SwRI IRD 2005--Capability Development for General Circulation Modeling of Titan's Atmosphere, 15-9453

Capability Development for General Circulation Modeling of Titan's Atmosphere, 15-9453

Background - Titan, Saturn's largest moon, has long been an interesting object to study because of its substantial atmosphere. Recent discoveries from NASA's Cassini mission have sparked a renewed interest in Titan. This project was undertaken to establish SwRI as an important member of the Titan community by developing a new state-of-the-art Titan atmospheric model.

Approach - The goal of this research is to create a General Circulation Model (GCM) of Titan's lower atmosphere. A GCM is a complex numerical code that solves coupled fluid dynamical and thermodynamic equations to simulate time-evolving atmospheric fields on a planetary body during the time period of a year or more. The bulk of the work on this project was to add the capability for haze and ethane/methane cloud microphysics. To date, Titan microphysics represented particles by a number of size bins. This method requires too many computations for a GCM, which may need hundreds of columns to cover a global scale. For the GCM, the appropriate microphysics equations were derived in the form of moments of a distribution (e.g., total number, area, mass).

The microphysics in the Titan GCM simulates the following processes: Small haze particles are introduced through a boundary condition at the model top to simulate photochemical production. Coagulation of these particles produces a distribution of large haze particles that have sufficient surface area to serve as condensation nuclei for clouds. The large haze particles fall into regions of the atmosphere where volatiles are supersaturated and nucleation produces a cloud particle distribution. Initially, an ethane cloud particle is formed; these fall to lower altitudes where methane nucleation occurs. Both types of clouds can grow or evaporate, depending on atmospheric conditions. The microphysics routines operate independently from the GCM on a column-by-column basis. The GCM communicates temperature, pressure, and particle and gas tracer mixing ratios to the microphysics driver routine. The microphysics then updates the temperature and tracers appropriately and returns the values to the GCM.

Accomplishments - The model has been run out to 15 Titan years (where 1 Titan year is about 30 Earth years) to look at the wind fields. Simulations with haze particles show an accumulation at the poles during solstice times and the appearance of haze particles at lower latitudes near equinox. This condition is consistent with the simulations of European modeling groups and the general consensus on Titan's circulation, pole-to-pole Hadley circulation during winter and summer switching to equator-to-pole cells at equinox. Cloud simulations have shown the formation of methane clouds at similar altitudes to those seen in Titan column models.

Haze particles in Titan's atmosphere are small and transport is mainly through atmospheric motions. In the GCM, the haze particles are initialized uniformly with latitude. The plots show the distribution of haze mass in the model atmosphere after the start of northern fall (L_S=180°) and northern winter (L_S=270°). Titan's atmospheric circulation results in an accumulation of haze at the poles from an equator-to-pole Hadley cell at equinox. At solstice the circulation is pole-to-pole.