Uranus atmospheric dynamics and circulation

Abstract

Uranus, with its 98° obliquity, apparently negligible internal heat source and methane-enriched, hydrogen-helium atmosphere, is a unique study in planetary meteorology. Although Voyager imaging measurements of cloud-tracked winds were disappointingly sparse, owing to the planet's low visual contrast and small number of discrete features, the available data indicate atmospheric rotation at mid latitudes nearly 200 m s^(-1) faster than that of the planetary magnetic field (presumably tied to the deep interior). Analysis of the dynamical deformation of the shape and size of isobaric surfaces measured by the Voyager radio-occultation experiment suggests a subrotating equator at comparable altitudes. Infrared temperature retrievals above the cloud deck indicate a smaller equator-to-pole contrast than expected for purely radiative-convective equilibrium, but show local variations implying a latitudinally correlated decrease with altitude in the cloud-tracked wind. While the speed of the differential motions is comparable to that on Jupiter and Saturn, the inferred shape of the zonal wind profile bears a surprising resemblance to the terrestrial circulation. If the observed flow is confined to the cloud layers, it implies an equator-to-pole temperature contrast that is not only much larger than that induced by direct solar heating but also of the opposite sign. Parametric models of the adjustment of the thermal structure by horizontal eddy fluxes successfully predict a reduced latitudinal contrast at infrared sounding levels, as compared with the radiative equilibrium value, but fail to reproduce the observed local variations in temperature and cloud-top winds. Despite the current uncertainties, the planet's unique external forcing and at least superficially simple flow structure (compared with the other Jovian planets) make the Uranian meteorology an important target for further observational and theoretical investigation.