Geophysical and Astrophysical Fluid Dynamics

Abstract:

We conducted experimental investigations on the effect of stable thermal conditions on rough-wall boundary layers, with a specific focus on their response to abrupt increases in surface roughness. For stably stratified boundary layers, a new analytical relation between the skin-friction coefficient, $C_f$ , and the displacement thickness was proposed. Following the sharp roughness change, the overshoot in $C_f$ is slightly enhanced in stably stratified layers when compared with that of neutral boundary layers. Regarding the velocity defect law, we found that the displacement thickness multiplied by $\sqrt{2/C_f}$ , performs better than the boundary layer thickness alone when describing the similarity within internal boundary layers for both neutral and stable cases. A non-adjusted region located just beneath the upper edge of the internal boundary layer was observed, with large magnitudes of skewness and kurtosis of streamwise and wall-normal velocity fluctuations for both neutral and stable cases. At a fixed wall-normal location, the greater the thermal stratification, the greater the magnitudes of skewness and kurtosis. Quadrant analysis revealed that the non-adjusted region is characterised by an enhancement/reduction of ejection/sweep events, particularly for stably stratified boundary layers. Spatially, these ejections correspond well with peaks of kurtosis, exhibit stronger intensity and occur more frequently following the abrupt change in surface conditions.

Abstract:

Earth is not the only terrestrial body in the solar system with subseasonal-to-seasonal climate oscillations. Though these worlds are not as well observed as Earth, Venus, Mars, and the Saturnian moon Titan each has multiple modes of variability. Mars climate analyses can be considered the most robust given the large quantity of data available, along with three reanalysis datasets. Venus also has had multiple orbiters monitor the climate, and the Cassini mission studied Titan for nearly a decade. Mars and Titan appear to have annular modes of variability in their zonal-mean zonal wind and in the zonal-mean eddy kinetic energy. Mars’s modes are most similar to Earth’s whereby the barotropic mode in the zonal wind captures latitudinal variation in the jet stream; Titan’s mode in the zonal wind describes vertical shifts in the jet. For both Mars and Titan, the baroclinic mode in the eddy kinetic energy quantifies storm track intensity, like Earth’s mode. Mars’s annular modes relate to the timing of large dust storms, and Titan’s annular modes appear related to methane convective events. Mars also has a Semi-Annual oscillation (SAO) in its mesosphere, with similarities to Earth’s stratospheric SAO. Mars’s zonal mean wind swaps between relative westward and eastward phases during solstices and equinoxes, respectively, due primarily to thermal tides. Separately, Venus has three seasonal modes: A 255-day oscillation in zonal wind which is similar to Earth’s Quasi-Biennial Oscillation (QBO) due to vacillations between when Kelvin or Rossby wave modes prevail; a 150-day oscillation in cloud optical depth which may be related to a cycle in eddy diffusion and radiative cooling in the upper-level sulfuric acid cloud deck; and a 50-day oscillation in cloud albedo which could be due to an as yet undetected oscillation in the source of Rossby waves below 35km altitude. Some of the Venusian modes may be related to a recharge-discharge oscillation of convection that has also been speculated to occur on Titan or exoplanets. Finally, we are beginning to glimpse the climate of exoplanets, and simulations in advance of new observations from space telescopes suggest that tidally locked (showing the same face to their star) planets may also have a QBO. Continued discovery and understanding of climate modes of variability remains predicated on continued atmospheric monitoring of these worlds, both from the ground on Earth, in situ on their surfaces, and particularly from orbit around them. The planetary science community must pay particular priority to maintaining monitoring efforts to ensure a robust understanding of these impactful atmospheric features. Future work should pursue a mechanistic understanding of the modes and seek to quantify how they interact.

Abstract:

Until recently, observations of the giant planets of our Solar System were confined to sampling relatively shallow regions of their atmospheres, leaving many uncertainties as to the dynamics of deeper layers. The Cassini and Juno missions to Saturn and Jupiter, however, have begun to address these issues, for example, by measuring their gravity and magnetic fields. The results show that the zonally coherent jets and cloud bands extend to levels where the electrical conductivity of the fluid becomes significant, whereas large-scale vortices, such as the Great Red Spot, are relatively shallow but may have deep-seated roots. The polar regions also exhibit intense cyclonic vortices that, on Jupiter, arrange themselves into remarkably regular “vortex crystals.” Numerical models seem able to capture some of this complexity, but many issues remain unresolved, suggesting a need for models that can represent both deep and shallow processes sufficiently realistically to compare with observations.

Abstract:

We investigate the spatial distribution and dynamics of the vortices in rotating Rayleigh–Bénard convection in a reduced Rayleigh number range $1.3\le Ra/Ra_{c}\le 83.1$ . Under slow rotations ( $Ra\approx 80\,Ra_{c}$ ), the vortices are distributed randomly, which is manifested by the size distribution of the Voronoi cells of the vortex centres being a standard $\varGamma$ distribution. The vortices exhibit Brownian-type horizontal motion in the parameter range $Ra\gtrsim 10\,Ra_{c}$ . The probability density functions of the vortex displacements are, however, non-Gaussian at short time scales. At modest rotating rates ( $4\,Ra_{c}\le Ra\lesssim 10\,Ra_{c}$ ), the centrifugal force leads to radial vortex motions, i.e. warm cyclones (cold anticyclones) moving towards (outwards from) the rotation axis. The horizontal scale of the vortices decreases with decreasing $Ra/Ra_c$ , and the size distribution of their Voronoi cells deviates from the $\varGamma$ distribution. In the rapidly rotating regime ( $1.6\,Ra_{c}\le Ra\le 4\,Ra_{c}$ ), the vortices are densely distributed. The hydrodynamic interaction of neighbouring vortices results in the formation of vortex clusters. Within clusters, cyclones exhibit inverse-centrifugal motion as they submit to the outward motion of the strong anticyclones, and the radial velocity of the anticyclones is enhanced. The radial mobility of isolated vortices, scaled by their vorticity strength, is shown to be a simple power function of the Froude number. For all flow regimes studied, we show that the number of vortices with a lifespan greater than $t$ decreases exponentially as $\exp ({-t/{\tau }})$ for large time, where $\tau$ represents the characteristic lifetime of long-lived vortices.