A mini-chemical scheme with net reactions for 3D general circulation models

Abstract

Context. Growing evidence has indicated that the global composition distribution plays an indisputable role in interpreting observational data. Three-dimensional general circulation models (GCMs) with a reliable treatment of chemistry and clouds are particularly crucial in preparing for upcoming observations. In attempts to achieve 3D chemistry-climate modeling, the challenge mainly lies in the expensive computing power required for treating a large number of chemical species and reactions. Aims. Motivated by the need for a robust and computationally efficient chemical scheme, we devise a mini-chemical network with a minimal number of species and reactions for H2-dominated atmospheres. Methods. We apply a novel technique to simplify the chemical network from a full kinetics model, VULCAN, by replacing a large number of intermediate reactions with net reactions. The number of chemical species is cut down from 67 to 12, with the major species of thermal and observational importance retained, including H2O, CH4, CO, CO2, C2H2, NH3, and HCN. The size of the total reactions is also greatly reduced, from ~800 to 20. We validated the mini-chemical scheme by verifying the temporal evolution and benchmarking the predicted compositions in four exoplanet atmospheres (GJ 1214b, GJ 436b, HD 189733b, and HD 209458b) against the full kinetics of VULCAN. Results. The mini-network reproduces the chemical timescales and composition distributions of the full kinetics well within an order of magnitude for the major species in the pressure range of 1 bar–0.1 mbar across various metallicities and carbon-to-oxygen (C/O) ratios. Conclusions. We have developed and validated a mini-chemical scheme using net reactions to significantly simplify a large chemical network. The small scale of the mini-chemical scheme permits simple use and fast computation, which is optimal for implementation in a 3D GCM or a retrieval framework. We focus on the thermochemical kinetics of net reactions in this paper and address photochemistry in a follow-up paper.

A Mini-Chemical Scheme with Net Reactions for 3D GCMs II. 3D thermochemical modelling of WASP-39b and HD 189733b

“…However, assuming we used the same chemical timestep as Drummond et al (2020) and that the picket fence scheme running every timestep is equivalent to their corr-k model every 5 timesteps, our scheme is approximating 4 to 6 times faster than Drummond et al (2020). Tsai et al (2022a) report an approximate 10 times speedup using the mini-chem network compared to a similar sized network to Venot et al (2019) on the same computing hardware. Overall, our experience coupling the scheme and results here show that mini-chem is suitable for running on modest computational resources and does not require a large leap in resources beyond the original GCM model requirements to run.…”

“…We have also made some stiff ordinary differential equation (ODE) solvers 'threadsafe', able to be run with OpenMP to increase the efficiency inside GCMs that use the OpenMP framework. Minichem currently has three miniature chemical networks available, HO, CHO and NCHO, with the CHO and NCHO networks making use of the net reaction rate tables tested and benchmarked in Tsai et al (2022a). A table of the reactions and reference used in the mini-chem networks are provided in Table 1.…”

“…Our mini-chem method (Tsai et al 2022a) reduces the number of species and GCM tracers to 12 (OH, H 2 , H 2 O, H, CO, CO 2 , O, CH 4 , C 2 H 2 , NH 3 , N 2 and HCN) with 10 reactions (Tab. 1) for the NCHO model.…”

“…However, such GCMs with comprehensive chemical network are too computationally expensive to conduct a wider range of numerical survey which is key to improve our understanding of 3D chemical transport. Therefore, developing a GCM framework with a more portable chemical scheme such as the Tsai et al (2022a) net reaction table scheme is urgently needed.…”

“…In this follow up paper to Tsai et al (2022a), we couple the mini-chem chemical network to the 3D Exo-FMS GCM in the hot Jupiter setup (Lee et al 2021). We then use the hot Jupiters WASP-39b and HD 189733b as testbeds of the network, examining the non-equilibrium chemistry behaviour of both simulations.…”