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

We study the momentum deposition in the thermosphere from the dissipation of small amplitude gravity waves (GWs) within a wave packet using a fully nonlinear two-dimensional compressible numerical model. The model solves the nonlinear propagation and dissipation of a GW packet from the stratosphere into the thermosphere with realistic molecular viscosity and thermal diffusivity for various Prandtl numbers. The numerical simulations are performed for GW packets with initial vertical wavelengths (lambda(z)) ranging from 5 to 50 km. We show that lambda(z) decreases in time as a GW packet dissipates in the thermosphere, in agreement with the ray trace results of Vadas and Fritts (2005) (VF05). We also find good agreement for the peak height of the momentum flux (z(diss)) between our simulations and VF05 for GWs with initial lambda(z) <= 2 pi H in an isothermal, windless background, where H is the density scale height. We also confirm that z(diss) increases with increasing Prandtl number. We include eddy diffusion in the model, and find that the momentum deposition occurs at lower altitudes and has two separate peaks for GW packets with small initial lambda(z). We also simulate GW packets in a non-isothermal atmosphere. The net lambda(z) profile is a competition between its decrease from viscosity and its increase from the increasing background temperature. We find that the wave packet disperses more in the non-isothermal atmosphere, and causes changes to the momentum flux and lambda(z) spectra at both early and late times for GW packets with initial lambda(z) >= 10 km. These effects are caused by the increase in T in the thermosphere, and the decrease in T near the mesopause.