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Project Progress

The disk-averaged spectrum of a planet is strongly influenced by the vertical distribution of temperature, atmospheric absorbing gases, and airborne particles. To model extrasolar planetary environments that may be telescopically observed, the VPL uses a 1D terrestrial climate model to describe the globally-averaged vertical thermal structure. The full model includes stellar radiative heating and thermal radiative cooling of the atmosphere and surface, vertical diffusive heat transport within the surface, vertical convective heat transport within the atmosphere, and the associated latent heat transport that results from the vertical mixing, condensation, evaporation, and precipitation of volatile species to produce clouds and rain. Our assumed initial atmospheric vertical profile is time-stepped to match the surface-atmosphere thermodynamic energy equation to radiative/convective/conductive equilibrium. Our modular formalism provides opportunities to deploy algorithms with a range of capabilities and accuracies.

Existing simplified versions of the climate model use an efficient approximate method to calculate radiative fluxes, and heating and cooling rates. These faster models facilitate code development and have adequate accuracy when applied to “Earth-like” (e.g Segura et al. 2003; 2005) and “Mars-like” environments, but they are not generally valid for other environments. We first attempted to replace this formalism with a more versatile and accurate model, the Spectral Mapping Atmospheric Radiative Transfer (SMART) model (Meadows and Crisp, 1996; Crisp, 1997) that combines a line-by-line description of gas absorption with a spectrum-resolving, multi-stream, multiple scattering model to generate stellar and thermal radiances, fluxes, and heating rates throughout realistic, vertically-inhomogeneous, scattering, absorbing and emitting planetary environments. This radiative transfer model has increased accuracy and range of validity, but its complexity and computational expense limited its utility in climate modeling experiments. It was therefore used to validate results from the simpler radiative transfer models, and to generate the high-resolution spectra used in biosignature analysis.

More recently, we improved SMART’s speed and efficiency to provide atmospheric radiative flux and heating rate calculations that are valid over a broad range of planetary environments. SMART now generates linearized “radiance Jacobians”, which specify the rate of change of the radiative fluxes at any level of the atmosphere as a function of the change in the atmospheric or surface state (temperature, pressure, or optical properties) of any other level. These Jacobians provide opportunities for dramatic increases in the speed of the code in climate modeling experiments. The fluxes and heating rates for the initial time step are based on a full-resolution SMART calculation. Wavelength-dependent fluxes for subsequent steps are approximated by a first order Taylor series expansion with respect to variations in the atmospheric state properties. These values can then be integrated to yield heating and cooling rates at a tiny fraction of the cost of a full SMART calculation. This new approach has been implemented in the VPL climate model, and is currently being tested to assess its accuracy and range of validity.