Mesosphere dynamics with gravity wave forcing: Part II. Planetary waves

We present results from a non-linear, 3D, time dependent numerical spectral model (NSM), which extends from the ground up to the thermosphere and incorporates Hines' Doppler spread parameterization for small-scale gravity waves (GWs). Our focus is the mesosphere where wave interactions play a prominent role. We discuss planetary waves (PWs) in the present paper (Part II) and diurnal and semi-diurnal tides in the companion paper (Part I). Without external, time-dependent energy or momentum sources, PWs are generated in the model for zonal wavenumbers 1-4, which have amplitudes in the mesosphere above 50 km as large as 30 m/s and periods between 2 and 15 days. The waves are generated primarily during solstice, which indicates that the baroclinic instability (associated with the GW driven reversal in the latitudinal temperature gradient) plays an important role. Results from a numerical experiment show that GWs are also significantly involved directly in generating the PWs. For the zonal wavenumber m = 1, the predominant wave periods at mid latitudes in summer cover a wide range around 7 days and peak in winter around 4 days. For m = 2, the periods in summer and winter are around 4 and 3 days, respectively. For m = 3, 4 the predominant wave periods are in both seasons close to 2 days. At low latitudes, these waves have the characteristics of Rossby gravity waves, with large meridional winds at the equator and propagating westward. A common feature of the PWs (m = I to 4) generated in the summer and winter hemispheres is that their vertical wavelengths throughout the mesosphere are large, which indicates that the waves are not propagating freely but are generated (and absorbed) throughout the region. Another common feature is that the PWs propagate preferentially westward in summer and eastward in winter. being launched from the westward and eastward zonal winds that prevail, respectively, in the summer and winter hemispheres at altitudes below 80 km. As shown in Part 1, the PWs generated in the model produce large amplitude modulations of the diurnal and semi-diurnal tides above 80 km and contribute to their seasonal variations. (C) 2001 Published by Elsevier Science Ltd