Featured Researcher
UC Los Angeles: Krista Soderlund
Numerical Models of Deep Convection for Atmospheric Dynamics on the Ice Giants
I am presently carrying out simulations on DataStar to test the hypothesis that convection in the deep molecular envelopes of the Ice Giants, Uranus and Neptune, controls the observed zonal flows and outward heat fluxes. As shown in Figures 1a and 1b, the Ice Giants have zonal (east-west) winds that flow westward near the equator, flanked on either side by eastward jets at high latitudes. Measurements of outward heat flux show that Neptune emits approximately 2.6 times more heat than it receives via insolation. This indicates a significant internal heat source. In contrast, the ratio of outward thermal emission to insolation is only 1.1 for Uranus. While this ratio is only slightly greater than unity, the internal heat flow may still be dynamically important. The driving mechanism for these winds and the source of the strongly differing heat flows, despite similar internal structures and zonal flows, are two of the fundamental unsolved questions in planetary atmospheric dynamics.
Figure 1: Observationally-inferred surface zonal wind profiles for (a) Uranus and (b) Neptune. Stars indicate data from Sukorianksy et al. (2002). c) Azimuthally-averaged surface zonal wind profiles from our deep convection model at t=0.17512, 0.19717, and 0.21448 viscous diffusion times (Aurnou et al., 2007). The velocity is given in Rossby number units, Ro=u/Wro.
Previous work by Aurnou et al. (2007) has shown that strong convective mixing can homogenize absolute angular momentum in a deep shell of fluid. This angular momentum homogenization produces a parabolic westward equatorial jet and strong eastward jets at high latitudes. The resulting zonal wind profile, illustrated in Figure 1c, shows good qualitative agreement between the turbulent convection model and the observational models, suggesting that deep convection may be an important dynamical process on the Ice Giants.
I am using DataStar to systematically test and extend the results of Aurnou et al. (2007) by simulating turbulent Boussinesq thermal convection in a rotating spherical shell. This study will help to determine how the zonal flow and heat flow patterns depend on the rotation rate, the competition between Coriolis and buoyancy forces, the physical properties of the fluid, shell geometry, and boundary conditions. The primary goal of this study is to determine whether the inferred internal heat fluxes for Uranus and Neptune are both capable of driving turbulent mixing in their molecular envelopes. These three-dimensional, strongly turbulent simulations require a large grid and, hence, significant computing resources. Consequently, DataStar and HPSS are both proving essential to the success of this project.
This work is supported by the Department of Defense National Defense Science and Engineering Graduate Fellowship Program and the NASA Planetary Atmospheres Program.
References:
Aurnou, J., Heimpel, M., and Wicht, J. The effects of vigorous mixing in a convective model of zonal flow on the Ice Giants, Icarus, doi:10.1016/j.icarus.2007.02.024 (2007).
Sukoriansky, S., Galperin, B., Dikovskaya, N, Universal spectrum of two-dimensional turbulence on a rotating sphere and some basic features of circulation on giant planets.Phys. Rev. Lett., 89, doi: 124501 (2002).
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