Search Results

You are looking at 1 - 10 of 108 items for

  • Author or Editor: C. Jones x
  • All content x
Clear All Modify Search
Sarah C. Jones

Abstract

The ability of dry tropical-cyclone-like vortices to resist vertical shear is discussed. An idealized model calculation is presented in which a dry vortex remains nearly upright during 4 days under the influence of environmental vertical shear. It is shown that the outer portion of the vortex tilts more strongly than the inner core and that the pattern of vertical velocity is related to the vertical tilt of the outer portion of the vortex. This result is discussed with relation to observations of the location of convection in tropical cyclones. An alternative definition of the vortex center is proposed for cases in which the vertical tilt of the vortex is of importance. The average vertical shear across the center of the vortex is shown to depend on both the vortex tilt and the presence of large-scale potential vorticity asymmetries in the outer regions of the vortex. The average vertical shear is a function of time and of the area of the circle over which the averaging is carried out. Thus, the initial environmental shear may not be a reliable measure of the vertical shear felt by the vortex at later times.

Full access
Chris Thorncroft and Sarah C. Jones

Abstract

The extratropical transitions of Hurricanes Felix and Iris in 1995 are examined and compared. Both systems affected northwest Europe but only Iris developed significantly as an extratropical system. In both cases the hurricane interacts with a preexisting extratropical system over the western Atlantic. The remnants of the exhurricanes can be identified and tracked across the Atlantic as separate low-level potential vorticity (PV) anomalies. The nature of the baroclinic wave involved in the extratropical transition is described from a PV perspective and shown to differ significantly between the two cases.

The role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated. Iris moved into a region of strong shear. The high PV tower of Iris developed a marked downshear tilt. Felix moved into a vertically sheared environment also but the shear was weaker than for Iris and the PV tower of Felix did not tilt much.

Iris maintained its warm-core structure as it tracked across relatively warm water. It moved into the center of a large-scale baroclinic cyclone. The superposition of the two systems gave rise to strong low-level winds. The resulting strong surface latent heat fluxes helped to keep the boundary layer equivalent potential temperature (θe) close to the saturated equivalent potential temperature of the underlying sea surface temperature. This high equivalent potential temperature air was redistributed in the vertical in association with deep convection, which helped maintain the warm core in a similar way to that in tropical cyclones.

Felix did not maintain its warm-core structure as it tracked across the Atlantic. This has been shown to be linked to its more poleward track across colder water. It is argued that negative surface fluxes of latent and sensible heat decrease the boundary layer θ e, resulting in low-cloud formation and a decoupling of the cyclone boundary layer from the the deep troposphere.

In order to forecast these events there is a need for skill in predicting both the nature of the large-scale baroclinic wave development and the structural evolution of the exhurricane remnants.

Full access
C. S. Jones and Paola Cessi

Abstract

The surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation (MOC); the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific. Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the “sinking” basin. The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation.

Full access
C. D. Stow and K. Jones

Abstract

A disdrometer for the measurement of the co-spectra of raindrop size and charge is described which can evaluate to a significant extent spurious data caused by unavoidable drop overlap within the charge detector volume. The sizes of individual drops in the range 0.1–2.5 mm radius and charges within the limits of magnitude 0.1–10 pC could be determined. The use of two size detectors enabled the measurement of drop velocity and the detection of drop overlap within the charge-sensitive volume to be made. Non-ideal data which arise from natural conditions, measuring sites, and from fundamental or unavoidable deficiencies in disdrometer design, could be tested and monitored automatically using a central processor under software control: TIME OUT, when a drop failed to pass through both size detectors; COINCIDENCE, caused by simultaneous occupation of both size detector volumes; SIZE MISMATCH, when the separate size measurements did not agree within limits predetermined by software; HIGH or LOW VELOCITY, when the actual drop velocity was not close to the terminal value expected from size measurement. The self-evaluating disdrometer cannot be designed to provide the minimum error content possible but offers the advantage of assessing the proportion of spurious data present; it is argued that this may be preferable to the situation in which the error content is lower but unknown. The performance of the instrument was assessed using individual drops generated in the laboratory and by exposure to natural rain falling through an aperture into a chamber shielded from wind and associated turbulence. The latter test was made at a non-ideal site in order to demonstrate the ability of the disdrometer to provide information on invalid data so that raw co-spectra may be corrected. In the preliminary tests described a substantial proportion of the drops possessed fall speeds significantly below their expected terminal velocity, in some cases as much as 30% less, and not more than 20% of the drops detected satisfied all criteria for acceptance. Further, an examination of drop arrival rates showed that not all data could be fitted to a Poisson-type distribution, either because of rapid changes in the mean arrival rate or on account of the clustering of drops. The potential seriousness of the drop overlap problem, which is fundamental to all methods of measurements, is emphasized in the trial analysis: uncertainties in the exact form of the size distribution, particularly for drops in the radius range 0.1–0.5 mm, render the design of any instruments of fixed entrance aperture size dubious; the co-spectra must be expected to show appreciable distortion unless data associated with drop overlap, particularly within the charge-sensitive volume, are excluded. Some improvements in the current disdrometer design are suggested.

