The interaction between Gravity Waves and Solar Tides: results from 4D Ray Tracing coupled to a Linear Tidal Model

Abstract:

Gravity waves (GWs) and solar tides (STs) are major components of the mesosphere-lower-thermosphere (MLT) dynamics. GWs are short scale free waves while STs are global scale and thermally driven. Mostly exited in the lower atmosphere and propagating upward, GWs and STs form the main constituents of the dynamical coupling between troposphere and MLT.

GWs and STs incorporate a huge range of scale, from small to global. But conventional GWs parameterizations used to describe this interaction neglect time-dependence and horizontal gradients of the background flow, with potentially fatal effects. We here consider a step-by-step approach to study the GWs and diurnal STs interaction.

GWs propagate in a time-dependent background flow, modeled by a new WKB GW model (ray tracer scheme) where the crossing of rays (caustic problem) is prevented. Climatological mean (which include stationary planetary waves) and diurnal tidal fields extracted initially from a general circulation model (HAMMONIA) compose the background flow. The deposition of momentum and buoyancy from the GWs propagation is calculated. The evaluated Rayleigh-friction and temperature-relaxation coefficients impose in turn a GW forcing on the propagation of diurnal STs in a climatological mean background flow. The propagation of STs is described in a linear version of a general circulation model (KMCM), allowing for planetary waves in the background state. The extracted STs are then used for a new computation of the GW fluxes. This is iterated a few times to obtain a converged result on GW fluxes and STs.

A simplified GWs ensemble is considered, homogenously launched in all directions, at a single launch level. STs are projected onto their migrating and non-migrating components DS₀, DE_[1,2,3] and DW_[1,2,3]. The temporal dependence of the background flow leads to a modulation of the GWs momentum deposition. Due to constructive and destructive interferences, planeraty waves play an active role in the modulation of the GW fluxes but they also influence the DS₀ and DW_[1,2] amplitudes. Because transient critical layers disappear, the horizontal and temporal dependence of the background flow reduce the amplitude of GW momentum and entropy. In concequence, the amplitude of the STs is also reduced. The seasonal variability of the GWs-STs interaction is studied as well.