[Paper Review] MS-GWaM: A 3-dimensional transient gravity wave parametrization for atmospheric models

Georg Sebastian Voelker, Gergely Bölöni+3 more|arXiv (Cornell University)|Sep 20, 2023

Geophysics and Gravity Measurements12 citations

TL;DR

MS-GWaM-3D extends the Multiscale Gravity Wave Model to full 3D transient propagation of internal gravity waves, coupling ray-traced waves with the ICON model and showing 3D wave effects on mesopause dynamics and mean flows.

ABSTRACT

Parametrizations for internal gravity waves in atmospheric models are traditionally subject to a number of simplifications. Most notably, they rely on both neglecting wave propagation and advection in the horizontal direction (single-column assumption) and an instantaneous balance in the vertical direction (steady-state assumption). While these simplifications are well justified to cover some essential dynamic effects and keep the computational effort small it has been shown that both mechanisms are potentially significant. In particular, the recently introduced Multiscale Gravity Wave Model (MS-GWaM) successfully applied ray tracing methods in a novel type of transient but columnar internal gravity wave parameterization (MS-GWaM-1D). We extend this concept to a three-dimensional version of the parameterization (MS-GWaM-3D) to simulate subgrid-scale non-orographic internal gravity waves. The resulting global wave model -- implemented into the weather-forecast and climate code ICON -- contains three-dimensional transient propagation with accurate flux calculations, a latitude-dependent background source, and convectively generated waves. MS-GWaM-3D helps reproducing expected temperature and wind patterns in the mesopause region in the climatological zonal mean state and thus proves a viable IGW parameterization. Analyzing the global wave action budget, we find that horizontal wave propagation is as important as vertical wave propagation. The corresponding wave refraction includes previously missing but well-known effects such as wave refraction into the polar jet streams. On a global scale, three-dimensional wave refraction leads to a horizontal flow-dependent redistribution of waves such that the structures of the zonal mean wave drag and consequently the zonal mean winds are modified.

Motivation & Objective

• Motivate improved representation of subgrid-scale internal gravity waves beyond single-column and steady-state assumptions.
• Develop a 3D transient gravity wave parameterization (MS-GWaM-3D) based on ray tracing and phase-space wave action.
• Couple the 3D wave field to the ICON model to evaluate effects on zonal winds and temperatures in the mesopause region.
• Incorporate two sources of gravity waves (background and convectively generated) with latitude-dependent launching.
• Assess the global distribution, budgets, and mean-flow impacts of 3D IGWs compared to 1D and no-IGW cases.

Proposed method

• Formulate 3D transient IGWs using Bretherton–Grimshaw modulation equations on a sphere with a phase-space (N) representation.
• Apply ray tracing with eikonal equations to compute wave-number and group-velocity evolution along rays.
• Use phase-space wave-action conservation to propagate wave packets and handle caustics.
• Compute IGW fluxes and their divergences to update mean-flow tendencies via Reynolds-stress-like and Coriolis terms.
• Incorporate two lower-boundary wave sources (latitudinally varying background and convective) with Desaubie-like spectral shapes and solstice-based modulation.
• Implement a saturation mechanism based on an integrated density perturbation criterion to regulate amplitudes.

Experimental results

Research questions

• RQ1 How does 3D transient propagation of internal gravity waves alter their energy and momentum fluxes compared to 1D/steady-state parameterizations?
• RQ2 What is the impact of 3D wave refraction on the redistribution of wave energy and on zonal-mean winds in the mesopause region?
• RQ3 How do horizontal wave propagation and phase-space dynamics influence the mean-flow forcing in the ICON model?
• RQ4 Do background and convectively generated IGWs, with latitude- and seasonally-dependent sources, reproduce observed mesopause features?
• RQ5 What are the limitations and sensitivities of MS-GWaM-3D near the poles and in highly unbalanced flow regimes?

Key findings

• Horizontal propagation is as important as vertical propagation for the wave-action budget.
• 3D refraction includes effects such as wave refraction into polar jet streams and leads to horizontal redistribution of wave energy.
• On a global scale, 3D wave propagation modifies the zonal-mean wave drag and alters zonal-mean winds.
• MS-GWaM-3D reproduces mesopause-region temperature and wind patterns in the climatological zonal mean state, consistent with expectations and comparable to MS-GWaM-1D.
• Comparisons with non-IGW runs and reference climatologies show MS-GWaM-3D generally affects the mean state only moderately, with notable differences in the Antarctic winter jet and mesopause region.
• The study demonstrates a viable IGW parameterization that captures 3D transient effects within the ICON framework.