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ABSTRACT
This idealized modeling study of moist baroclinic waves addresses the formation of moist ascending airstreams, so-called warm conveyor belts (WCBs), their characteristics, and their significance for the downstream flow evolution. Baroclinic wave simulations are performed on the f plane, growing from a finiteamplitude upper-level potential vorticity (PV) perturbation on a zonally uniform jet stream. This nonmodal approach allows for dispersive upstream and downstream development and for studying WCBs in the primary cyclone and the downstream cyclone. A saturation adjustment scheme is used as the only difference between the dry and moist simulations, which are systematically compared using a cyclone-tracking algorithm, with an eddy kinetic energy budget analysis, and from a PV perspective. Using trajectories and a selection criterion of maximum ascent, forward- and rearward-sloping WCBs in the moist simulation are identified. No WCB is identified in the dry simulation. Forward-sloping WCBs originate in the warm sector, move into the frontal fracture region, and ascend over the bent-back front, where maximum latent heating occurs in this simulation. The outflow of these WCBs is located at altitudes with prevailing zonal winds; they hence flow anticyclonically (''forward'') into the downstream ridge. In case of a slightly weaker ascent, WCBs curve cyclonically (''rearward'') above the cyclone center. A detailed analysis of the PV evolution along the WCBs reveals PV production in the lower troposphere and destruction in the upper troposphere. Consequently, WCBs transport low-PV air into their outflow region, which contributes to the formation of distinct negative PV anomalies. They, in turn, affect the downstream flow and enhance downstream cyclogenesis.
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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...