferred occurrence of cold air outbreaks and the incidence of polar lows. This effect may be enhanced just downstream of areas of strongest oceanic heat loss to the atmosphere.

A satellite‐derived climatology of polar‐low evolution in the North Pacific

AbstractA climatology of polar‐low evolution for seven low‐sun seasons (November to March) in the North Pacific is presented. The data were collected by the subjective interpretation of Defense Meteorological Satellite Program (DMSP) infra‐red imagery, using a cloud vortex classification scheme designed especially for use with medium to high resolution imagery. For the study period, about two‐thirds of all polar lows that formed over the North Pacific evolved to a higher stage. Approximately one‐half of those (one‐third of the total) passed through all stages of development from cyclogenesis to decay. The more common form of cyclogenetic polar low in the North Pacific, the comma cloud, continued to develop two‐thirds of the time, while the less common spiraliform only evolved to a higher stage of development in two‐fifths of the cases. Those polar lows that formed in certain key areas were likely to evolve to a higher stage, while other specific locations discouraged continued development. No readily discernible difference between gross size and cloud characteristics of evolving and non‐evolving polar lows was present in the satellite data, suggesting that additional data are needed to forecast correctly which will develop into mature cyclones.

Analysis of Karman vortex clouds revealed by satellite images

The analysis of the Karman vortex cloud street in the wake of Cheju Island Korea in geostationary meteorological satellite imageries together with meteorological data indicate following features. (1) The frequency of formation depends upon that of a cold-air outbreak. Frequency of a well developed one during a cold season of Dec., Jan., Feb. and March is in average 10. (2) Favorable meteorological conditions for the formation is a sharp air temperature inversion below the top of the highest mountain in the center of the island and a steady cold northerly wind. (3) Displacement speed of the vortices has an inverse relationship with the size of b/a where a is the distance of successive vortex pair and b is the distance between the vortices consisting a pair. (4) The time required to reach mature stage of a vortex pair is around three hours.

Intermediate-scale sea ice ? Atmosphere interactions over high southern latitudes in winter

Associations between polar air cloud vortices (“polar lows”), as an indicator of intermediate-scale atmospheric activity, and the Antarctic sea ice, are examined for the Southern Hemisphere winter (June–September). Seven consecutive winters, spanning a period of marked interannual variability of the atmospheric circulation and sea ice (1977–83), are analyzed using sets of DMSP (Defense Meteorological Satellite Program) imagery. Relatively high frequencies of polar lows are found in ice-edge and adjacent ocean latitudes. There is some evidence for an equatorward shift in the latitude of maximum monthly polar low occurrence during the June to September period. Polar low incidence over the Southern Hemisphere on interannual time scales shows a close association with positive sea ice anomalies in the longitudes of more frequent cold air outbreaks from higher latitudes. This is particularly apparent for winters of strongly anomalous circulation, such as FGGE (1979) and the major ENSO of 1982–83. However, for individual cases on daily to weekly time scales, the feedback of cold air — sea ice advance — polar low development is not always evident, and implies that additional processes may contribute to polar air cyclogenesis in the marginal ice zone.

Prediction of vortex shedding from bluff bodies in the presence of a sound field

The separated flow around a rectangular cylinder, in the presence of a transverse duct resonant acoustic mode, is modelled using a vortex method. The instantaneous transfer of power between the mean flow and the acoustic field is predicted using Howe's theory of aerodynamic sound. Whether the net acoustic energy per cycle generated is positive or negative depends on the phase of the acoustic cycle at which vortex clouds arrive at the trailing edge of the cylinder.

Eddy transport of sensible heat and the life history of synoptic systems: A statistical analysis for the Southern Hemisphere winter

The efficiency of successive satellite-observed stages in the development of extratropical frontal cyclones for transporting eddy sensible heat at 850 mb on the Southern Hemisphere, is determined for the five winters (June through September) of 1973–1977. An inventory of cloud vortex data is allied with independent estimates of the zonally-averaged transient eddy heat flux,\([\overline {v' T'} ]\), using a statistical technique known aspath analysis. Path analysis is an improvement over single equation linear regression models since it controls for the temporally continuous nature of cyclone evolution. The results indicate that different cloud vortex types are associated with characteristic regimes of\(\overline {v' T'} \). The quite strong poleward flux of heat effected in the cyclogenetic stages weakens substantially by the onset of dissipation. These results are in general agreement with those for the summer and transition seasons derived using linear regression by Tucker (1979). However, a significant netequatorward flux of\(\overline {v' T'} \) is found to be connected with the short-lived mature (spiral) cloud vortex type. In association with a southward displacement of the maximum latitude of cyclogenesis and a reduced crosslatitude motion of systems in winter, this apparently results in the strong lower-level heat flux convergence characteristic of the circumpolar trough at that time. A systematic error, by year, in the statistical model specification of heat transport related to cloud vortex type is found to be associated with phase of the Southern Oscillation (SO). Incorporating this effect into the model greatly improves the goodness-of-fit of the heat flux estimates and indicates that the efficiency of the mcridional eddy heat transport by cyclones is greatest (least) when the SOI is low (high). The implications of these results in the context of other, related studies are discussed.

