Influence of upstream diabatic heating upon an Alpine event of heavy precipitation

Narrow extended troughs associated with elongated streamers of potential vo rticity (PV) are a frequent feature of upper-tropospheric flow and they can instigate or modify surface weather systems. In this study consideration i s given to the generation of one particular PV streamer that was itself a d ynamical 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 w estern North Atlantic. Diagnostic trajectory analysis and heuristic simulat ions with a limited-area NWP model suggest that the streamer's origin is in fluenced crucially by the following chain of physical processes: cloud-diab atic heating associated with the upstream cyclogenesis enhances a downstrea m negative PV anomaly in the upper troposphere, and the flow associated wit h this anomaly subsequently helps transform a farther-downstream preexistin g broad positive PV anomaly into an elongated streamer. Hence the case stud y extends back in time the causal chain that heralds the occurrence of heav y precipitation events in the Alpine region and moreover the existence of t he chain has further implications for the classification of extratropical c yclones.