Numerical modeling study of the momentum deposition of small amplitude gravity waves in the thermosphere

摘要

We study the momentum deposition in the thermosphere from the dissipation ofsmall amplitude gravity waves (GWs) within a wave packet using a fullynonlinear two-dimensional compressible numerical model. The model solves thenonlinear propagation and dissipation of a GW packet from the stratosphereinto the thermosphere with realistic molecular viscosity and thermaldiffusivity for various Prandtl numbers. The numerical simulations areperformed for GW packets with initial vertical wavelengths (λ_z)ranging from 5 to 50 km. We show that λ_z decreases intime as a GW packet dissipates in the thermosphere, in agreement with theray trace results of Vadas and Fritts (2005) (VF05). We also find goodagreement for the peak height of the momentum flux (z_diss) between oursimulations and VF05 for GWs with initial λ_z ≤ 2π H inan isothermal, windless background, where H is the density scale height. Wealso confirm that z_diss increases with increasing Prandtl number. Weinclude eddy diffusion in the model, and find that the momentum depositionoccurs at lower altitudes and has two separate peaks for GW packets withsmall initial λ_z. We also simulate GW packets in anon-isothermal atmosphere. The net λ_z profile is a competitionbetween its decrease from viscosity and its increase from the increasingbackground temperature. We find that the wave packet disperses more in thenon-isothermal atmosphere, and causes changes to the momentum flux andλ_z spectra at both early and late times for GW packets withinitial λ_z ≥ 10 km. These effects are caused by theincrease in T in the thermosphere, and the decrease in T near the mesopause.