Recent developments and engineering applications of the vortex cloud method

Following a brief summary of the vortex cloud method, a comparison of two models is made for the case of flow past an isosceles triangular wedge. The simpler and more commonly used method assumes potential flow under the influence of two vortex sheets progressively shed from the sharp edge separation points. The second method involves full vortex cloud theory (Navier-Stokes solution) for which vorticity is shed continuously from the whole body surface. On the basis of reasonable agreement between these methods a hybrid method has been developed combining potential flow over the upper surface with sharp edge separation and full vortex cloud theory over the lower and rear surfaces. Success for the wedge flow confirms the suitability of this method to aerofoils for which the upper surface is prone to ‘numerical separation’ by full vortex cloud theory. Application of the hybrid method to NACA 0025 with an airbrake flap results in considerable improvement of predicted surface pressure, lift coefficient, and drag coefficient.

Satellite-derived attributes of cloud vortex systems and their application to climate studies

Defense Meteorological Satellite Program (DMSP) visible and infrared mosaics are analyzed in conjunction with synoptic meteorological observations of sea level pressure (SLP) and upper-air height to derive composite patterns of cyclonic cloud vortices for the Northern Hemisphere. The patterns reveal variations in the structure and implied dynamics of cyclonic systems at different stages of development that include 1) increasing vertical symmetry of the lower-level and upper-air circulations and 2) decreasing lower-tropospheric thicknesses and temperature advection, associated with increasing age of the vortex. Cloud vortices are more intense in winter than in summer and typically reach maximum intensity in the short-lived prespiral signature stage. There are major structural differences among frontal wave, polar air, and “instant occlusion” cyclogenesis types. Cyclones in the dissipation stage may reintensify (deepen), as denoted by the appearance in the imagery of an asymmetric cloud band or a tightened spiral vortex. The satellite-derived statistics on cloud vortex intensity, which are seasonal- and latitude- as well as type-dependent, are applied to a preliminary examination of the synoptic manifestations of seasonal climate variability. An apparently close relationship is found, for two winter and spring seasons, between Northern Hemisphere cyclonic activity and variations in cryosphere variables, particularly the extent of Arctic sea ice. The results may indicate that increased snow and ice extent accompany a southward displacement of cyclonic activity and/or a predominance of deeper systems. However, there is also a strong regional dependence to the ice-synoptics feedback. This study demonstrates the utility of high resolution meteorological satellite imagery for studies of climate variations (climate dynamics).

Classification of mesoscale vortices in polar airstreams and the influence of the large-scale environment on their evolutions

Satellite imagery was used to detect and study 133 mesoscale cloud vortices which occurred in the northeastern Atlantic Ocean and adjacent seas between 1 December 1981 and 5 January 1982. Only vortices occurring in cold air, separate from synoptic-scale frontal cloud bands, were considered. The complete history of the evolution of each cloud vortex was recorded in terms of its location, size, cloud pattern, and cloud type (shallow or deep; stratiform or convective). Once all vortices were documented, conventional meteorological data and analyses were consulted. One objective of the study was to determine whether satellite imagery could be used to estimate the central pressure of the cyclones accompanying the mesoscale vortices. Other objectives were to determine if there were significant differences in (1) the mesoscale cloud structure or (2) the large-scale environment of vortices which remained insignificant versus those that developed into polar lows. Results indicate that the deficit pressure of the polar vortex could be estimated from satellite imagery and that there were recognizable differences between developing and non-developing vortices. Only a few cloud configurations accompanied polar lows, whereas the occurrence of certain other configurations at early stages in the vortex existence indicated that these vortices would remain insignificant. The synoptic-scale environment appeared to play a considerable role in determining which incipient vortices continued to develop. Moderate baroclinicity (horizontal temperature gradients and vertical wind shear), relatively weak winds, large lapse rates, and favorable vorticity advections accompanied developing polar lows. Non-developing vortices had unfavorable vorticity advections. Of these, some vortices were able to grow initially to moderate disturbances which remained steady thereafter. The apparent difference between these and insignificant vortices was that the steady vortices were more baroclinic and had steeper lapse rates. DOI: 10.1111/j.1600-0870.1985.tb00276.x

Synoptic cryosphere—atmosphere interactions in the Northern Hemisphere from DMSP image analysis†

