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About the Project

NAWDIC (North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign) is an international field campaign focusing on mid-latitude dynamics with the aim to provide required observations for understanding the tropopause structure and downstream high impact weather (HIW) in the eastern North Atlantic winter. NAWDIC will build directly on insights of the North Atlantic Wave guide and Downstream impact EXperiment (NAWDEX; Schäfler et al. 2018) and is scheduled to take place in the winter of 2024 or 2025.

Scientific aims:
The tropopause structure is important for the extratropical large-scale circulation, as it is closely related to the strength of the jet stream, forming a wave guide for Rossby waves (e.g. Grams and Archambault 2016; Quinting and Vitart 2019). Two of the key findings of NAWDEX are (i) previously unobserved neartropopause meso-scale features related to diabatic processes that enhance the jet stream wind speeds (Harvey et al. 2019), and (ii) observational evidence of insufficient representation of strong humidity, temperature and wind gradients at the tropopause already in the analysis and early forecast of state-ofthe- art NWP systems (Schäfler et al. 2019).
The tropopause structure has a significant influence on the formation of HIW. On the one hand, it affects the propagation and amplification of Rossby waves which can later trigger downstream HIW. A more direct – still understudied – effect is the air mass transport in so-called dry intrusions (DIs), which connect the near-tropopause air with weather phenomena at the surface downstream in a Lagrangian sense. Within DIs, air masses descend slantwise from near the tropopause towards the cold front of the cyclone downstream, where they are climatologically associated with stronger fronts and enhanced vertical mixing in the boundary layer (e.g. Raveh Rubin and Catto 2019). Yet, it remains unclear under which conditions DIs trigger convection, strong and damaging winds, and how they affect the distribution of clouds and

heavy precipitation.

Observational targets:

To evaluate and improve NWP model representation of tropopause structure and PBL processes, observations of temperature and water vapor variations on the sub-grid scale are vital, but these are routinely not available. NAWDIC aims to systematically sample the near-tropopause structure with a particular focus on its sharpness in the vertical and horizontal directions in regions from where DIs originate. We complement this by following and probing the descending DI air streams from above until they reach and interact with the PBL in regions of exceptional surface heat and moisture fluxes. Finally, we plan probing with high spatio-temporal resolution in downstream regions prone to HIW related to strong winds and deep convection.

Links to HIWeather:

NAWDIC aims is to enhance our understanding of how extratropical HIW emerges downstream of particular large-scale environments and study the multi-scale mechanisms involved. Furthermore, NAWDIC will provide new observations in high resolution to evaluate NWP model performance. As such, NAWDIC lies at the heart of the Predictability and Processes theme. Two of the project leaders, Andreas Schäfler and Shira Raveh-Rubin, are members of the P&P task team.

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