Spatio-Temporal Gravity Wave Source Variability (SV) - Results

Here all information of the project will be published:

Peter Preusse, Manfred Ern, Forschungszentrum Jülich GmbH, Jülich
Ulrich Achatz, Gergely Bölöni, Goethe Universität Frankfurt, Frankfurt/Main

PhD Students/Postdocs:

Isabell Krisch, Cornelia Strube, Forschungszentrum Jülich GmbH; Dr. Young-Ha Kim, Goethe Universität Frankfurt


The goal of the project Source Variability (SV) in the MS-GWaves researchers group is to understand the role of gravity wave (GW) sources in shaping the global distribution of GWs. For this we combine observations and modeling. Observations represent reality. However, in order to gain understanding we need a theory and, for quantitative understanding, a process model which formulates our concepts and which is to be tested against the observations. Numerical models add information, which is not avaible from the measurements. Therefore in SV three different sets of global distributions are compared 1.) a combination of GW source models and GW propagation, 2.) GWs resolved by the UA-ICON GCM and 3.) remote sensing observations from various satellites. By confronting distributions from GW source models with observations we can on the one hand determine free parameters of the models and, on the other hand, assess at which altitudes, regions and seasons which sources are dominant for GW momentum flux (GWMF) and the drag exerted on the main circulation. With increasing resolution, UA-ICON (cf. GWING) resolves a larger part of the GW spectrum. Whether these distributions are realistic, is tested by comparison versus satellite observations. Sources in ICON can be identified by tracing GWs backwards to potential source processes as well as by comparison to the dedicated source models. This allows us to diagnose the reason for deviations and to develop strategies for improvement. A focus will be on the interaction of spontaneous imbalance (SI) and orography in exciting GWs, which we will study by 3D observations taken with GLORIA ( Finally, at all resolutions of ICON we aim at a realistic representation of GWs either by resolved waves or parametrized gravity waves.

In the first phase of the project we expanded our data base (with a traditional stronghold of infrared limb sounders) by new 3D analyses of infrared nadir observations of the satellite instrument AIRS (cf. Figure 1 showing data for January 2009). We confined the GW source model for waves emitted from mesoscale convective complexes (MCCs) by remote sensing observations and used this knowledge to better understand the global distribution of GWs in different seasons. In addition, the role of GWs in sudden stratospheric warmings was investigated from limb sounder observations. Preliminary comparisons between ICON in a low resolution run (40 km) and infrared limb observations show agreement of salient features but also deviations, in particular for presumably convective GWs.

In the second phase we will further improve our understanding of the contribution of different sources to the global distribution, but shift our focus also on the variability of GWs in the atmosphere. The project SV will provide closure to MS-GWaves by integrating the individual sources in a fully-coupled way into ICON. This is based on the MS-GWaM GW propagation model developed by 3DMSD, which is capable of determining the direct transient interaction of GWs with the background flow.

The work in SV is performed in close cooperation with the projects 3DMSD, GWING and SI as  well as PACOG.

Fig 1: One example for observed global distributions are the maps of GW amplitudes and GWMF for January 2009 (result from the first phase of SV). Using retrievals developed in Juelich, the infrared nadir sounder AIRS provides 3D information on atmospheric temperatures. For vertical wavelengths longer than 12km and horizontal wavelengths longer than 40km they allow to infer temperature amplitudes and the 3D wave vector. From these GWMF and its direction, and thus net momentum fluxes, are calculated, here for 36km altitude. Zonal GWMF is directed opposite to the prevailing background winds, i.e. westward in the winter polar jet and eastward in the summer subtropics. In the winter polar night jet, the distribution is given by a combination of mainly orographic sources and a strong global wave 2, in the summer subtropics (around 20 S) GW hot spots correspond to subtropical convection. Adapted from Ern et al. (2017).