Abstract
Narrow extended troughs associated with elongated streamers of potential vorticity (PV) are a frequent feature of upper-tropospheric flow and they can instigate or modify surface weather systems. In this study consideration is given to the generation of one particular PV streamer that was itself a dynamical precursor of a heavy precipitation event in the Alpine region.
It is shown that the streamer's parturition over the eastern North Atlantic is linked to a prior event of cyclo- and frontogenesis upstream over the western North Atlantic. Diagnostic trajectory analysis and heuristic simulations with a limited-area NWP model suggest that the streamer's origin is influenced crucially by the following chain of physical processes: cloud-diabatic heating associated with the upstream cyclogenesis enhances a downstream negative PV anomaly in the upper troposphere, and the flow associated with this anomaly subsequently helps transform a farther-downstream preexisting broad positive PV anomaly into an elongated streamer. Hence the case study extends back in time the causal chain that heralds the occurrence of heavy precipitation events in the Alpine region and moreover the existence of the chain has further implications for the classification of extratropical cyclones.
Corresponding author address: Dr. A. C. Massacand, Institute for Atmospheric Science, ETH, Hönggerberg HPP, CH-8093 Zurich, Switzerland. Email: massacand@atmos.umnw.ethz.ch
