MS-GWaM - A Three Dimensional Transient Parameterization for Internal Gravity Waves in Atmospheric Models

Internal gravity waves (IGWs) are important distributors of energy and momentum in a stratified atmosphere. While they are mostly excited at lower altitudes their effects are most important between the upper troposphere to the mesopause (~85km). While propagating-both in the vertical and the horizontal-nonlinear IGWs can exert a wave drag on the large-scale winds, interact with the large-scale potential temperature, and influence transport and mixing of atmospheric constituents such as aerosols or greenhouse gases.

In state-of-the art weather prediction models subgrid-scale IGWs are typically parameterized neglecting both the horizontal wave propagation (single-column assumption) and the transient wave behavior including wave-mean-flow interactions and time dependent wave generation (steady-state assumption). The potential importance of the horizontal wave propagation and wave transience has, however, been shown in various theoretical, numerical and experimental studies.

The transient Multi Scale Gravity Wave Model (MS-GWaM)-implemented in the whole-atmosphere model UA-ICON-aims to improve these shortcomings by allowing for transient and three dimensional wave propagation. Being based on a multi scale WKBJ analysis of the compressible atmosphere, MS-GWaM includes various non-orographic wave sources, non-dissipative wave-mean-flow interactions as well as wave breaking. An efficient parallelization of the Lagrangian ray-tracing scheme allows for simulations with transient wave effects in reasonable computation times.

While satisfactorily reproducing the observed zonal-mean wind and potential temperature climatology the model results reveal new insight into the detail of the role of IGWs in the atmosphere. In particular, probability density functions of wave momentum fluxes exhibit the typical observed long tails (i.e. wave intermittency) which cannot be reproduced with steady-state parameterizations. Moreover, the three dimensional distribution of wave momentum and wave action fluxes differ greatly when relaxing the single-column assumption. As an example the well known three dimensional scattering of IGWs into polar jets can be shown.