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1. Introduction
While the two most prominent conceptual models of extratropical cyclone life cycles, the Norwegian (Bjerknes 1919) and the Shapiro–Keyser (Shapiro and Keyser 1990) models, focus on the surface cyclonic and frontal evolution, respectively, the conveyor belt concept (Harrold 1973; Carlson 1980; Young et al. 1987; Browning 1990; Browning and Roberts 1994) aims to describe the key aspects of the three-dimensional airflow in developing extratropical cyclones. The most intense ascending airstream in extratropical cyclones is the warm conveyor belt (WCB), which typically rises from the boundary layer to the upper troposphere while moving poleward. Browning (1986) identified two types of WCB ascent, characterized by a rearward- and forward-sloping orientation relative to the moving cold front. The two types were shown to occur preferentially with ana (rearward) and kata (forward) cold-frontal configurations. Following Browning and Roberts (1994), WCBs are classified into types W1 and W2, depending on their frontal region of ascent. The WCB ascent is associated with cross-isentropic flow propelled by the release of latent heat due to the condensation of water vapor in the lower troposphere and ice phase processes (mainly depositional growth of snow) in the upper troposphere (Joos and Wernli 2012). This latent heating in turn acts as a source and sink of potential vorticity (PV; e.g., Hoskins et al. 1985), as discussed in more detail below. WCBs are responsible for the major part of precipitation in the extratropical storm-track areas (Browning 1990). Eckhardt et al. (2004) showed that, in the Northern Hemisphere, they most frequently originate in the western North Pacific and North Atlantic, in the latitude band between 25° and 40°N. Sinclair et al. (2008) emphasized the role of WCBs for boundary layer venting, including the potential transport of pollutants into the free atmosphere. Because of their outflow at the level of the upper-tropospheric jet stream, WCBs have the potential to modify the downstream flow evolution. Case study analyses have shown that the WCB outflow can strongly amplify upper-level ridges, which in turn can lead to downstream Rossby wave breaking (Massacand et al. 2001; Grams et al. 2011). This study investigates the structure of WCBs, the associated PV modification, and their downstream impact for the first time within idealized baroclinic wave simulations. The following subsections provide relevant background information...