Quantifying gravity wave forcing using scale invariance

Han-Li Liu

doi:10.1038/s41467-019-10527-z

Abstract

Despite the increasing resolution, forcing on the mean circulation by resolved waves in general circulation models is not yet converging. Parameterization of the forcing remains a major source of model uncertainty. This study examines the scale invariance of zonal spectra of momentum flux and wave forcing, and shows that it can be used to quantify the forcing by unresolved waves with knowledge of the resolved ones in global models. The result reveals the leading order importance of the small-scale wave forcing, which is in general agreement with that required for obtaining the zonal mean wind climatology. It is also found that wave and mean flow interaction is important in maintaining the robust spectral structure. This method may provide a strategy to design physically consistent and scale-aware parameterization schemes for scale invariant quantities, when a model has sufficient resolution to partially resolve their spectra.

Highlights

Despite the increasing resolution, forcing on the mean circulation by resolved waves in general circulation models is not yet converging
Scale invariance of kinetic energy (KE) spectrum is well established from atmospheric wind observations[1] and can be reproduced in high-resolution general circulation models (GCMs) over part of the observed scale ranges, as allowed by model resolution[2,3,4,5,6,7]
There are competing theories for the observed ~−5/3 slope of the KE spectrum over the mesoscale range, high-resolution numerical studies provide support for the interpretation in terms of gravity waves: they have shown that the KE spectrum of the flow divergence modes, presumably associated with gravity waves, follows a ~−5/3 slope and that it becomes increasingly dominant over the rotational modes at higher altitudes[2,6,9]

Summary

Introduction

Despite the increasing resolution, forcing on the mean circulation by resolved waves in general circulation models is not yet converging. The KE spectrum is resolved at smaller scales and the gravity waves from high-resolution GCMs are rather realistic in comparison with satellite, airborne, and ground-based observations[7,19,20,21], the spectral structures of momentum flux and forcing have not been addressed in the literature, and the mean forcing by resolved waves is still too weak in the middle and upper atmosphere. This is reflected in the excessively strong stratospheric and mesospheric zonal winds and high wind reversal altitudes[7] and in the unrealistic periodicity of quasibiennial oscillation (QBO)[22] in GCM simulations, probably because the unresolved waves still have significant contribution to the global momentum budget[23]. Atmosphere Community Climate Model (WACCM) with a horizontal resolution of ~25 km[7], the model could effectively resolve waves with horizontal wavelengths down to ~250 km

Objectives

Methods

Results

Conclusion