Mesosphere dynamics with gravity wave forcing: Part I. Diurnal and semi-diurnal tides

Abstract

We present results from a non-linear, 3D, time dependent numerical spectral model, which extends from the ground up to the thermosphere and incorporates Hines’ Doppler spread parameterization for small-scale gravity waves (GWs). Our focal point is the mesosphere, which is dominated by wave interactions. We discuss diurnal and semi-diurnal tides in the present paper (Part I) and planetary waves (PWs) in a companion paper (Part II). To study the seasonal variations of tides, in particular with regard to GW forcing, numerical experiments are performed that lead to the following conclusions: (1) The large semi-annual variations in the diurnal tide (DT), with peak amplitudes observed around equinox, are produced to a significant extent by GW interactions that involve, in part, PWs. (2) The DT, like PWs, is amplified by GW momentum deposition, which reduces also the vertical wavelength. (3) Variations in eddy viscosity associated with GW interactions may also influence the DT. (4) The semidiurnal tide (SDT), and its phase in particular, is strongly influenced by the zonal mean circulation. (5) Without the DT present, the SDT is amplified by GWs; but the DT filters out GWs such that the wave interaction significantly reduces the amplitude of the SDT during equinox, effectively producing a strong non-linear interaction between the DT and the SDT. (6) PWs generated internally by the baroclinic instability and GW forcing produce large amplitude modulations of the DT and SDT.