Abstract
Abstract. Using a fully nonlinear two-dimensional (2-D) numerical model, we simulated gravity waves (GWs) breaking and their contributions to the formation of large winds and wind shears in the mesosphere and lower thermosphere (MLT). An eddy diffusion coefficient is used in the 2-D numerical model to parameterize realistic turbulent mixing. Our study shows that the momentum deposited by breaking GWs accelerates the mean wind. The resultant large background wind increases the GW's apparent horizontal phase velocity and decreases the GW's intrinsic frequency and vertical wavelength. Both the accelerated mean wind and the decreased GW vertical wavelength contribute to the enhancement of wind shears. This, in turn, creates a background condition that favors the occurrence of GW instability, breaking, and momentum deposition, as well as mean wind acceleration, which further enhances the wind shears. We find that GWs with longer vertical wavelengths and faster horizontal phase velocity can induce larger winds, but they may not necessarily induce larger wind shears. In addition, the background temperature can affect the time and height of GW breaking, thus causing accelerated mean winds and wind shears.
Highlights
Knowledge of wind structures in the mesosphere and lower thermosphere (MLT) has advanced in recent years with sounding rocket measurements and radar and lidar remotesensing techniques
We focus on the effects of the initial background temperature and gravity waves (GWs) scales on the formation of large winds and wind shears in the MLT region
Using a fully nonlinear 2-D numerical model, which includes a proper eddy diffusion coefficient of 100 m2 s−1 and standard molecular diffusion, we simulated the GW breaking in a non-isothermal compressible atmosphere, and their contributions to the formation of large winds and wind shears in the MLT region
Summary
Knowledge of wind structures in the mesosphere and lower thermosphere (MLT) has advanced in recent years with sounding rocket measurements and radar and lidar remotesensing techniques. In the chemical release experiment by sounding rockets at different latitudes, longitudes, seasons, and local times, large horizontal winds and strong vertical shears have been measured (e.g., Wu and Widdel, 1992; Larsen, 2000, 2002; Larsen et al, 2005; Larsen and Fesen, 2009; Koizumi et al, 2009). Liu (2007) found that the theoretical maximum shears allowed by the background temperature and dynamical stability are similar to those obtained by sounding rocket measurements These large shears cannot be reproduced in global models because the waves are either unresolvable (e.g., GWs) or underestimated (e.g., tides) due to coarse model resolutions (Larsen and Fesen, 2009). A positive feedback process is formed among GW instability and breaking, momentum deposition, mean flow acceleration, decreased GW vertical wavelength, and enhanced wind shear This positive feedback process is difficult to quantify in a global model because resolved GWs are absent.
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