Development of an Improved General Circulation Model for Uranus

The year 2007 marks the passage of the first readily observable Uranian equinox. Due to the extreme axial tilt (980) of Uranus, the dead of a Uranian winter or summer has one pole pointing directly at the sun, resulting in one hemisphere bathed in near perpetual sunlight, the other in near continuous darkness. This was the case when Voyager II arrived at Uranus in 1986, with the southern hemisphere near summer solstice. The result was a visually bland planet, with little observable wave, cloud, vortex, or other similar atmospheric activity. It is this vision of Uranus that came to dominate our view of this planet. Now, 21 years later (a Uranian orbit requires about 84 Earth years), the planet is approaching equinox, meaning much of the planet is receiving roughly nine hours of light and nine hours of darkness each day. This has prompted a much higher level of activity than previously observed on Uranus in the form of visible cloud formations, waves, and even the first large vortex feature or spot ever observed on the planet. The intensity of observation continues to grow as more and more of the northern hemisphere is revealed for the first time, generating the largest burst of data about this planet since Voyager II. This new vision of Uranus is also prompting greater consideration for an orbital mission to this planet (like Galileo for Jupiter and Cassini for Saturn), since our only close encounter appears to have occurred at an inauspicious time in terms ofUranian atmospheric activity. To take advantage of this new found interest, this project proposes to use the well-established EPIC General Circulation Model to conduct observation-driven simulations of the Uranian atmosphere. To achieve this requires that EPIC contain adequate geophysical fluid dynamics, cloud physics, thermodynamics, and radiative transfer models, all of which are critical to properly representing what appear to be solar-driven atmospheric fluid vortices and cloud formation. EPIC currently has a well-tested geophysical fluid dynamics core and less well-tested cloud physics and thermodynamics models, but it is completely without a radiative transfer model. It is this weakness that this project is targeted to address through a collaboration with Dr. Jim Friedson at NASA-JPL, an expert in radiative transfer models for the outer planets. The addition of this model will create an improved GCM, one that is uniquely capable of simulating the observed long-term, radiation-driven activity that is coming to dominate our view of Uranian dynamics.