Geophysical and Astrophysical Fluid Dynamics

Abstract:

We investigate the dynamics of Jupiter's upper troposphere and lower stratosphere using a General Circulation Model that includes two-stream radiation and optional heating from below. Based on the MITgcm dynamical core, this is a new generation of the Oxford Jupiter model [Zuchowski, L.C. et al., 2009. Plan. Space Sci., 57, 1525--1537, doi:10.1016/j.pss.2009.05.008]. We simulate Jupiter's atmosphere at up to 0.7 degree horizontal resolution with 33 vertical levels down to a pressure of 18 bar, in configurations with and without a 5.7 W/m2 interior heat flux. Simulations ran for 130000-150000 days to allow the deep atmosphere to come into radiative equilibrium. Baroclinic instability generates alternating, eddy-driven, midlatitude jets in both cases. With interior heating the zonal jets migrate towards the equator and become barotropically unstable. This generates Rossby waves that radiate away from the equator, depositing westerly momentum there via eddy angular momentum flux convergence and spinning up a super-rotating 20 m/s equatorial jet throughout the troposphere. There are 30-35 zonal jets with latitudinal separation comparable with the real planet, and there is strong eddy activity throughout. Without interior heating the jets do not migrate and a divergent eddy angular momentum flux at the equator spins up a broad, 50 m/s sub-rotating equatorial jet with weak eddy activity at low latitudes.

Abstract:

We examine the ammonia and water cycles in Jupiter's upper troposphere and lower stratosphere during spin-up of a multiple zonal jet circulation using the Oxford Jupiter GCM. Jupiter's atmosphere is simulated at 512 x 256 horizontal resolution with 33 vertical levels between 0.01 and 18 bar, putting the lowest level well below the expected water cloud base. Simulations with and without a 5.7 W/m2 interior heat source were run for 130000-150000d to allow the deep atmosphere to come into radiative-convective-dynamical equilibrium, with variants on the interior heating case including varying the initial tracer distribution, particle condensate diameter, and cloud process timescales. The cloud scheme includes simple representations of the ammonia and water cycles. Ammonia vapour changes phase to ice, and reacts with hydrogen sulphide to produce ammonium hydrosulphide. Water changes phases between vapour, liquid, and ice depending on local environmental conditions, and all condensates sediment at their respective Stokes velocities. With interior heating, clouds of ammonia ice, ammonium hydrosulphide ice, and water ice form with cloud bases around 0.4 bar, 1.5 bar, and 3 bar respectively. Without interior heating the ammonia cloud base forms in the same way, but the ammonium hydrosulphide and water clouds sediment to the bottom of the domain. The liquid water cloud is either absent or extremely sparse. Zonal structures form that correlate regions of strong latitudinal shear with regions of constant condensate concentration, implying that jets act as barriers to the mixing. Regions with locally high and low cloud concentrations also correlated with regions of upwelling and downwelling, respectively. Shortly after initialisation, the ammonia vapour distribution up to the cloud base resembles the enhanced concentration seen in Juno observations, due to strong meridional mean circulation at the equator. The resemblance decays rapidly over time, but suggests that at least some of the relevant physics is captured by the model. The comparison should improve with additional microphysics and better representation of the deep ammonia reservoir.

Abstract:

The energetics of possible global atmospheric circulation patterns in an Earth-like atmosphere are explored using a simplified GCM based on the University of Hamburg’s Portable University Model for the Atmosphere (designated here as PUMA-S), forced by linear relaxation towards a prescribed temperature field and subject to Rayleigh surface drag and hyperdiffusive dissipation. Results from a series of simulations, obtained by varying planetary rotation rate Ω with an imposed equator-to-pole temperature difference, were analysed to determine the structure and magnitude of the heat transport and other contributions to the energy budget for the time-averaged, equilibrated flow. These show clear trends with rotation rate, with the most intense Lorenz energy cycle for an Earth-sized planet occurring with a rotation rate around half that of the present day Earth (i.e. Ω* = Ω/ΩE = 1/2, where ΩE is the rotation rate of the Earth). KE and APE spectra, EK(n) and EA(n) (where n is total spherical wavenumber), also show clear trends with rotation rate, with n^-3 enstrophy-dominated spectra around Ω* = 1 and steeper (~ n^-5) slopes in the zonal mean flow with little evidence for the n^-5/3 spectrum anticipated for an inverse KE cascade. Instead, both KE and APE spectra become almost flat at scales larger than the internal Rossby radius, Ld, and exhibit near-equipartition at high wavenumbers. At Ω* << 1, the spectrum becomes dominated by KE with EK(n) ~ (2 - 3)EA(n) at most wavenumbers and a slope that tends towards n^-5/3 across most of the spectrum. Spectral flux calculations show that enstrophy and APE are almost always cascaded downscale, regardless of rotation rate. KE cascades are more complicated, however, with downscale transfers across almost all wavenumbers, dominated by horizontally divergent modes, for Ω* ≲ 1/4. At higher rotation rates, transfers of KE become increasingly dominated by rotational (horizontally non-divergent) components with strong upscale transfers (dominated by eddy-zonal flow interactions) for scales larger than Ld and weaker downscale transfers for scales smaller than Ld.

Abstract:

The solar system presents us with a number of planetary bodies with shallow atmospheres that are sufficiently Earth-like in their form and structure to be termed "terrestrial. " These atmospheres have much in common, in having circulations that are driven primarily by heating from the Sun and radiative cooling to space, which vary markedly with latitude. The principal response to this forcing is typically in the form of a (roughly zonally symmetric) meridional overturning that transports heat vertically upward and in latitude. But even within the solar system, these planets exhibit many differences in the types of large-scale waves and instabilities that also contribute substantially to determining their respective climates. Here we argue that the study of simplified models (either numerical simulations or laboratory experiments) provides considerable insights into the likely roles of planetary size, rotation, thermal stratification, and other factors in determining the styles of global circulation and dominant waves and instability processes. We discuss the importance of a number of key dimensionless parameters, for example, the thermal Rossby and the Burger numbers as well as nondimensional measures of the frictional or radiative timescales, in defining the type of circulation regime to be expected in a prototypical planetary atmosphere subject to axisymmetric driving. These considerations help to place each of the solar system terrestrial planets into an appropriate dynamical context and also lay the foundations for predicting and understanding the climate and circulation regimes of (as yet undiscovered) Earth-like extrasolar planets. However, as recent discoveries of "super-Earth" planets around some nearby stars are beginning to reveal, this parameter space is likely to be incomplete, and other factors, such as the possibility of tidally locked rotation and tidal forcing, may also need to be taken into account for some classes of extrasolar planet.

Abstract:

Saturn’s polar stratosphere exhibits the seasonal growth and dissipation of broad, warm vortices poleward of ~75° latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini’s reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn’s long-lived polar hexagon—which was previously expected to be trapped in the troposphere—can influence the stratospheric temperatures some 300 km above Saturn’s clouds.