Energy deposition of thermal tides

Abstract:

The main role of vertically propagating waves in the general circulation is to transfer pseudo momentum from the region of generation to the region of wave breaking. The most prominent examples in atmospheric dynamics are planetary Rossby waves forced in the troposphere, which drive a poleward residual circulation in the winter stratosphere, and mesoscale gravity waves with tropospheric origin, which drive a summer-to-winter-pole circulation in the mesopasue region. In addition, the role of energy deposition by gravity waves has long been recognized to contribute substantially to the energy budget above the stratopause. In atmospheric circulation models, gravity waves are usually parameterized. Their energy deposition can be computed along with the momentum deposition and the turbulent diffusivity associated with wave breaking. In particular, the energy deposition is expressed in terms of secondary moments of the parameterized waves. Therefore, one is tempted to assume that the energy deposition of waves that are resolved in circulation models, e.g., Rossby waves and thermal tides, is automatically taken into account. This assumption is, however, flawed. We show that the energy deposition by resolved waves corresponds to the shear production (frictional heating) of the subgrid-scale turbulence model by which these waves are damped. Computational results from an atmospheric circulation model with energetically consistent treatment of momentum diffusion and frictional heating show that the energy deposition of thermal tides is substantial above the mesopause. This effect is either incomplete or even ignored in conventional atmospheric models that resolve the mesopause region. An idealized sensitivity experiment furthermore shows that thermal tides lead to a significant downward shift of gravity-wave breaking in the upper mesosphere.