Abstract
This study is restricted to an assessment of the degree to which non-local thermodynamic equilibrium (non-LTE) is important in cloudy planetary atmospheres, although the formalism is developed in a fashion general enough to serve as a guide for similar studies involving various low-density systems of astrophysical interest. The atmosphere is assumed to be plane-parallel, semi-infinite and vertically homogeneous, and is considered to consist of conceptually separate but spatially coexistent cloud and gas subsystems. Certain physical requirements are imposed: 1) the cloud subsystem opacity is made consistent with the Mie theory; 2) the gas subsystem opacity is restricted to be gray over the visible spectrum and to obey a modified picket fence model in the thermal infrared; and 3) the restriction of LTE is relaxed.
Various microscale heating and cooling mechanisms are then considered. Radiative and conductive heat transfer processes are examined in detail, whereas heat transfer processes by convection and advection are found to be essentially ignorable in the present context. It is verified a posteriori that the importance of latent heat transfer is negligible. It is found for intermediate time scales that the gas subsystem is thermally inert, and that the cloud subsystem is in a state of radiative-conductive equilibrium.
The equation of radiative transfer is solved in the two-stream approximation for two representative models, and the results are tabulated. It is found that departures from LTE fail to be of practical importance by about two orders of magnitude for the cases considered. From scaling arguments it is further concluded that effects due to non-LTE (in the sense under consideration) are of negligible practical concern for cloudy planetary atmospheres under all conditions.