Full access
C. S. Jones and Paola Cessi

Abstract

The meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Flow is forced at the surface by longitude-independent wind stress, freshwater flux, and fast temperature relaxation to prescribed profiles. The only longitudinal asymmetry is that one basin is twice as wide as the other. Two states, a preferred one with sinking in the narrow basin and an asymmetrically forced one with sinking in the wide basin, are compared. In both cases, sinking is compensated by upwelling everywhere else, including the passive basin. Despite the greater area of the wide basin, the residual overturning transport is the same regardless of the location of sinking. The two basins exchange flow at their southern edge by a geostrophic transport balanced by the difference in the depth of isopycnals at the eastern boundaries of each basin. Gnanadesikan’s model for the upper branch of the MOC is extended to include two basins connected by a reentrant channel and is used to illustrate the basic properties of the flow: the layer containing the surface and intermediate water is shallower in the active basin than in the passive basin, and this difference geostrophically balances an exchange flow from the passive to the active basin. The exchange flow is larger when sinking occurs in the narrow basin. A visualization of the horizontal structure of the upper branch of the MOC shows that both the gyres and the meridional flow are important in determining the flow field.

Full access
C. S. Jones and Paola Cessi

Abstract

The salt transport by the wind-driven gyres and the meridional overturning circulation (MOC) is studied in an idealized-geometry primitive equation ocean model. Two narrow continents, running along meridians, divide the model domain into two basins of different widths connected by a re-entrant channel south of 52.5°S. One of the continents, representing the Americas, is longer than the other, representing Europe/Africa. Two different configurations of the model are used: the “standard” one, in which the short continent is west of the wide basin, and the “exchanged” one, in which the short continent is west of the narrow basin. In both cases, deep water is formed in the basin to the west of the short continent. Most residual transport of the MOC’s upper branch enters this basin by flowing along open streamlines that pass westward south of the short continent before proceeding northward. The meridional salt transport in the upper ocean of the sinking basin is decomposed into two portions: transport along open streamlines and transport by closed streamlines (gyres). In the Northern Hemisphere of the basin in which deep water is formed, the total northward salt transport per unit width along open streamlines is larger in the standard configuration than in the exchanged configuration. This larger salt transport is caused by two factors: a larger northward advection of salt by the interbasin transport and a larger cross-streamline salt transport out of the subpolar gyre. It is concluded that increasing interbasin flow south of Africa would likely bring more salt into the Atlantic Ocean.

Full access
Charles Jones and Bryan C. Weare

Abstract

This paper examines whether or not low-level moisture convergence and surface latent heat flux act as forcing mechanisms of the Madden and Julian oscillation (MJO), as it is proposed by the theories of wave-CISK (conditional instability of the second kind) and evaporation-wind feedback. The mean brightness temperature of cloudy pixels at 11 μm, obtained from five years of International Satellite Cloud Climatology Project data, is used as a proxy for tropical convective activity. Five years of European Centre for Medium-Range Weather Forecasts analyses are used to estimate surface latent heat fluxes and moisture divergence integrated in the low levels of the troposphere.

