A Lagrangian perspective on the Alpine Foehn

Abstract

Mountains exert a strong influence on the atmosphere that manifests itself across the entire spectrum of spatial and temporal scales. For example, downslope winds frequently develop in the lee as a response to the flow over an orographic barrier. In the Alpine region, Foehn constitutes the generic term for such winds. Foehn is perceived as a strong and often gusty wind that brings a a rapid temperature increase and low relative humidity to the respective valleys. Its striking characteristics lead to important impacts, such as an enhanced risk of forest fires. Accordingly, the phenomenon is not only well known to the local population, but has also received long-standing scientific attention. Nevertheless, the causes of the Foehn air warming, the mechanisms controlling its rapid and strong descent into the valleys, and the striking variability associated with different Foehn types, are still heavily debated and, therefore, build the motivation for this thesis. While the existing literature mainly addressed these topics from a Eulerian perspective, a Lagrangian perspective is adopted here, following air parcels as they traverse the Alps. Thereby, the focus lies on South Foehn, which corresponds to a southerly Foehn flow arriving in northern Alpine valleys. What causes the Foehn air warming? To address this first question, we conducted 15 kilometer-scale simulations with the mesoscale COSMO model and combined these hindcasts with online trajectories calculated during the model integration. The warming and its contributing processes were quantified by applying a Lagrangian heat budget along the online trajectories arriving in northern Alpine valleys. Overall, adiabatic descent predominantly warms the Foehn air parcels, while diabatic processes typically account for a smaller fraction of the warming. But still, diabatic heating is the leading mechanism for about one fifth of the air parcels. The diabatic heating is primarily attributable to upstream latent heat release as the air parcels rise towards the Alpine crest. Turbulent mixing and radiative processes, in turn, only affect the heat budget in individual cases. Besides, some air parcels actually experience a net cooling while surmounting the Alps. This net cooling is a consequence of their particularly low-level origin over the Po Valley, which induces an adiabatic cooling during the ascent to the crest that exceeds the concurrent diabatic heating in magnitude. However, the exact partitioning of the net temperature change into an adiabatic and a diabatic contribution strongly depends upon the studied case, the considered region, and it also varies with time during a Foehn event. More specifically, for events lacking precipitation on the Alpine south side, the bulk of the warming is caused by adiabatic descent. Latent heating plays a substantial role in warming Foehn air parcels during moist events, especially when they arrive in western Foehn valleys on the Alpine north side. In contrast, adiabatic warming typically remains the dominant warming mechanism for eastern Alpine valleys. In essence, it is the net vertical displacement of air parcels across the Alps that determines the relative importance of the two warming mechanisms, as well as the magnitude of the net warming. Air parcels rising strongly on the Alpine south side are simultaneously associated with strong microphysical heating, yet a diminished adiabatic contribution, and vice versa. Furthermore, the warming mechanisms are clearly linked to distinct upstream airflows. In a detailed case study, the near-surface flow south of the Alps is found to be deflected westwards and barrier winds transport air parcels towards the Alpine concavity. These air parcels experience particularly strong diabatic heating as they rise towards the crest and subsequently arrive in the western valleys. Further aloft, trajectories approach the Alps quasi-horizontally from the south. Accordingly, they are subject to slight diabatic heating or even diabatic cooling and predominantly reach the eastern valleys. What flavors of Foehn can we distinguish and how do their characteristics differ? In the second part, using five years of operational COSMO-1 analyses, we established a climatology of different Foehn types. To this end, we classified a total of 2329 Foehn hours at Altdorf in the Swiss Reuss Valley by the means of a decision tree. It distinguishes the different types of Foehn by considering wind speed and direction above Altdorf, precipitation intensity upstream, and the extent of precipitation beyond the Alpine crest. First, three main Foehn types (Deep Foehn, Shallow Foehn, Gegenstrom Foehn) are identified based on the Alpine-scale flow and then systematically studied with respect to their dynamic and thermodynamic characteristics. Deep Foehn occurs most frequently and is associated with a deep southwesterly synoptic flow, as the Alps are located ahead of an upper-level trough. During Shallow Foehn, the temperature contrast of the low-level air masses on the two sides of the Alpine barrier provokes a southerly gap flow. Above crest level, the weak synoptic forcing results in essentially calm conditions. Gegenstrom Foehn, in turn, is characterized by strong westerlies in the mid-troposphere, while southerly winds prevail further below, passing through major gaps along the Alpine transect. In addition, it is differentiated between three Deep Foehn subtypes (Dry Foehn, Moist Foehn, Dimmer Foehn). The subtypes primarily differ with respect to the position and the depth of the upper-level trough. Moist Foehn is the predominant Deep Foehn subtype. If the trough is weak and instead an upper-level ridge extends from the Mediterranean towards the Alpine region, the formation of precipitation on the Alpine south side is inhibited (Dry Foehn). In case of a deep trough close to the Alps, however, intense precipitation and at times spillover of precipitation beyond the main Alpine crest is detected (Dimmer Foehn). Backward trajectories from Altdorf, calculated for each of the 2329 Foehn hours, reveal an upstream flow pattern resembling the airstreams that were identified in the detailed case study. The air parcels originating in the eastern part of the Po Valley rise steeply towards the crest and are subject to intense diabatic heating. In contrast, the air parcels from higher altitudes south of the Alps experience weak diabatic heating or even diabatic cooling. The first category of air parcels primarily contributes to Moist Foehn and Dimmer Foehn, while the second category is most important for Dry Foehn, Shallow Foehn and Gegenstrom Foehn. Finally, the prevalence of a certain Foehn type at Altdorf is shown to influence the local conditions in the Reuss Valley and the Foehn frequency at other stations north of the Alps. Where does Foehn air preferentially descend and how is this descent characterized on a regional to local scale? In the third part of this thesis, we employed the online trajectories from the 15 simulations to identify strong descent along the parcels’ pathways across the Alps. Spatially confined hotspots of descent are primarily located in the immediate lee of local mountain peaks and chains, whereby the small-scale elevation differences of the underlying terrain largely determine its magnitude. Along the Raetikon, a regional mountain range adjacent to the Rhine Valley, one particularly prominent hotspot stands out, which is further examined in two case studies. During periods of strong descent, local peaks along the Raetikon excite gravity waves that propagate downstream and extend towards the axis of the Rhine Valley, therefore causing descent into the northern tributaries of the Raetikon and into the Rhine Valley. Apart from propagating gravity waves, other effects likewise influence the descent activity. First of all, a topographic concavity redirects air parcels towards the floor of the Rhine Valley and thereby promotes strong descent. Secondly, nocturnal cooling can inhibit the formation of pronounced gravity waves and thus impede the descent of Foehn air parcels into the valley atmosphere. Finally, periods of enhanced descent correspond, at least to some extent, to periods of stronger Foehn winds at the floor of the Rhine Valley. In summary, this thesis provides novel insights on several of the long-standing conundrums in Foehn research. Contrasting the past attempts to build an all-encompassing theory on Foehn, we demonstrate the wide spectrum of processes governing Foehn flows. The warming processes are quantified and the descent of Foehn air is characterized in a series of case studies, and a climatology of different Foehn types is established. All of these research topics strongly benefited from adopting the Lagrangian perspective, which not only complements but also substantially extends the previously predominant Eulerian perspective on Foehn research.