Introduction
The Venus PCM (Planetary Climate Model) [1,2] has been using a pre-computed net exchange rate (NER) matrix formalism to calculate the infrared radiative transfer in the wavelength range, from 1.7 to 250 μm. This implementation allows the Venus PCM to compute temperature self-consistently. In this work, we present recent improvements done to the infrared radiative transfer, in order to improve the Venus PCM performances and reliability. The influence from different optical parameters on the deep atmosphere (below the clouds) temperature and layer stability has also been evaluated to help better understand the thermodynamic processes under the cloud.
Update of the opacity properties
When calculating the NER coefficients, correlated-k distribution for gas opacity, cloud opacities, and collision induced absorptions (CIA) for CO2 and H2O are integrated [3]. As the dominant gas compound, CO2 dominates most of the energy exchange. In this work, the high-resolution spectrum data of different molecules are updated to the latest version. An up-to-date combination of empirical corrections of sub-lorentzian far-wing profiles of CO2 is considered [4,5,6]. CIA have been updated to their latest measurements[7], in particular in the key spectral region between 3 and 10 microns. The influence of this update on the energy exchange, as discussed in [8], will be presented.
Impact on the temperature and layers
Simulations
We use the Venus PCM in a 1-dimensional configuration, using a fixed solar input corresponding to the global average of solar radiation received by Venus, and run simulations for 2000 Venus days to reach equilibrium. The details of the opacity in the windows located from 2 to 10 microns is crucial as these spectral regions control the energy exchanges in the deep atmosphere and strongly affect the temperature profile from cloud base to surface[8]. After the update of the CIA and CO2 line shape, our results indicate that these windows are not opaque enough to reproduce the observed temperature profile, given the solar heating rate distribution used[9][10], as shown in Figure 1.
Figure 1. Temperature profile applying different continuum settings (unit: cm-1 amg-2 , added in wavenumber from 500 cm-1 to 3570 cm-1, and in altitude from 30km ~ 50km) from the 1D configuration of the Venus PCM after 2000 Venus days of simulations, Haus’ table is used to get the solar input, the correlated-k distribution is calculated from the latest version high-resolution spectrum data and the newest CIA data are taken from [7].
Cloud-base temperature
The temperature at the cloud base is slightly under-estimated in our simulations. It mostly depends on the amount of solar energy absorbed in the middle cloud and below, as this energy is balanced by thermal emission mostly to space in the 10-30 micron spectral region at the top of the convective layer (interface between upper and middle cloud).
Temperature profile in the stable layer below the cloud base
With the obtained shape of the opacity in the 2-10 micron spectra region, energy exchanges between lower layers and the cloud base are too efficient in the stable layer (30-50 km), leading to under-estimated temperatures in the deep atmosphere. Some tests are done to evaluate the impact of additional continuum added to slightly close these windows (3-10 microns) on the temperature below the cloud. This additional continuum might be related to the lower haze, so we apply it from 30 to 50 km only. This hypothesis remains to be more fully assessed. The impact of this additional continuum is illustrated in Figure 1. In our results, this additional opacity seems to be also needed below 30 km to reach the observed T profile.
Comparison with the radiative transfer model developed at CPS (Japan) by Takahashi et al [11][12][13] is also done. Differences may be explained by several factors, but here we illustrate the comparison in Figure 2 by comparing extinction coefficients obtained in their work at 5x106 Pa with the extinction computed at the same level in our work.
Figure 2. The absorption coefficient of the CPS radiative transfer model (adapted from figure 11 of [12]), compared with the one obtained in the Venus PCM, both at the 5x106 Pa level (roughly 10 km altitude).
References
[1] Lebonnois, S., Sugimoto, N., & Gilli, G. (2016). Icarus, 278, 38-51, doi : 10.1016/j.icarus.2016.06.004
[2] Garate-Lopez, I., & Lebonnois, S. (2018). Icarus, 314, 1-11, doi : 10.1016/j.icarus.2018.05.011
[3] Eymet, V., Fournier, R., Dufresne, J. L., Lebonnois, S., Hourdin, F., & Bullock, M. A. (2009). Journal of Geophysical Research: Planets, 114(E11), doi : 10.1029/2008JE003276
[4] Tran, H., Boulet, C., Stefani, S., Snels, M., & Piccioni, G. (2011). Journal of Quantitative Spectroscopy and Radiative Transfer, 112(6), 925-936, doi : 10.1016/j.jqsrt.2010.11.021
[5] Tonkov, M. V., Filippov, N. N., Bertsev, V. V., Bouanich, J. P., Van-Thanh, N., Brodbeck, C., ... & Le Doucen, R. (1996). Applied optics, 35(24), 4863-4870.
[6] Pollack, J. B., Dalton, J. B., Grinspoon, D., Wattson, R. B., Freedman, R., Crisp, D., ... & Tipping, R. (1993). Icarus, 103(1), 1-42, doi : 10.1006/icar.1993.1055
[7] Tran, H., Hartmann, J. M., Rambinison, E. & Turbet, M. (2024). Icarus 422, 116265, doi: 10.1016/j.icarus.2024.116265
[8] Lebonnois, S., Eymet, V., Lee, C., & Vatant d'Ollone, J. (2015). Journal of Geophysical Research: Planets, 120(6), 1186-1200, doi : 10.1002/2015JE004794
[9] Haus R, Kappel D, Arnold G.(2015). Planetary and Space Science, 117:262-294. doi:10.1016/j.pss.2015.06.024
[10] Garate-Lopez I, Lebonnois S. (2018) . Icarus;314:1-11. doi:10.1016/j.icarus.2018.05.011
[11] Takahashi, Y. O., Hayashi, Y.-Y., Hashimoto, G. L., Kuramoto, K. & Ishiwatari, M. (2023). Journal of the Meteorological Society of Japan 101, 39–66, doi: 10.2151/jmsj.2023-003.
[12] Takahashi, Y. O. et al. (2024). Journal of the Meteorological Society of Japan, 102, 469–483, doi: 10.2151/jmsj.2024-025.
[13] Our team would like to express our deep sorrow, as Yoshiyuki O Takahashi passed away on April 30th, this year.
How to cite: Han, P. and Lebonnois, S.: Effect of the different opacity factors on deep atmosphere temperature and layers in the Venus PCM, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-591, https://doi.org/10.5194/epsc-dps2025-591, 2025.
