This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article. Venus is a slowly rotating planet with a thick atmosphere (~9.2 MPa at the surface). The ground- and satellite-based observations have shown atmospheric superrotation (atmospheric rotation much faster than the solid surface rotation), cloud patterns (e.g., Y-shaped clouds), and polar vortices (polar dipole, cold collar, and fine structures). The Venusian atmospheric circulation, controlled by the planet’s radiative forcing and astronomical parameters, is quite different from the Earth’s one. Since the meteorological data have been accumulated, understanding of the atmospheric circulation has been gradually enriched with the help of theories of geophysical fluid dynamics and meteorology. Observations and fundamental dynamics of the atmospheric circulation are overviewed in this article. In the cloud layer (49-70 km altitude) far from the surface, planetary-scale brightness variations unique to Venus and thermally induced meridional winds have been observed, along with superrotational flows of >100 m/s. The fully developed superrotation ~60-times faster than the planetary rotation is maintained by meridional circulation and waves. Thermal tides, Rossby wave, Kelvin wave, and gravity wave play important roles in some promising mechanisms for maintaining the fast atmospheric rotation. In the lower atmosphere below the cloud layer, unlike the Earth, the general circulation is still unknown, because there is a lack of the global simultaneous observation. Thus, in addition to the limited observations, development of the atmospheric modeling and understanding of the fluid dynamics help to elucidate the atmospheric circulation system. Recently, general circulation models have simulated the dynamical and thermal structures of the Venus atmosphere. The recent advances and future perspectives are marshaled, along with the outstanding issues.
Timothy E. Dowling
Jet streams, “jets” for short, are remarkably coherent streams of air found in every major atmosphere. They have a profound effect on a planet’s global circulation and have been an enigma since the belts and zones of Jupiter were discovered in the 1600s. Collaborations between observers, experimentalists, computer modelers, and applied mathematicians seek to understand what processes affect jet size, strength, direction, shear stability, and predictability. Key challenges include nonlinearity, nonintuitive wave physics, nonconstant-coefficient differential equations, and the many nondimensional numbers that arise from the competing physical processes that affect jets, including gravity, pressure gradients, Coriolis accelerations, and turbulence. Fortunately, the solar system provides many examples of jets, and both laboratory and computer simulations allow for carefully controlled experiments. Jet research is multidisciplinary but is united by a common language, the conservation of potential vorticity (PV), which is an all-in-one conservation law that combines the conservation laws of mass, momentum, and thermal energy into a single expression. The leading theories of how jets emerge out of turbulence, and why they are invariably zonal (east-west orientated), reveal the importance of vorticity waves that owe their existence to conservation of PV. Jets are observed to naturally group into equatorial, midlatitude, and polar types. Earth and Uranus have weakly retrograde equatorial jets, but most planets exhibit strongly prograde (superrotating) equatorial jets, which require eddies to transport momentum up-gradient in a manner that is not obvious but is beginning to be understood. Jupiter and Saturn exhibit multiple alternating jets spanning their midlatitudes, with deep roots that connect to their interior circulations. Polar jets universally exhibit an impressive inhibition of meridional (north-south) mixing, and the seasonal nature of the polar jets on Earth, Mars, and Titan contrasts with the permanence of those on the giant planets, including Saturn’s beautiful north-polar hexagon. Intriguingly, jets in atmospheres have strong analogies with jets in nonneutral plasmas, with practical benefits to both disciplines.