Gravity waves in convection-permitting simulations

In a new study published in the Journal of the Atmospheric Sciences Dr Claudia Stephan and Dr Hauke Schmidt from the department “The Atmosphere in the Earth System” at the Max Planck Institute for Meteorology (MPI-M) together with scientists from Forschungszentrum Jülich and the German Meteorological Service (DWD) present the first intercomparison of resolved gravity wave momentum flux (GWMF) in global convection-permitting simulations and those derived from satellite observations. Increasing computing power is starting to enable global simulations at horizontal grid resolutions of a few kilometers, permitting the explicit simulation of large fractions of the gravity wave spectrum. Thus, in principle, cumbersome gravity wave parameterizations might no longer be required. But are the explicitly simulated gravity waves sufficiently realistic and do different high-resolution models produce similar waves?

Atmospheric gravity waves are mostly generated in the troposphere, for instance by flow over mountains or by convection. They can propagate horizontally and vertically and are particularly important in the middle atmosphere, where their dissipation is associated with the deposition of gravity wave momentum. The resulting drag defines the large-scale global circulation and thermal structure of the middle and upper atmosphere. A realistic representation of wave drag in numerical models has posed a difficult problem because gravity waves are typically too small in scale to be explicitly describable in weather prediction and climate models with grid boxes of the order of several tens to hundred kilometers. Moreover, gravity wave parameterizations suffer from a lack of observational constraints as well as physical inconsistencies between parameterizations and reality.

The published analysis is based on six simulations that were performed as part of the DYAMOND initiative for one month of August. The models used are ICON (horizontal grid spacings of 5 km and 2.5 km, respectively), NICAM (7 km and 3.5 km) and IFS (9 km and 4 km). The GWMF was computed from model data using two different techniques: three-dimensional sinusoidal wave fits (S3D) and a calculation based on the local values of wind and temperature quadratics.

Unlike most climate models that use parameterizations for gravity waves, the DYAMOND simulations reproduce detailed observed features of the global GWMF distribution. This can be attributed to realistic gravity waves from convection, orography and storm tracks. Yet, the GWMF magnitudes differ substantially between simulations. Differences in the strength of convection may help explain differences in the GWMF between simulations of the same model in the summer low latitudes where convection is the primary source. When convection itself is not parameterized, a coarser grid mesh is associated with greater precipitation variability and larger GWMF.* Across the models considered here, there is no evidence for a systematic change with resolution. Instead, GWMF is strongly affected by model formulation. The results imply that validating the realism of simulated gravity waves across the entire resolved spectrum will remain a difficult challenge not least because of a lack of appropriate observational data.

More Information:

Dyamond initiative

Original publication:

Stephan, C. C., C. Strube, D. Klocke, M. Ern, L. Hoffmann, P. Preusse, and H. Schmidt (2019): Intercomparison of gravity waves in global convection-permitting models, J. Atmos. Sci., 124, doi: 10.1175/JAS-D-19-0040.1

*The effects of convective parameterization on GWMF were addressed in a related study published in the “Journal of Geophysical Research – Atmospheres” by the same authors:
Stephan, C. C., C. Strube, D. Klocke, M. Ern, L. Hoffmann, P. Preusse, and H. Schmidt, (2019): Gravity waves in global high-resolution simulations with explicit and parameterized convection. J. Geophys. Res. Atmos., 124, doi: 10.1029/2018JD030073


Dr Claudia Stephan
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 124
Email: claudia.stephan@we dont want