The interaction of warm conveyor belt outflows with the upper-level waveguide: a four-type climatological classification

Abstract

Warm conveyor belts (WCBs) are coherent airstreams in extratropical cyclones, characterized by rapid ascent, intense latent heating, and cross-isentropic flow, reaching upper-tropospheric levels in their outflow. The divergent outflow of the WCB with low potential vorticity (PV) influences the upper-level PV distribution, thereby modifying the Rossby waveguide and amplifying the non-linear flow evolution. For instance, the interactions of WCB outflows with the waveguide can initiate block formation and Rossby wave breaking, potentially leading to high-impact weather events in the regions of the interaction and downstream. This study introduces a diagnostic approach to classify the combined WCB-waveguide flow patterns and the associated waveguide disturbance into four distinct types: (i) weak/no interaction, (ii) ridge, (iii) block, and (iv) tropospheric cutoff interactions. Using ERA5 reanalysis data, we present the first systematic climatology (1980–2022) of these interaction types, quantifying their frequency and environmental conditions. The Lagrangian method is based on 5 d backward trajectories from the upper tropospheric waveguide region, which fulfill typical WCB criteria. They are classified into four types based on the presence of ridges, blocks, and cutoffs at their starting points. The method is applied globally and in all seasons, but this paper focuses mainly on the Northern Hemisphere winter (DJF). The WCB identification and interaction classification method is illustrated first for previously documented cases of WCB outflows associated with waveguide disturbances. The climatological analysis in DJF shows that tropospheric WCB outflows most frequently lead to ridge interaction (58.7 %), followed by no interaction (27.7 %), and rarely proceed to block and cutoff interactions (9.7 % and 3.9 %, respectively), with each interaction type occurring in preferred regions. The climatology highlights that the latitude of the WCB outflow clearly differs between the interaction types, whereas the latitudinal distribution of the WCB inflow is fairly similar across the four types. As the amplitude of the waveguide disturbance increases from type (i) to (iv), the associated WCB outflows occur further poleward and westward, have a stronger negative PV anomaly, and reach lower pressure levels. The preceding and prevailing ambient large-scale flow conditions also significantly differ between the interaction types, indicating the large influence of the synoptic flow situation on how WCBs and the upper-level waveguide interact with each other. Weak/no interactions occur in situations with weak synoptic activity and an undisturbed zonally oriented waveguide, while the other interaction types are typically preceded by upper-level ridges and strong synoptic activity.