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ABSTRACT
This study continues the investigation of airstreams in idealized moist baroclinic waves and addresses the formation of the cold conveyor belt (CCB), its linkage to the warm conveyor belt (WCB), and their impact on the development of a midlatitude cyclone. The CCB is identified as a coherent bundle of trajectories, characterized by weak ascent and a strong increase of potential vorticity (PV) along the flow, in contrast to the WCB, defined as the trajectories with maximum ascent. The authors illuminate the role of the two conveyor belts in the formation of two strong PV anomalies that form in the upper (WCB, negative PV anomaly) and lower troposphere (CCB, positive PV anomaly), respectively, and thereby establish a link between these airstreams and relevant aspects of the dynamics of extratropical cyclones. The CCB moves close to the surface along the colder side of the bent-back front and experiences a PV increase as it passes below a region of maximum latent heat release at midtropospheric levels. Accordingly, it arrives with high PV values at the tail of the bent-back front where the most intense low-level winds occur. The WCB, which rises above the bentback front, causes the formation of the midtropospheric heating rate maximum and thereby not only influences the upper-level downstream development, but also drives the increase of PV along the CCB and, in consequence, indirectly drives the formation of the low-level jet at the tail of the bent-back front.
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1. Introduction
a. The Lagrangian perspective on extratropical cyclones
Studies of coherent "air currents" have a long tradition in the field of dynamic meteorology, dating back at least to the model of warm and cold air currents by Bigelow (1902). Bigelow described alternating streams of rising warm and moist tropical air masses east of a surface depression and sinking polar air masses west of the depression as coherent currents. One convenient possibility to study and visualize the motion of such coherent air masses that ascend or descend within a propagating system is the calculation of trajectories. The calculation of trajectories was already applied by Shaw and Lempfert (1906) to observations close to the surface, and it has become a widely used diagnostic of midlatitude flow systems ever since. Several studies...