Ray‐tracing simulation of the global propagation of inertia gravity waves through the zonally averaged middle atmosphere

Abstract

The global propagation of large‐horizontal‐wavelength inertia gravity waves through a zonally averaged geostrophic wind and temperature climatology of the middle atmosphere is investigated using numerical ray‐tracing techniques. Strong meridional shear in the zonal wind acts to refract waves horizontally (azimuthally), leading to changes in the local propagation azimuth, horizontal wavelength, and ground‐based horizontal phase speed of the wave. Unlike other identified mechanisms which can produce such changes, this refraction effect is linear and does not require (nor forbid) that the wave amplitude be unstable for it to occur. Careful treatment of wave amplitudes indicates that such refracted waves are suppressed in amplitude far less by radiative damping and saturation than previous modeling has suggested, due to the use of a more accurate radiative damping parameterization. Refraction therefore is potentially important in modifying the waves' characteristics as they propagate through the middle atmosphere, and simulations reveal modifications which agree broadly with some observational data. Multiray simulations, using a horizontally isotropic source “spectrum” of waves with equal initial amplitudes, produce wave amplitudes at a height of 60 km which exhibit variations with latitude, season, and vertical wavenumber which have much in common with observations. A dearth of waves with large horizontal wavelengths also arises, in agreement with collated observations in the upper middle atmosphere. Significant differences in the latitude‐season structure of the mean wave amplitudes of the northern and southern hemisphere are simulated. The simulations reproduce the well‐known change in mean azimuthal wave propagation from eastward in summer to westward in winter, but the simulated westward phase invariably exceeds 6 months in duration, and in places the eastward summer phase is very weak and of limited duration. In addition, a smaller but nonetheless distinct poleward component to the mean propagation directions arises, due in part to horizontal refraction. Comparison of measured and simulated distributions of wave propagation azimuths reveals good agreement at some locations. Mid‐latitude winter distributions around North America are well simulated when only c = 0 waves are retained. However, significant differences also occur, which may reflect the importance of features omitted from this model, such as zonal atmospheric variability and/or wave source effects. These and other limitations of the present model are highlighted; it is recommended that subsequent simulations incorporate zonal refraction due to planetary Rossby wave structure, and that higher‐frequency waves be included so that the impact of refraction on the vertical flux of horizontal wave momentum can be assessed.