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Project Progress

As the first step in our efforts to simulate the environments of extrasolar terrestrial planets, a simple 1-dimensional radiative convective equilibrium (RCE) model (c.f. Crisp, 1986; 1989) will be used to study the globally averaged, thermal structures of their atmospheres and surfaces. An existing RCE model (Crisp 1990; Gerstell, Crisp, and Crisp, 1995) is being upgraded to simulate a broad range of plausible environments. This model originally combined band modeling methods with a 2-stream multiple scattering algorithm to estimate radiative heating rates. This approach produced reliable results in some cases, but could produce significant errors in atmospheres where both multiple scattering and line absorption by gases contributed to the radiation field. Second, unlike most climate models, which use a simple convective adjustment to simulate the effects of vertical convection, the RCE model adopted here uses mixing length theory to simulate convection. This approach produces much more realistic temperature profiles near planetary surfaces. However, even though this approach could also provide a self-consistent method for transporting water vapor and other volatiles from the surface to other atmospheric layers, the existing model did not include this capability.

The RCE model upgrades implemented this year address both of these issues. The Spectral Mapping Atmospheric Radiative Transfer (SMART) algorithm has been installed in place of the simpler band modeling and 2-stream methods. The mixing length approach was also extended to transport volatiles as well as heat, and a simple, non-precipitating cloud model was added, so that clouds could form and dissipate as the atmospheric thermal structure evolved toward equilibrium.

Preliminary tests of this model show that additional modifications will be needed to improve its capability, stability, and ease of use. The mixing length scheme is currently being upgraded by adopting the vertical transport scheme from the University of Helsinki 1-dimensional planetary boundary layer model (Savijarvi 1995; 1999). The cloud model is being upgraded to include precipitation as well as condensation and evaporation of clouds.