Comparison of General Circulation Models of the Venus upper atmosphere

TL;DR

This study conducts an inter-comparison of three 3D GCMs for Venus' upper atmosphere (Venus PCM, VTGCM, TUGCM) to assess their realism up to the exosphere and to guide mission planning. By evaluating thermal, dynamical, and composition outputs against limited observations, it identifies that dayside temperatures are generally overestimated due to underrepresented atomic oxygen cooling and that EUV-spectral input largely drives solar-cycle sensitivity. The work highlights key model differences in CO${_2}$ photochemistry, NLTE radiative cooling, NIR heating, and gravity-wave drag as major sources of discrepancy, and proposes concrete recommendations to standardize inputs and improve O abundance representation. The Venus Climate Database framework is leveraged to enable reproducible inter-model analysis, with VTGCM showing the best partial agreement with mass density data but still requiring refinement. Overall, the paper delineates practical steps to enhance Venus upper-atmosphere modeling and emphasizes the need for dedicated measurements of atomic oxygen and related radiative processes.

Abstract

In the context of future Venusian missions, it is crucial to improve our understanding of Venus upper atmosphere through 3D modeling, notably for spacecraft orbit computation. This study compares three General Circulation Models (GCMs) of the Venusian atmosphere up to the exosphere: the Venus Planetary Climate Model (Venus PCM), the Venus Thermospheric Global Model (VTGCM) and the Tohoku University GCM (TUGCM), focusing on their nominal simulations (e.g. composition, thermal structure and heating/cooling rates). Similarities and discrepancies among them are discussed in this paper, together with data-models comparison. The nominal simulations analyzed in this study fail to accurately reproduce the daytime observations of Pioneer Venus, notably overestimating the exospheric temperature. This is linked to an underestimation of the atomic oxygen (O) abundance in the three GCMs, and suggests the need of additional O production in the thermosphere. The selection of solar spectrum is also the main reason for the discrepancies between the models in terms of temperature dependence on solar activity. A list of recommendations is proposed aiming at improving the modeling of Venus' upper atmosphere, among them: 1. Standardize the EUV-UV solar spectrum input. 2. Update the near-infrared heating scheme with Venus Express-Era data. 3. Reassess Radiative cooling schemes. 4. Investigate the underestimated atomic Oxygen abundance.

Figures (13)

• Figure 1: Diurnal structure of the temperatures for equatorial latitude (30ºS-30ºN) for Venus PCM (top), VTGCM (middle) and TUGCM (bottom) at high solar activity (E10.7 = 200 s.f.u; leftside) and at low solar activity (E10.7 = 70 s.f.u; rightside). The approximate altitude of each model for each solar activity is marked in white for pressure levels of 10, 1, 0.1, $10^{-2}$, $10^{-3}$, $10^{-4}$ and $10^{-5}$ Pa.
• Figure 2: Vertical profile of the EUV heating (top, leftside), NIR heating (top, rightside), conduction heating/cooling (bottom, leftside) and radiative cooling (bottom rightside) rates [K/s] at high solar activity (E10.7 = 200 s.f.u) obtained for equatorial latitude (30ºS-30ºN) at noon for VPCM, VTGCM and TUGCM. The cooling/heating processes have negative/positive value. Cooling processes are plotted with dashed lines. Blue, red and green lines correspond to VTGCM, VPCM and TUGCM.
• Figure 3: Diurnal structure of the zonal wind for equatorial latitude (30ºS-30ºN) for Venus PCM (top), VTGCM (middle) and TUGCM (bottom) at high solar activity (E10.7 = 200 s.f.u; leftside) and at low solar activity (E10.7 = 70 s.f.u; rightside).
• Figure 4: Vertical profile of the dayside thermosphere composition (CO${_2}$ in blue, N${_2}$ in purple, CO in orange and O in red) around the equator (30ºS-30ºN) for each GCM at high solar activity (E10.7 = 200 s.f.u). Solid lines correspond to VPCM. Dashed lines correspond to TUGCM. Dotted lines correspond to VTGCM. Note that TUGCM does not include N${_2}$.
• Figure 5: Vertical profile of EUV heating rate obtained at noon for equatorial latitude (30ºS-30ºN) and for different solar activity and for each GCM (Solid line: high solar activity; dashed line: low solar activity; VPCM in red, VTGCM in blue and TUGCM in green).