THOR: A New and Flexible Global Circulation Model to Explore Planetary Atmospheres

TL;DR

THOR is a novel, flexible 3D global circulation model that solves non-hydrostatic Euler equations on an icosahedral grid, enabling detailed planetary atmosphere simulations with GPU acceleration.

Contribution

It introduces a non-hydrostatic, grid-imprinted, GPU-optimized global circulation model that lifts common atmospheric assumptions and is validated on Earth and exoplanet benchmarks.

Findings

Successfully reproduces Earth's atmospheric circulation.

Accurately simulates hot Jupiter-like atmospheres.

Demonstrates computational efficiency on GPUs.

Abstract

We have designed and developed, from scratch, a global circulation model named THOR that solves the three-dimensional non-hydrostatic Euler equations. Our general approach lifts the commonly used assumptions of a shallow atmosphere and hydrostatic equilibrium. We solve the "pole problem" (where converging meridians on a sphere lead to increasingly smaller time steps near the poles) by implementing an icosahedral grid. Irregularities in the grid, which lead to grid imprinting, are smoothed using the "spring dynamics" technique. We validate our implementation of spring dynamics by examining calculations of the divergence and gradient of test functions. To prevent the computational time step from being bottlenecked by having to resolve sound waves, we implement a split-explicit method together with a horizontally explicit and vertically implicit integration. We validate our global circulation model by reproducing the Earth and also the hot Jupiter-like benchmark tests. THOR was designed to run on Graphics Processing Units (GPUs), which allows for physics modules (radiative transfer, clouds, chemistry) to be added in the future, and is part of the open-source Exoclimes Simulation Platform (ESP; www.exoclime.org).