Spectral analysis of latent heat fluxes over the Indian and Pacific Oceans shows significant spectral peaks in the frequency band of the MJO. These peaks are due mainly to the oscillation in the surface wind speed rather than in the specific humidity difference. Principal component analysis and tagged correlation patterns of filtered time series 20–70 days are used to investigate the relationships between anomalies in convection, surface latent heat fluxes, and low-level moisture divergence. The correlation patterns show that negative anomalies of latent heat fluxes are systematically observed to the east, whereas positive anomalies are observed to the west of the region of convection. Positive anomalies of surface latent heat flux tag time variations in convection by about 4 days. This result contrasts with the basic requirement of the evaporation-wind feedback theory, which claims that evaporation anomalies are positive on the eastern side of the convective region. In contrast, tag correlation patterns indicate that the region of maximum low-level moisture convergence is located to the east of the region of convection, and low-level moisture convergence leads time variations in convective activity by about 2 days. This observational result supports the frictional wave-CISK theory as a mechanism of the MJO.

Full access
Paola Cessi and C. S. Jones

Abstract

The interbasin exchange of the meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Gnanadesikan’s conceptual model for the upper branch of the MOC is extended to include two basins of different widths connected by a reentrant channel at the southern edge and separated by two continents of different meridional extents. Its analysis illustrates the basic processes of interbasin flow exchange either through the connection at the southern tip of the long continent (cold route) or through the connection at the southern tip of the short continent (warm route). A cold-route exchange occurs when the short continent is poleward of the latitude separating the subpolar and subtropical gyre in the Southern Hemisphere (the zero Ekman pumping line); otherwise, there is warm-route exchange. The predictions of the conceptual model are compared to primitive equation computations in a domain with the same idealized geometry forced by wind stress, surface temperature relaxation, and surface salinity flux. Visualizations of the horizontal structure of the upper branch of the MOC illustrate the cold and warm routes of interbasin exchange flows. Diagnostics of the primitive equation computations show that the warm-route exchange flow is responsible for a substantial salinification of the basin where sinking occurs. This salinification is larger when the interbasin exchange is via the warm route, and it is more pronounced when the warm-route exchange flows from the wide to the narrow basin.

Full access
Leonhard Scheck, Sarah C. Jones, and Vincent Heuveline

Abstract

In this study the structure and evolution of singular vectors (SVs) for stable and unstable hurricane-like vortices in background flows with horizontal shear are investigated on f and β planes using a nondivergent barotropic model. With increasing shear strength, the singular values for stable vortices increase and the sensitive regions extend farther away from the vortex. The formation of β gyres leads to significant changes in the SV structure but has only weak influence on the singular values. For sufficiently strong anticyclonic shear, the initial SVs are aligned with streamlines connected to stagnation points. The evolved SVs are dominated by dipole structures, indicating a displacement of the vortex. The displacement is caused by the circulation associated with the initial SV perturbation outside of the vortex core, which grows by untilting and unshielding. This process is strongly enhanced by anticyclonic background shear. For both cyclonic and anticyclonic shear, the displacement by the perturbation circulation causes an additional displacement that is proportional to the shear strength. The shear-enhanced barotropic growth mechanism in stable vortices results in singular values that are comparable to those for unstable vortices without background shear. Perturbation growth involving the normal mode in barotropically unstable vortices suffers from background shear. The shear-induced modifications of the outer vortex regions cause a strong decrease of the singular value with increasing shear. For sufficiently strong shear, the SVs for unstable vortices grow by the same mechanism as for stable vortices.

Full access
B. C. Kenney and Ian S. F. Jones

Abstract

Experiments were performed in Lake Huron in which the relative diffusion of a dye plume and the lateral component of turbulent velocity were measured simultaneously. The standard deviation of the plume width varied in the same manner as the lateral turbulent intensity for cases of “regular” diffusion, although the precise relationship could not be established bemuse Eulerian, rather than lagrangian, time scales of the current fluctuations were measured.

Rapid vertical diffusion was associated with horizontal striations in the plume, with a quasi-periodic structure being observed simultaneously in the lake current. These striations were always aligned with the wind and rapidly followed shifts in the wind direction. It appears that these quasi-periodic current structures ensure “accelerated diffusion” when there is a variation in the direction of the current with depth. Since there is a strong association among the surface wind, the striations and current skewness, it is postulated that the surface wind plays a significant role in diffusion processes in the top few meters of the lake.

Full access