Resolved gravity waves in km-scale models: A stratospheric perspective

Gravity waves are ubiquitous in the Earth’s atmosphere. While they are generated predominantly in the troposphere by e.g., flow over orography and strong convective events in the tropics, their largest impact on the circulation is in the middle atmosphere. There they attain large amplitudes and on breaking/saturation drive important features that modulate atmospheric teleconnections, which are a major source of predictability for surface weather and climate.  Such features in the stratosphere include the quasi-biennial oscillation, the polar vortices, and the subtropical jets above the tropopause.

Because gravity waves have a broad wavelength spectrum, most models rely on gravity wave parametrizations to simulate their effect on the large-scale flow. However, global models that explicitly resolve gravity waves are now possible due to recent technological advancements. Using ECMWF IFS global simulations at horizontal grid-spacing ranging from 10 km to 1 km, in which gravity waves are partially or fully resolved, this talk elucidates the following questions relating to the representation of gravity waves in the stratosphere: i) At what horizontal grid-spacings do we expect to resolve the whole gravity wave spectrum so that parameterizations of gravity waves are no longer needed? And ii) What can we learn from km-scale models with regards to the parametrisation design at lower resolution? Finally, the impact of making the hydrostatic approximation (as is currently done at ECMWF) on resolved gravity waves is discussed.