Search Results

You are looking at 1 - 10 of 26 items for

  • Author or Editor: G. P. Williams x
  • Refine by Access: All Content x
Clear All Modify Search
G. P. Williams

Abstract

To extend studies of the dynamics of thin atmospheric layers, the generation and equilibration of multiple anticyclonic vortex sets associated with long solitary baroclinic Rossby waves are examined numerically using a primitive equation model with Jovian parameters subject to a simple heating function. We seek primarily to model the three main groups of anticyclones seen on Jupiter, namely, the Great Red Spot, the three White Ovals, and the dozen or so Small Ovals that occur at latitudes of −21°, −33°, and −41°, respectively. The motions are confined to thin upper layers by exponential vertical structures that favor absolute vortex stability. Calculations are also made to examine the regeneration, intrazonal and interscale interactions, and propagation rates of vortices.

Vortex sets resembling the three main Jovian groups in scale, form, and number can be simultaneously generated and maintained in a steady configuration by a heating that produces stable westerly and weakly unstable easterly jets. The steady configuration occurs when an optimal number of vortices exists in a balance between a weak heating and a weak dissipation. Vortex behavior can be more complex in the heated system because the generation of new storms offsets the tendency to merge into fewer vortices. The solutions also show that intrazonal vortex interactions can lead, in some situations, to the destruction of anticyclones modeling the Great Red Spot.

Full access
G. P. Williams

Abstract

The vertical of Jupiter's atmosphere is probed and isolated by evaluating the stability characteristics of planetary vortices over a wide parameter range. The resulting structures lead to simulating the genesis of single and multiple vortex states in Part I of this paper and the genesis of an equatorial superrotation and midlatitudinal multiple jets in Part II.

The stability and genesis of baroclinic Rossby vortices, the vortices associated with long solitary Rossby waves in a stratified fluid, are studied numerically using a primitive equation model with Jovian and oceanic parameters and hypo-thermal structures. Vortex stability, that is, coherence and persistence, depends primarily upon latitude location and vertical structure and is used to deduce possible stratifications for Jupiter's atmosphere. The solutions suggest that Jupiter's large-scale motions are confined to a layer of depth h and are bounded by an abyss with an impermeable interface at a depth H, such that h/H≤1/20. Consequently, they also extend earlier results derived with the reduced-gravity, shallow-water model, particularly the explanation for the origin, uniqueness, and longevity of the Great Red Spot (GRS).

Beginning at the equator, stable anticyclones are seen to exist only when they have the Hermitian latitudinal form, the Korteweg-deVries longitudinal form, the confined exponential vertical structure exp(Nz/H), and the amplitude range as prescribed by the analytical theory of Marshall and Boyd for N=8. Soliton interactions occur between equatorial vortices of similar horizontal and vertical form.

In middle and low latitudes, shallow anticyclones with an exponential structure of N=20 exist quasi-stably for a variety of sizes. Such vortices remain coherent but tend to migrate equatorward (where they disperse) at rates that depend upon their size, location, and vertical structure: large and medium anticyclones propagate primarily westward while migrating slowly, whereas small storms just migrate rapidly and then collapse. The migration of these large, shallow vortices can be reduced, but not stopped, in low latitudes by an easterly jet with the same vertical structure.

Anticyclones are stabler when they are thinner relative to the abyss. Thus, when N=60, their migration is sufficiently slow that it can be stopped by a weak easterly jet. Furthermore, absolute stability sets in when N=90 and migration ceases completely for the large, thin anticyclones that now just propagate westward. Such flows may also be usefully represented by a vertical structure that is linear in z for the velocity and static stability in the thin upper layer and vanishes in the abyss.

Large, thin (N≥90) anticyclones can exist indefinitely either freely or when embedded within an anticyclonic zone of alternating jet streams of similar vertical structure. This holds true for the confined linear-z representation also. The permanence of GRS-like, low-latitude vortices in Jovian flow configurations occurs in a variety of lengthy calculations with thin structures. Ocean vortices are less persistent because the thermocline is relatively thick.

The baroclinic instability of easterly jets is nonquasigeostrophic and takes on the form of solitary rather than periodic waves when the jets have a thin exponential (N≥90) or confined linear-z structure. Such nonlinear waves develop into vortices that exhibit a variety of configurations and evolutionary paths. In most cases multiple mergers tend toward an end state with a single large vortex. Two types of merging occur in which a stronger vortex either catches a weaker one ahead of it or reels in a weaker one from behind. This duality occurs because propagation rates depend as much on local as on global conditions. In a further complication, vortices generated by an unstable easterly tend to have an exponential structure for exponential jets but a first baroclinic eigenmodal structure for confined linear-z jets.