Abstract A climatology of Northern Hemisphere cyclonic cloud vortices is developed from high-resolution Defense Meteorological Satellite Program (DMSP) infrared imagery for mid-season months. The technique which is described involves pattern recognition using a detailed vortex classification system. Variations in hemispheric frequencies of successive vortex types are dominantly seasonal rather than latitudinal and imply a close association with surface (mainly cryosphere) variations. More extensive sea ice or snow cover in April and January is associated with increased cyclogenesis, indicating enhanced surface-atmosphere feedback. A significant relationship exists between cloud-vortex variations and the sea ice boundary, but not with the continental snowline

Satellite climatological aspects of the “polar low” and “instant occlusion”

Middle resolution DMSP (Defense Meterological Satellite Program) visible and infrared imagery are analyzed in conjunction with surface and upper-air synoptic observations for two years' (1977, 1978/79) mid-season months (January, April, July, October) to derive synoptic climatological information on the “polar low” and “instant occlusion” phenomena. Polar-air vortices occur most frequently over the oceans in winter. Regional differences in the dominance of cloud signature sub-types confirm variations in the dynamics involved in polar-air cyclogenesis, as noted by previous workers. The “comma cloud” appears to occur more frequently in the North Pacific, while the spiraliform vortex predominates in the North Atlantic. The latter seems to be at least partly attributable to the effect of the eastern Arctic sea-ice boundary, as demonstrated by an analysis of imagery for two winters (D. J. F). each representative of an opposing mode of the North Atlantic Oscillation (1974/75. 1976/77). Composite synoptic “models” of polar lows reveal surface-pressure departures similar to those for cyclogenesis of the frontal wave type. However, a cold core and little vertical tilt distinguish the polar low in the mean. The “instant occlusion” exhibits surface-pressure departures that are deeper and comparable to the mature/dissipating stages of wave cyclone evolution. Spatial patterns of the upper-air anomalies associated with instant occlusions show two distinct vortices: one apparently derived from the polar low, the other from the incipient frontal wave with which it has merged. The seasonal and latitude-dependent cyclone intensity statistics (all types) are applied to the cloud vortex inventory of the North Atlantic-Europe sector for January 1979, The results support the role of the polar low as a synoptic indicator of anomalous winter circulation in the extratropics. DOI: 10.1111/j.1600-0870.1985.tb00442.x

A study of pneumatic nebulisation systems for inductively coupled plasma emission spectrometry

The analytical performance of different pneumatic nebulisers and cloud chambers for inductively coupled plasma emission spectrometry is reported. A vortex cloud chamber and a double-pass cloud chamber were compared for use with concentric glass nebulisers. An all-plastic double-pass cloud chamber was preferred. Two new nebulisers for solutions containing high levels of dissolved solids or slurries are described. One of these, machined entirely from inert plastic, gave an improved performance compared with a glass concentric nebuliser and no problems were encountered with the nebulisation of solutions containing 20%m/V of dissolved solids or slurries.

Winter depression tracks and climatological jet streams in the Southern Hemisphere during the FGGE year

AbstractA band‐pass filter transparent to fluctuations in the 2£5‐6 day period range is applied to a set of twice‐daily analyses covering the 1979 (FGGE year) winter of the Southern Hemisphere. Fields analysed include MSL pressure, 500mb height, 300mb wind, and the meridional heat flux due to transient eddies at 850mb. Variance maxima in these filtered fields indicate regions of high cyclonic activity, with favoured cyclolysis areas in the vicinity of the Antarctic coastline also showing up in the MSL pressure statistics. Using these results, supplemented by data on the distribution of cyclogenesis, we are able to identify the major depression tracks. In this analysis, extensive use is made of studies relating satellite‐observed cloud vortices to the various stages of cyclonic development. In general, it appears there was significantly more activity in the eastern hemisphere than in the western during the 1979 winter.By evaluating the terms in the time‐averaged zonal momentum equation (using the unfiltered data set), it is possible to assess the role of transient eddies in the maintenance of the time‐mean zonal flow. This analysis shows that Coriolis acceleration associated with poleward time‐mean ageostrophic flow at the tropopause level dominates the forcing of the jet centred around 25°S, but that convergence of eddy momentum flux maintains the higher latitude jet around 50°S. However, a significant contribution to the latter jet comes from poleward ageostrophic flow south of Africa.

Monthly variability of satellite‐derived cyclonic activity for the southern hemisphere winter

AbstractA satellite‐derived climatology of cloud vortices for the Southern Hemisphere winter (June to September) indicates that significant within‐season changes occur in the areal frequencies of extratropical depressions. This variability is found to be most strongly associated with the climatic frontal zones, while on a zonally‐averaged basis, the semi‐annual oscillation of the circumpolar vortex appears to dominate the latitudinal changes in cyclonic activity between June and September. Surface‐atmosphere interactions evidently play a key role in the wintertime circulation at middle and high latitudes. Cyclogenesis over the Southern Oceans is observed to be linked closely to the baroclinity associated with the Oceanic Polar Front, while on a regional scale, the presence of a time‐averaged zone of in situ cyclogenesis over sub‐antarctic latitudes of the South Pacific is found to be augmented by the influence of the unique winter season sea ice regime in that sector.

