Is it north or west foehn? A Lagrangian analysis of Penetration and Interruption of Alpine Foehn intensive observation period 1 (PIANO IOP 1)

Abstract

Abstract. A case study of a foehn event in the Inn Valley near Innsbruck, Austria, that occurred on 29 October 2017 in the framework of the first intensive observation period (IOP) of the Penetration and Interruption of Alpine Foehn (PIANO) field campaign is investigated. Accompanied with northwesterly crest-level flow, foehn broke through at the valley floor as strong westerly winds in the morning and was terminated in the afternoon by a cold front arriving from the north. The difference between local and large-scale wind direction raises the question of whether the event should be classified as north or west foehn – a question that has not been convincingly answered in the past for similar events based on Eulerian approaches. Hence, the goal of this study is to assess the air mass origin and the mechanisms of foehn penetration to the valley floor based on a Lagrangian perspective. For this purpose a mesoscale simulation with the Weather Research and Forecasting (WRF) model and a backward trajectory analysis with LAGRANTO are conducted. The trajectory analysis shows that the major part of the air arriving in Innsbruck originates 6 h earlier over eastern France, crosses the two mountain ranges of the Vosges and the Black Forest, and finally impinges on the Alps near Lake Constance and the Rhine Valley. Orographic precipitation over the mountains leads to a net diabatic heating of about 2.5 K and to a moisture loss of about 1 g kg−1 along the trajectories. A secondary air stream originates further south over the Swiss Plateau and contributes about 10 % to 40 % of the trajectories to the foehn air in Innsbruck. Corresponding trajectories are initially nearly parallel to the northern Alpine rim and get lifted above crest level in the same region as the main trajectory branch. Air parcels within this branch experience a net diabatic heating of about 2 K and, in contrast to the ones of the main branch, an overall moisture uptake due to evaporation of precipitation formed above these air parcels. Penetration into the Inn Valley mainly occurs in the lee of three local mountain ranges – the Lechtal Alps, the Wetterstein Mountains, and the Mieming Chain – and is associated with a gravity wave and a persistent atmospheric rotor. A secondary penetration takes place in the western end of the Inn Valley via the Arlberg Pass and Silvretta Pass. Changes in the upstream flow conditions cause a shift in the contributions of the associated penetration branches. From a Lagrangian perspective this shift can be interpreted on the valley scale as a gradual transition from west to northwest foehn despite the persistent local west wind in Innsbruck. However, a clear classification in one or the other categories remains subjective even with the Lagrangian approach and, given the complexity of the trajectory pattern, is nearly impossible with the traditional Eulerian view.