Effective vertical diffusion by atmospheric gravity waves

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Wednesday, December 16, 2020

Han-Li Liu has observed that atmospheric gravity waves 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 gravity waves 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 gravity waves 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.

Graphic image depicting effective diffusion coefficient
Effective diffusion coefficient by (a) resolved waves, (b) unresolved waves, derived using the method described in the paper and (c) the sum of the two. (d): monthly mean parameterized eddy diffusion coefficient for July used in current WACCM.

Publication: Geophysical Research Letters
Author: Han-Li Liu