Effective Vertical Diffusion by Atmospheric Gravity Waves

• Published in: Geophysical research letters Vol. 48; no. 1
• Main Author: Liu, Han‐Li
• Format: Journal Article
• Language: English
• Published: 16.01.2021
• Subjects: eddy diffusion; gravity wave; parameterization; scale‐invariance
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091474

Quantification of heat and constituent transport by gravity waves (GWs) in global models is challenging due to limited model resolutions. Current parameterization schemes suffer from oversimplification and often underestimate the transport rate. In this study, a new approach is explored to quantify the effective vertical eddy diffusion by using a high‐resolution Whole Atmosphere Community Climate Model (WACCM) simulation based on scale invariance. The WACCM simulation can partially resolve the mesoscale GW spectrum down to 250 km horizontal wavelength, and the heat flux and the effective vertical eddy diffusion by these waves are calculated directly. The effective vertical diffusion by the smaller‐scale, unresolved waves, is then deduced based on scale invariance, following the method outlined by H.‐L. Liu (2019) in quantifying GW momentum flux and forcing. The effective vertical diffusion obtained is generally larger than that obtained from parameterizations, and is comparable with that derived from observations in the mesosphere and lower thermosphere region. Plain Language Summary Atmospheric gravity waves (GWs) may transport heat and chemical species in the vertical direction. Such transport, often measured in terms of an effective diffusion over the large‐scale background atmosphere, can be important in controlling the exchange of energy and mass between the lower and upper atmosphere, but quantification of the transport process is challenging because GWs are not well resolved or not resolved at all in global models. Previous formulation to approximate the transport tends to oversimplify the process, and can lead to model biases. In high‐resolution models, the larger scale part of the GWs are resolved and the transport by these waves can be directly calculated from simulation results. This study shows that the transport flux of heat follows scale invariance—a statistical similarity over scales—within the resolved mesoscale range. This scale invariance is used to derive the transport flux by the unresolved waves. It is shown that the transport by the unresolved waves can contribute significantly to the total wave transport. The effective diffusion coefficient derived from this study is comparable to values obtained from observations. Key points Vertical heat flux follows scale invariance with a shallow spectrum Effective vertical diffusion by resolved and unresolved waves are calculated using scale invariance The diffusion coefficient is comparable with values obtained from observations