Convection, fronts, flow over mountains, and spontaneous adjustments occurring at the tropospheric jet streams generate vertically propagating gravity waves (GWs) in the troposphere and lower stratosphere (Smith 1979, Gill 1982, Baines 1995, Fritts and Alexander 2003, Sutherland 2010, Nappo 2012, Plougonven and Zhang 2014). Generally, we differentiate between orographic GWs and non-orographic GWs (OGWs and NOGWs). Through their far-field interactions, GWs constitute an important coupling mechanism in the Earth’s atmosphere and the momentum deposition is described as orographic and non-orographic GW drag (OGWD, NOGWD), respectively. The associated redistribution of momentum and energy controls the global middle atmospheric circulation (Dunkerton 1978, Lindzen 1981, Alexander et al. 2010).

 Comparisons of stratospheric zonal-mean absolute GW momentum fluxes (GWMFs) from global circulation model (GCM) predictions and global satellite data reveal a large variability between currently used GW parametrizations. Some features are reproduced reasonably well in terms of magnitudes and the latitudinal dependence, but there are still large deviations. Particular deviations are the "gap" in simulated zonal mean GWMFs near 60° S in winter (Geller et al. 2013) and the large simulated maxima above mountainous regions which are not observed by satellites such as SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) and HIRDLS (High Resolution Dynamics Limb Sounder) (Geller et al. 2013). Most GCMs exhibit large systematic biases in the simulation of the Southern Hemisphere (SH) polar stratospheric circulation. These biases are known as the "cold pole bias" occurring, e.g., in the Canadian Middle Atmosphere Model (CMAM) or the Whole Atmosphere Community Climate Model (WACCM) (SPARC CCMVal 2010, Butchart et al. 2011) as they underestimate the polar stratospheric temperatures. Due to isolation of the cold stratospheric air and the weak planetary wave activity, there are significant late-biases in the simulated spring-time breakups of the SH vortex. Most likely, the simplified representations of GWs is a major cause of the biases in the simulation of SH stratospheric climate.
Summarizing and referring to a statement by T. G. Shepherd, the problem of parameterizing unresolved GWs and their sensitive interaction with large-scale dynamics is among the most uncertain aspects of climate modeling (Shepherd 2014).

 OGWs likely play an important role but also other processes could explain the "missing drag at 60° S" (de la Cámara et al. 2016). Generally, the physical origin of the observed belt of enhanced GWMF around 60° S can be explained by different mechanisms: (i) The downwind advection and meridional refraction of OGWs from the Southern Andes and the Antarctic Peninsula into the PNJ (Sato et al. 2009). (ii) Unresolved OGWs from small islands (Alexander et al. 2009, Alexander and Grimsdell 2013). (iii) The generation of secondary GWs generated locally in the breaking region of these primary OGWs (Hindley et al. 2015). (iv) NOGWs from sources associated with winter storm tracks over the Southern Ocean (Hendricks et al. 2014). (v) a zonally uniform distribution of small amplitude waves from non-orographic mechanisms such as spontaneous adjustment and jet instability around the edge of the stratospheric PNJ (Hindley et al. 2015).

 The project WASCLIM investigates the impact of GWs on the SH stratospheric circulation and tropospheric climate by means of field observations, high-resolution numerical modelling, the analysis of meteorological reanalyses, and climate modelling. The development and implementation of a simple parametrization for the horizontal propagation of GWs, and, eventually, climate sensitivity simulations will quantify the resulting effects on tropospheric climate and climate change. In the project, long-term ground-based measurements with autonomous temperature-lidar CORAL (Compact Rayleigh Autonomous Lidar) at the southern tip of South America are continued. They are complemented by episodic airborne measurements, amongst others, with the new ALIMA (Airborne Lidar for Studying the Middle Atmosphere) instrument and the infrared limb imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) onboard the German research aircraft HALO (High Altitude and Long Range Research Aircraft) and ground-based meteor radar observations.

 The region of the Andes and the Antarctic Peninsula is a hotspot of stratospheric GW activity. A data set for different GW sources will be assembled and high-resolution ICON (= global forecast model of the German Weather Service) simulations will complement observational data in a consistent way. This data set together with the drag climatology derived from the reanalyses will be used to develop a source-transfer parametrization mimicking the horizontal GW propagation. Moreover, ICON simulations will be used to evaluate its performance in EMAC (= global chemistry climate model of German research institutions) by means of nudged EMAC simulations for the specific episode of the aircraft campaign. The statistical mean representation of GWs in the modified EMAC model will be conducted based on long-term reanalyses-data.