Research in Engineering and Aviation

Orographic Cloud Formation and Evolution about Ice Giant Vortices

March 2011

Author(s): Warning, S.W., R.P. LeBeau, and Cs. Palotai

Orographic Cloud Formation and Evolution about Ice Giant Vortices, 36th Annual Dayton-Cincinnati Aerospace Science Symposium, Dayton, OH, March 1.


Large geophysical vortices provide a useful and interesting examination into the computational simulations of atmospheric dynamics and meteorological phenomena.  Vortex features observed on Uranus and Neptune have exhibited similar changes to that of Earth hurricanes, responding to internal and external conditions by changing strength and location both in shape and in meridional drift.  The most notable of these dynamic vortices was the original Great Dark Spot (GDS-89) observed by Voyager II in 1989, which through eight months of observation drifted towards the equator by ten degrees in latitude and oscillated in shape over an eight-day period during the month of closest observation.  The Ice Giant Dark Spots are also notable for co-existing with a bright methane ice cloud that moves with the vortex as it travels through the atmosphere. These clouds have been characterized as likely orographic clouds, generated as the atmosphere is pushed over the vortex. Both observations and meteorological simulations have indicated these persistent companion clouds may increase the stability of these vortices, allowing them to persist longer than vortices without companions.  Clouds may also mark the existence of a vortex able to maintain a long-term orographic cloud, but does not provide sufficient contrast the planet’s natural hue in order to be visible. This presentation will examine the link between these clouds and vortices, both behaviors seen on the surface and through vertical layers.  A numerical model called the Explicit Planetary Isentropic-Coordinate General Circulation Model (EPIC GCM) is used since observation data is limited.  This data provides the basic knowledge needed to initialize the model, such as zonal wind distribution and vertical temperature-pressure profiles.  Vortices are also initially set, but are allowed to progress computationally.  A methane microphysics model tracks the distribution of this trace gas and allows for the development of cloud features.  Results are compared to observations in order to confirm validation of the simulation techniques.  The process of orographic cloud formation on Earth and on the Ice Giants is likewise similar, even if the atmospheric chemistry and forcing mechanisms are different. The ultimate goal is the creation of a validated ice giant model that can be used to link data from isolated observation windows into a more continuous evolution of the atmospheres of these planets.