Single vortex states resembling the ORS, with sizes ranging from 15° to 50° in longitude and with temperature gradients, velocities, and propagation rates near the observed range, can be generated either directly through the growth of a local front in a marginally unstable easterly jet or indirectly through a series of mergers of the multiple vortices generated by a more unstable easterly jet. Sets of vortices can be produced simultaneously in the anticyclonic zones centered about latitudes −21°, −33°, and −41°, and have the same relative scales as Jupiter's GRS, Large Ovals, and Small Ovals. Thin anticyclones can also be generated at the equator by the action of vortices lying in low latitudes. Equally realistic long-lived vortices can also be generated by jets with structures matching the recent Galileo spacecraft observations by using other hyperbolic forms and greater depth scales.

Full access
G. P. Williams

Abstract

Altering the tropospheric static stability changes the nature of the equatorial superrotation associated with unstable, low-latitude, westerly jets, according to calculations with a dry, global, multilevel, spectral, primitive equation model subject to a simple Newtonian heating function. For a low static stability, the superrotation fluxes with the simplest structure occur when the stratospheric extent and horizontal diffusion are minimal. Barotropic instability occurs on the jet's equatorward flank and baroclinic instability occurs on the jet's poleward flank. Systems with a high static stability inhibit the baroclinic instability and thereby reveal more clearly that the barotropic instability is the primary process driving the equatorial superrotation. Such systems produce a flatter equatorial jet and also take much longer to equilibrate than the standard atmospheric circulation.

Full access
G. P. Williams

Abstract

Studies of the dynamical response of thin atmospheric layers overlying thick envelopes are extended to examine how multiple jets, such as those seen on Jupiter and Saturn, can be generated and maintained. The jets are produced by baroclinic instabilities and are examined numerically using a primitive equation model subject to simple heating functions. The motions are confined to a thin upper layer by a heating that produces a flow with either an exponential vertical structure or one that is linear aloft while vanishing below. The motions are driven by latitudinal heating distributions with a variety of global and local components.

The calculations show that jets roughly resembling the main Jovian ones in amplitude, scale, and form can be generated and maintained in a steady configuration when the flow has the confined linear structure. When the flow has the exponential structure, however, the jets migrate slowly but continuously equatorward while being regenerated in higher latitudes. For both structures, the flow is sensitive to the heating distribution in low latitudes where jets form only if a significant baroclinicity exists in that region; such jets can also be barotropically unstable and can generate a superrotating current at the equator. In midlatitudes, except for being confined to an upper layer, the baroclinic instabilities resemble the standard forms seen in terrestrial models with high rotation rates.

Additional calculations show that superrotating equatorial currents can also be generated for deep layers or for Earth's atmosphere if the initial instabilities are developed in low latitudes. Broad easterly currents such as Neptune's can also be generated by elementary heating distributions, provided that the heated layer becomes progressively thicker with latitude. Finally, the hexagonal shape that high-latitude jets sometimes assume on Saturn when viewed in a polar projection can be attributed to nonlinear waves associated with baroclinic instabilities.

Full access
G. P. Williams

Abstract

Baroclinically unstable zones in midlatitudes normally produce medium-scale planetary waves that propagate toward the equator where they generate easterlies while transferring westerly momentum poleward, so that the jet lies in higher latitudes than in the corresponding axisymmetric (eddy-free) state. When the baroclinically unstable zone is moved into low latitudes, however, the equatorward side of the jet can also produce a barotropic instability whose large-scale eddies lead to a strong superrotating westerly current at the equator; the jet remains close to its axisymmetric location. For the earth, the transition between these two regimes occurs when the jet lies close to 30°, according to calculations with a global, multilevel, spectral, primitive equation model that examines superrotating flows for a wide range of rotation rates. The existence of a stable superrotating regime implies that an alternative climate could occur, but only under novel conditions.

