NAWDIC planning wiki¶

This wiki page serves as a platform to prepare the future North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign (NAWDIC)

Context¶

The North Atlantic Wave Guide and Downstream Impact Experiment (NAWDEX; Schäfler et al. 2018; http://nawdex.org, http://www.pa.op.dlr.de/nawdex/, ) in 2016 observed diabatic processes and investigated their impact on the tropopause structure. NAWDIC will build directly on insights of NAWDEX and expand to providing required observations for understanding the tropopause structure and downstream high impact weather (HIW) in the eastern North Atlantic winter. While we envisage NAWDIC as an international campaign, it is initiated through its German HALO-aircraft component, scheduled to take place in the winter (within the January-March window) of 2024 or 2025. Here we outline our initial suggestions.

Scientific background¶

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. Martius et al. 2010; Grams and Archambault 2016; Quinting and Vitart 2019). Two of the key findings of NAWDEX are (i) previously unobserved near-tropopause meso-scale features related to diabatic processes that enhance the jet stream wind speeds (Harvey et al. 2020; Oertel et al. 2020), 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-of-the-art NWP systems (Schäfler et al. 2019).

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 (Browning 1997), where they are climatologically associated with stronger fronts, enhanced vertical mixing in the boundary layer and heavier frontal precipitation (Catto and Raveh-Rubin 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. Indeed, the structure of the tropopause and the interaction of DIs with the planetary boundary layer (PBL), through their impact on surface fluxes, are potential sources of errors and uncertainty in NWP models (Magnusson and Sandu 2019).

Scientific goal¶

NAWDIC aims to provide high-res observations to enhance our understanding of the tropopause structure, DI-PBL interaction and their role as precursors of HIW. Aiming to further improve their representation in NWP models, model output will be evaluated by assimilation of the added observations to identify model biases and new benefits.

Observational aims¶

NAWDIC will 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 (region 1 in Fig. 1). We complement this by following and probing the descending DI airstreams from above until they reach and interact with the PBL in regions of exceptional surface heat and moisture fluxes (region 2). Finally we plan observations with high spatio-temporal resolution in downstream regions prone to HIW (region 3 in Fig. 1).