Photochemical and Climate Models

Developed by Dr. James Kasting, colleagues and students

There are two models available:

- 1D Radiative/Convective Climate Model

- 1D Photochemical Model

These models have versions for high CO2 atmospheres and for high O2 atmospheres.

General characteristics of this model:
The time-stepping procedure and the solar (visible/near-IR) portion of the radiation code are from the model of Pavlov et al. (2000). The code incorporates a δ2-stream scattering algorithm (Toon et al., 1989) to calculate fluxes and uses correlated-k coefficients to parameterize absorption by O3, CO2, H2O, O2, CH4, C2H6, and NO2 in each of 38 spectral intervals in the visible/near-IR and 55 spectral intervals in the thermal-IR (Kasting and Ackerman, 1986; Mischna et al., 2000). 4-term sums are used for CO2 and H2O in the visible; 8-term sums are used for these gases in the thermal-IR. 6-term sums are used for CH4 and C2H6 at all wavelengths. At thermal IR wavelengths this model uses two different subroutines depending on the composition of the atmosphere:

* For low CO2 atmospheres (present Earth): The model uses the RRTM ( code for the infrared. It can give accurate solutions only for low CO2 atmospheres (up to 100 the present level of CO2, that is, a mixing ratio of 3.55×10-2, and up to 500 ppm of CH4). But it is very fast and highly accurate for these atmospheres. Get the compressed model here.

* For high CO2 atmospheres (early Earth, Mars): The IR subroutine is described in Kasting et al. 1984a, 1984b and Mischna et al., 2000. It uses the Toon et al. (1989) 2-stream model with an LTE source function. This version is set up to include only CO2 and H2O. This keeps it relatively fast, as it needs to make just 8×8 = 64 2-stream calculations within each thermal-IR spectral interval. The code can be modified to include CH4, C2H6, and NO2 but it will then run slower by a factor of 6 (when including CH4), and somewhat more when including C2H6. Get the compressed model here.

General characteristics of this model:
The model solves the continuity equation at each height for each of the long-lived species, including transport by eddy and molecular diffusion. The combined equations are cast in centered finite difference form. Boundary conditions for each species are applied at the top and botton of the model atmosphere, and the resulting set of coupled differential equations is integrated to steady state using the reverse Euler method. Photolysis rates for various gas-phase species were calculated using a δ two-stream routine (Toon et al., 1989) that accounts for multiple scattering by atmospheric gases and by aerosols.

* For high O2 atmospheres (present Earth): It was originally developed by Kasting et al. (1985), is more fully detailed in Pavlov and Kasting (2002). It solves for 55 different chemical species that are linked by 217 separate reactions. The altitude range extended from 0 to 64 km in 1-km increments. Get the compressed model here.

* For high CO2/high CH4 atmospheres (early Earth, Mars): Contains 73 chemical species involved in 359 reactions and spanned the region from the planetary surface up to 100 km in 1-km steps.This version of the model was originally developed by Pavlov et al. (2001) and was subsequently modified by Kharecha et al. (2005). Get the compressed model here.

Under revision

Except for the high-CO2 version of the photochemical model, there rest of the codes have four possible choices of incoming stellar flux: the Sun, an F2V star (σ Bootis, HD128167), a K2V (ε Eridani, HD22049) star, and an M star (GJ 888, AD Leonis). The spectra of these stars were gathered and coadded by Martin Cohen (U. California at Berkeley) for the Virtual Planetary Laboratory (VPL), a project of the NASA Astrobiology Institute. The models are adapted to choose a stellar flux using the variable STARR. The high resolution spectra of these stars can be found on the VPL Stellar Spectra Pages.
To bin new stellar flux to be used by the climate model you can use this fortran program.
To bin new stellar flux to be used by the photochemical model you can use this fortran program.
Please read this file before using the binning programs.

REFERENCES (Kasting and colleagues papers available here)

** Kasting et al. (1984a) Icarus 57, 335
** Kasting et al. (1984b) J. Atmos. Chem. 1, 403-428.
** Kasting, J.F. and Ackerman, T.P. (1986) Climatic consequences of very high CO2 levels in the earth s early atmosphere. Science 234, 1383-1385.
** Kharecha, P., Kasting, J.F., and Siefert, J.L (2005) A coupled atmosphere-ecosystem model of the early archean Earth. Geobiology 3, 53-76.
** Mischna et al. "Influence of Carbon Dioxide Clouds on Early Martian Climate" Icarus 145: 546-554 (2000).
** Mlawer, E.J., Taubman, S.J., Brown, P.D., Iacono, M.J., and Clough, S.A. (1997) Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model of the longwave. J. Geophys. Res. 102, 16663 -16682.
** Pavlov, A.A. and Kasting, F. (2002) Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere. Astrobiology 2, 27-41.
** Pavlov, A.A., Kasting, J.F., Brown, L.L., Rages, K.A., and Freedman, R. (2000) Greenhouse warming by CH4 in the atmosphere of early Earth. J. Geophys. Res. 105, 11981-11990.
** Segura, A.; K. Krelove, J. F. Kasting, D. Sommerlatt V. Meadows, D. Crisp, M. Cohen and E. Mlawer (2003) Ozone Concentrations and Ultraviolet Fluxes on Earth-like Planets Around Other Stars. Astrobiology, 3(4), 689-708.
** Antigona Segura, James F. Kasting, Victoria Meadows, Martin Cohen, John Scalo, David Crisp, Rebecca A.H. Butler and Giovanna Tinetti . Biosignatures from Earth-Like Planets Around M Dwarfs. Astrobiology, 5(6), 706-725. 2005.
** Segura, A., V. S. Meadows, J. F. Kasting, M. Cohen, D. Crisp. Abiotic Formation of O2 and O3 in High-CO2 Terrestrial Atmospheres. A. Astronomy and Astrophysics, Volume 472, Issue 2, pp.665-679, 2007.
** Toon, Owen B.; McKay, C. P.; Ackerman, T. P.; Santhanam, K. Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres.Journal of Geophysical Research (ISSN 0148-0227), vol. 94, Nov. 20, 1989, p. 16287-16301.

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Edited by Antigona Segura. February, 2009
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