Full access
G. P. Williams

Abstract

The possibility that the tropopause could be lower during an ice-age cooling leads to an examination of the general sensitivity of global circulations to the tropopause height by altering a constant stratospheric temperature Ts in calculations with a dry, global, multilevel, spectral, primitive equation model subject to a simple Newtonian heating function. In general, lowering the tropopause by increasing the stratospheric temperature causes the jet stream to move to lower latitudes and the eddies to become smaller. Near the standard state with Ts = 200 K, the jets relocate themselves equatorward by 2° in latitude for every 5 K increase in the stratospheric temperature. A double-jet system, with centers at 30° and 60° latitude, occurs when the equatorial tropopause drops to 500 mb (for Ts = 250 K), with the high-latitude component extending throughout the stratosphere.

The eddy momentum flux mainly traverses poleward across the standard jet at 40°, in keeping with the predominantly equatorward propagation of the planetary waves. But when the jet lies at 30° (for Ts = 225 K) the flux converges on the jet in keeping with planetary waves that propagate both equatorward and poleward. Two sets of such wave propagation occur in the double-jet system. As the troposphere becomes even shallower, the flux reverts to being primarily poleward across the jet (for Ts = 260 K) but then becomes uniquely primarily equatorward across the jet (for Ts = 275 K) before the circulation approaches extinction. Thus the existence of a predominantly poleward flux in the standard state appears to be parametrically fortuitous.

Full access
David P. Marshall, Richard G. Williams, and Mei-Man Lee

Abstract

The dynamical control of the eddy-induced transport is investigated in a series of idealized eddy-resolving experiments. When there is an active eddy field, the eddy-induced transport is found to correlate with isopycnic gradients of potential vorticity, rather than gradients of layer thickness. For any unforced layers, the eddy transfer leads to a homogenization of potential vorticity and a vanishing of the eddy-induced transport in the final steady state.

Full access
G. S. Kent, E. R. Williams, P-H. Wang, M. P. McCormick, and K. M. Skeens

Abstract

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5–18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Niño-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the South American and Central African landmasses, where the percentage of cloud occurrence in the upper troposphere can exceed 75%. Cloud occurrence at all altitudes within the Tropics, over both land and ocean, increases with the underlying surface temperature at a rate of approximately 13%°C−1. Extrapolated threshold temperatures for the formation of cloud are about 5°C lower than those found from nadir viewing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. ENSO cycle changes in cloud occurrence are observed, not only over the Tropics but also over the subtropics, indicating a difference in the meridional Hadley circulation between ENSO warm and cold years. Sunrise–sunset cloud differences indicate that large-scale variations, whose form resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-tropospheric ice and its positive correlation with surface temperature on all timescales are generally consistent with the behavior of lightning and the global electrical circuit.

Full access
Todd P. Lane, Robert D. Sharman, Stanley B. Trier, Robert G. Fovell, and John K. Williams

Anyone who has flown in a commercial aircraft is familiar with turbulence. Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using examples, this article demonstrates how these advances have stemmed from improved turbulence observing and reporting systems, the establishment of archives of turbulence encounters, detailed case studies, and high-resolution numerical simulations. Some of the important phenomena that have recently been identified as contributing to near-cloud turbulence include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands. The consequences of these phenomena for developing new en route turbulence avoidance guidelines and forecasting methods are discussed, along with outstanding research questions.

Full access
A. Bodas-Salcedo, P. G. Hill, K. Furtado, K. D. Williams, P. R. Field, J. C. Manners, P. Hyder, and S. Kato

Abstract

The Southern Ocean is a critical region for global climate, yet large cloud and solar radiation biases over the Southern Ocean are a long-standing problem in climate models and are poorly understood, leading to biases in simulated sea surface temperatures. This study shows that supercooled liquid clouds are central to understanding and simulating the Southern Ocean environment. A combination of satellite observational data and detailed radiative transfer calculations is used to quantify the impact of cloud phase and cloud vertical structure on the reflected solar radiation in the Southern Hemisphere summer. It is found that clouds with supercooled liquid tops dominate the population of liquid clouds. The observations show that clouds with supercooled liquid tops contribute between 27% and 38% to the total reflected solar radiation between 40° and 70°S, and climate models are found to poorly simulate these clouds. The results quantify the importance of supercooled liquid clouds in the Southern Ocean environment and highlight the need to improve understanding of the physical processes that control these clouds in order to improve their simulation in numerical models. This is not only important for improving the simulation of present-day climate and climate variability, but also relevant for increasing confidence in climate feedback processes and future climate projections.

Full access