Revisiting the latent heating contribution to foehn warming: Lagrangian analysis of two foehn events over the Swiss Alps

Abstract

Foehn flows are typically associated with warm air temperatures. Though several theories for the so‐called foehn air warming have been developed over the past century, no conclusion about the most important mechanism has been reached. The development of new methods to calculate accurate air‐mass trajectories over complex topography has opened up a new perspective on this question. Air‐mass‐trajectories derived from wind‐field data from COSMO model simulations with 20 s temporal resolution are used in this study to investigate the origin of the foehn air and the contribution of adiabatic and diabatic processes for two foehn events in the Swiss Alps, with a focus on the Rhine valley. The first foehn event investigated has no precipitation on the upstream side of the Alps. The majority of air parcels stem from upstream altitudes above 1.8 km and most of the foehn air warming is due to adiabatic descent (∼79%). In the second event investigated, significant upstream precipitation occurred. For this case, a significantly larger fraction of the foehn air parcels originate within the lowest 2 km of the upstream atmosphere (up to 70%). Adiabatic descent accounts for the largest part of the temperature change (∼70%), while moist diabatic processes explain about 60% of the potential temperature change. The vertical displacement across the Alpine range is correlated with the diabatic temperature change: parcels strongly heated by condensation, deposition and freezing are in general found at high altitudes above the foehn valley, while parcels affected by diabatic cooling through evaporation, sublimation and melting arrive closer to the valley floor. The high‐resolution trajectories also indicate a much more complicated vertical and horizontal flow pattern than generally assumed, with several distinct air streams upstream of the mountain range and vertical ‘scrambling’ of air masses.

West Antarctic surface melt event of January 2016 facilitated by föhn warming

“…First, the isentropic drawdown is the dominant factor over both Mountain Ranges X and Y in this study. Several studies demonstrate the importance of this mechanism for mountains ranges in the Alps, Iceland, and Antarctica (Seibert, ; Grosvenor et al, ; Elvidge et al, ; Würsch and Sprenger, ; Miltenberger et al, ). Elvidge and Renfrew () indicated that radiative heating did not contribute as much to warming as other mechanisms.…”

“…In the textbook föhn effect case, relatively warm and moist air blows nearly perpendicular to the topographic barrier, and then either flows over the mountain or is blocked, leading to the cold air damming on the upwind side. However, the föhn effect is more complicated in the real world, and there is no general theory that can be applied to each case (Miltenberger et al, ). The strength of the lee‐side warming is significantly related to the dominant mechanisms generating the föhn effect.…”

“…However, isentropic drawdown is the most common mechanism (Elvidge and Renfrew, ). On the windward side, potentially warmer air at higher levels passes over the mountain without being blocked like the air with lower potential temperature at lower levels (Elvidge et al, ; Miltenberger et al, ). Then, the descending air increases the surface temperature on the lee side via adiabatic warming.…”

Kilometer-Scale Climate Models: Prospects and Challenges

“…An offline analysis would follow the traditional approach of saving all necessary fields on disk (e.g., with a temporal resolution of 1 h) and then running the diagnostic. In contrast, an online analysis would be run as part of the main model forward integration, allowing for an almost arbitrary temporal resolution of input fields-for example, online forward trajectory calculations (Miltenberger et al 2016). In the following, two applications are considered, with differing requirements in terms of the temporal resolution and data volume of the input fields.…”

The spatial distribution and temporal variability of föhn winds over the Larsen C ice shelf, Antarctica

“…Advection of potentially warmer and drier air from aloft towards the surface generates the föhn conditions (Elvidge & Renfrew, ). Turbulent sensible heating, mechanical mixing of air on the lee slope, and increased incoming solar radiation from cloud‐clearing can also enhance föhn warming (Miltenberger et al ). In this study, two methods of detection have been used to identify föhn conditions from observations and from the Antarctic Mesoscale Prediction System (AMPS).…”