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

We present results from a non-linear, 3D, time dependent numerical spectral model, which extends from the ground up to the thermosphere and incorporat es Hines' Doppler spread parameterization for small-scale gravity waves (GW s). Our focal point is the mesosphere, which is dominated by wave interacti ons. 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 se asonal variations of tides, in particular with regard to GW forcing, numeri cal experiments are performed that lead to the following conclusions: (1) T he large semi-annual variations in the diurnal tide (DT), with peak amplitu des observed around equinox, are produced to a significant extent by GW int eractions that involve, in part, PWs. (2) The DT, like PWs, is amplified by GW momentum deposition, which reduces also the vertical wavelength. (3) Va riations in eddy viscosity associated with GW interactions may also influen ce the DT. (4) The semidiurnal tide (SDT), and its phase in particular, is strongly influenced by the zonal mean circulation. (5) Without the DT prese nt, the SDT is amplified by GWs; but the DT filters out GWs such that the w ave interaction significantly reduces the amplitude of the SDT during equin ox, effectively producing a strong non-linear interaction between the DT an d the SDT. (6) PWs generated internally by the baroclinic instability and G W forcing produce large amplitude modulations of the DT and SDT. (C) 2001 P ublished by Elsevier Science Ltd.