A synoptic climatology of satellite-observed extratropical cyclone activity for the Southern Hemisphere winter

Twice-daily infra-red satellite imagery for the winters (June to September) of 1973–77 are used to investigate cyclonic activity over the Southern Hemisphere. The analysis uses a previously developed satellite-observed cloud vortex classification scheme and “mean” patterns are derived for the different signature types. It is found that cyclogenesis is pronounced in both lower-middle and high latitudes in winter and conforms generally to the locations of the major hemispheric long wave troughs. Cyclolysis is observed to be most significant in certain sectors of the Antarctic Ocean and to exhibit little interannual variation in spatial patterns for the period studied. High frequencies of lower-latitude “cut-off” cyclones occur in the Australian-New Zealand region during winter. The results of this analysis are compared with previous non-satellite and satellite-derived studies for the Southern Hemisphere.

Transient synoptic systems as mechanisms for meridional transport: An observational study in the southern hemisphere

AbstractA classification of satellite cloud vortices compiled for the southern hemisphere by Streten and Troup is used to provide an inventory of the incidence of various stages in the life‐cycle of depressions as a function of latitude. These figures are then allied with estimated meridional transient eddy fluxes of both heat and momentum to assess the varying effectiveness of the systems during their life‐cycles as mechanisms of meridional transport, in comparison with the net contribution from other transient systems (travelling long waves and temporal variations in stationary waves). The developing stage of the cloud vortex is shown to be associated with a strong poleward heat flux, decreasing with the age of the system, with only a small (poleward) contribution from other transient mechanisms. For momentum, the developing stage is associated with a moderate poleward flux and the mature stage with relatively little net flux, while strong equatorward transport occurs during the decaying stage. A major poleward momentum flux is attributed to other transient mechanisms, particularly in the upper troposphere. The results are consistent with the hypothesis that the occurrence at preferred latitudes of different stages in the life‐cycle of systems is a major factor controlling the latitude profile of meridional transient eddy flux. A postulated meridional eddy flux distribution within a frame of influence for a mid‐latitude disturbance during its life‐cycle, when related to the broad‐scale zonal current, can account for salient features in the observed global distribution of meridional eddy fluxes of both heat and momentum. An indirect toroidal circulation is inferred when such a system interacts with the mean zonal current, and other synoptic implications are discussed.

Transient synoptic systems as mechanisms for meridional transport: an observational study in the southern hemisphere

A classification of satellite cloud vortices compiled for the southern hemisphere by Streten and Troup is used to provide an inventory of the incidence of various stages in the life-cycle of depressions as a function of latitude. These figures are then allied with estimated meridional transient eddy fluxes of both heat and momentum to assess the varying effectiveness of the systems during their life-cycles as mechanisms of meridional transport, in comparison with the net contribution from other transient systems (travelling long waves and temporal variations in stationary waves). The developing stage of the cloud vortex is shown to be associated with a strong poleward heat flux, decreasing with the age of the system, with only a small (poleward) contribution from other transient mechanisms. For momentum, the developing stage is associated with a moderate poleward flux and the mature stage with relatively little net flux, while strong equatorward transport occurs during the decaying stage. A major poleward momentum flux is attributed to other transient mechanisms, particularly in the upper troposphere. The results are consistent with the hypothesis that the occurrence at preferred latitudes of different stages in the life-cycle of systems is a major factor controlling the latitude profile of meridional transient eddy flux. A postulated meridional eddy flux distribution within a frame of influence for a mid-latitude disturbance during its life-cycle, when related to the broad-scale zonal current, can account for salient features in the observed global distribution of meridional eddy fluxes of both heat and momentum. An indirect toroidal circulation is inferred when such a system interacts with the mean zonal current, and other synoptic implications are discussed.

Formation of Vortex Streets in Shear Flow

Main features of the formation of vortex streets from the free shear layers emanating from two-dimensional bluff bodies placed in shear flow are obtained by means of the Abernathy-Kronauer model which is extended to include the effect of the background vorticity on the vortex formation. The calculations show that the background vorticity promotes the rolling up of the vortex sheet of the same sign whereas it decelerates that of the vortex sheet of the opposite sign. The vortex clouds in the low-velocity side of shear flow fall behind and will eventually be overtaken by the vortex clouds in the high-velocity side. Thus the steady configuration of the conventional Karman vortex street is no longer possible in shear flow. These features of the theoretical vortex patterns are experimentally verified by a flow-visualization technique.

Vortex cloud street during AMTEX 75

Vortex cloud street during AMTEX 75