Mean-flow effects of thermal tides in the mesosphere and lower thermosphere

Abstract

his study addresses the heat budget of the mesosphere and lower thermosphere with regard to the energy deposition of upward propagating waves. To this end we recapitulate the ener-getics of gravity waves, using an anelastic version of the Primitive Equations. This leads to an expression for the energy deposition of waves that are usually resolved in general circulation models. The energy deposition is shown to be mainly due to the frictional heating, and additionally due to the negative buoyancy production of wave kinetic energy. The frictional heating includes contributions from horizontal and vertical momentum diffusion, as well as from ion drag. We apply this formalism to analyze results from a mechanistic middle atmosphere general circulation model that includes energetically consistent parameterizations of diffusion, gravity waves, and ion drag. We estimate 1) the wave driving and energy deposition of thermal tides, 2) the model response to the excitation of thermal tides, and 3) the model response to the combined energy deposition by parameterized gravity waves and resolved waves. It is found that thermal tides give rise to a significant energy deposition in the lower thermosphere. The temperature response to thermal tides is positive. It maximizes at polar latitudes in the lower thermosphere due to poleward circulation branches that are driven by the predominantly westward Eliassen-Palm-ux divergence of the tides. In addition, thermal tides give rise to a downward shift and reduction of the gravity-wave drag in the upper mesosphere. Including the energy deposition in the model causes a substantial warming in the upper mesosphere and lower thermosphere.