Modeling seasonal change on Uranus with the EPIC GCM

Uranus is unique among the gas giant planets as it emits little to no internal heat; the planet's atmospheric energy balance is largely mediated by incoming solar radiation. This fact, combined with its axial tilt of 97°, suggests seasonal forcing of atmospheric dynamics should be intense. Considerable changes in cloudiness and banding were observed between Voyager 2 era images of the atmosphere during solstice and recent HST and Keck images during equinox. Observational analyses do not provide definitive evidence whether the zonal wind profile also changed between these epochs. To determine the sensitivity of the zonal wind field to seasonal forcing, we simulated the planet using the Explicit Planetary Isentropic Coordinate (EPIC) general circulation model (GCM). To drive atmospheric dynamics, we implemented a correlated-k radiative transfer routine that varies both diurnally and seasonally to simulate shortwave absorption by methane. Our radiative model also simulates longwave emission due to H2-H2 collision induced opacity and methane line emission. Our radiative method was validated by its strong conformance to the Voyager 2 observed temperature-pressure profile within a stand-alone one-dimensional model of a vertical column of Uranus' atmosphere. We implemented this radiative model within the EPIC GCM. Starting from a state of zero winds, we ran a series of spin-up experiments in both two-dimensional and three-dimensional mode during equinox, solstice, as well as throughout the course of the Uranian orbit. Our generated zonal winds showed considerable differences in the planet's circulation between seasons. The equinoctial spin-up experiments generated two symmetric high latitude prograde zonal jets. The solsticial spin-up experiments generated two antisymmetric jets - one prograde and one retrograde - at low latitudes. Our spin-up experiments over the course of the Uranian orbit generated a consistent flipping chirality of the zonal antisymmetry after each solstice. We conclude that these experiments explain the overall zonal symmetry of the observed zonal wind profile of Uranus, while tempering the profile with some degree of asymmetry after each solstice.