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 the formation of blocks and of 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 WCB-waveguide interactions into four distinct types based on the intensity of the WCB-related waveguide disturbance: (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 the different interaction types, quantifying their frequency and the environmental conditions. The Lagrangian method is based on five-day backward trajectories from the upper tropospheric waveguide region, which fulfill typical WCB criteria. They are classified into different interaction 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 that influenced the upper-level dynamics. The climatological analysis in DJF shows that 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 and end-of-ascent clearly differ between the interaction types, whereas the latitudinal distribution of the WCB inflow and the start-of-ascent is fairly similar across the four types. As the intensity of the interaction 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 ambient large-scale flow conditions also significantly differ between the interaction types, indicating the large influence of the preexisting synoptic flow situation on how WCBs interact with the upper-level waveguide. Weak/no interactions occur in situations with weak synoptic activity and an undisturbed zonally oriented waveguide, while the intense interactions are typically preceded by upper-level ridges and strong synoptic activity.