Observations of Gravity Waves from Space - A Study in Support of the PREMIER Earth Explorer Candidate
Preusse, Peter; Hoffmann, Lars; Lehmann, Catrin
Forschungszentrum Juelich, GERMANY
PREMIER is one of three candidates studied in phase A for ESA's Earth Explorer 7 mission. One science aim are mesoscale atmospheric gravity waves (GWs). Gravity waves are an important dynamical coupling mechanism and a large source of uncertainty in climate and weather prediction as they are challenging to measure and to model. For instance, GWs contribute more than 50% of the driving of the QBO, of the predicted trend of the Brewer-Dobson circulation and of the mesospheric circulation. They thus have large influence also on radiative forcing and stratosphere-troposphere dynamical coupling. For instance, European winter temperatures differ up to 2K depending on the phase of the QBO, and NAO coupling via the stratosphere changes precipitation patterns in future climate predictions.
The PREMIER mission will deploy a 2D infrared limb imager feasible through recent advances in detector technology. This facilitates the retrieval of high spatial resolution 3D temperature distributions along the orbital track. A dedicated ESA study ("Observation of Gravity Waves from Space", contract number: 22561/09/NL/AF) was carried out in order to demonstrate that this is a significant step forward for GW research allowing to infer direction resolved GW momentum flux not accessible from space yet. In particular, zonal mean zonal net momentum fluxes at different altitudes can be inferred, which makes PREMIER unparalleled and irreplaceable for GW research.
The study comprises a review of the current state of the art in GW research, a full end-to-end simulation of the processing, the development of a validation concept and, finally, a demonstration of the scientific potential. In particular, requirements for the accuracy of GW momentum flux are deduced. The end-to-end simulation is based on simulated radiance measurements through atmospheric model data and encompasses the development of a 2D tomographic retrieval dedicated for mesoscale GWs, the separation of global scale background and mesoscale GWs and the analysis of the resulting temperature residuals by a 3D wave analysis. The errors of the inferred momentum flux values are assessed for the whole processing chain as well as for the contributions of the single steps and compared to the measurement requirements. Validation of GW momentum flux poses new challenges to validation.
Backward raytracing of GWs identified in the simulated PREMIER data is used to characterize sources of GWs resolved by the ECMWF general circulation model. For instance, we can identify orographic forcing at the south-tip of Greenland, at Tierra del Fuego and at the Antarctic peninsula. These waves contribute an important part of the zonal mean GW momentum flux at these latitudes. They propagate downstream and are observed up to several thousand km to the lee of the mountains. A further prominent source is a winter storm approaching the Norwegian coast. Convectively generated GWs have phase speeds slower than expected and seem to be primarily generated on top of the convection, compatible to the moving mountain model. It is discussed whether this points to a weakness of ECMWF in the generation of GWs by convection.
The study team consisted of the following members:
M. Joan Alexander, Dave Broutman, Hye-Yeong Chun, Anu Dudhia, Albert Hertzog, Michael Hoepfner, Young-Ha Kim, William Lahoz, Jun Ma, Manuel Pulido, Martin Riese, Harjinder Sembhi, Sabine Wüst, Verena Alishahi, Michael Bittner, Manfred Ern, So-Young Kim, Verena Kopp, John C. McConnell, Juan Ruiz, Guillermo Scheffler, Kirill Semeniuk, Viktoria Sofieva, and Francois Vial