Thermodynamics and airstreams of a south foehn event in different Alpine valleys

Jansing, Lukas; Sprenger, Michael

doi:10.1002/qj.4285

Cited by 5 publications

(12 citation statements)

References 43 publications

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“…Several recent studies point to the existence and varying importance of different airstreams contributing to Alpine foehn events (Miltenberger et al, 2016;Jansing and Sprenger, 2022;Saigger and Gohm, 2022). These airstreams differ with respect to the horizontal and vertical origin of foehn air parcels and the thermodynamic evolution along their transport pathway.…”

Section: Lagrangian Analysismentioning

confidence: 99%

“…However, systematically assessing the penetration pathways of shallow-foehn air parcels is beyond the scope of this study and could only be accomplished using online trajectories (e.g. Jansing and Sprenger, 2022) or offline trajectories calculated with a much higher temporal input frequency of the 3D wind fields (e.g. Saigger and Gohm, 2022).…”

Section: Impact Of Foehn Types In Altdorfmentioning

confidence: 99%

“…Detailed descriptions of the synoptic evolution of so-called deep-foehn events can be found in the literature (e.g. Burri et al, 1999;Richner et al, 2006;Hächler et al, 2011;Jansing and Sprenger, 2022). With respect to deep foehn, Burri et al (1999) differentiate between "prefrontal foehn", where short-lived foehn episodes are terminated by rapidly propagating cold fronts (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Hächler et al, 2011), and long-lasting foehn situations related to stationary upper-level troughs ("trough foehn" situations), where the upstream trough over western Europe can remain quasistationary for up to several days (e.g. Jansing and Sprenger, 2022).…”

Section: Introductionmentioning

confidence: 99%

“…While the foehn hours are grouped depending on the minimum altitude of the trajectories over the Po Valley, they refrain from an explicit characterization of different foehn types for Altdorf. All of the recent Alpine south-foehn studies (Würsch and Sprenger, 2015;Miltenberger et al, 2016;Jansing and Sprenger, 2022) invoke the Lagrangian perspective to highlight the complex upstream flow pattern. The lowlevel easterly flow over the Po Valley is usually decoupled 1116 L. Jansing et al: Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data from the southwesterlies aloft (see also Rotunno and Houze, 2007), which leads to the formation of distinct airstreams contributing to the foehn flow.…”

Section: Introductionmentioning

confidence: 99%

See 4 more Smart Citations

Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data

Jansing¹,

Papritz²,

Dürr³

et al. 2022

Weather Clim. Dynam.

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Abstract. It has long been recognized that a rich variety of Alpine south-foehn flavours exist that are related to varying flow conditions above crest level, the presence and intensity of orographic precipitation on the Alpine south side, and the Po Valley stratification. This study presents a systematic 5-year climatology of different foehn types. The classification relies on 2329 foehn hours, which are diagnosed using a station-based foehn index for Altdorf in the Swiss Reuss Valley. Operational analyses at 1 km horizontal resolution are employed to classify foehn hours with a decision tree that is based on foehn forecasting experience. Mean wind direction and speed in a circle with 100 km radius centred around Altdorf are considered to differentiate between three main foehn types (deep foehn, shallow foehn, gegenstrom foehn). In addition, upstream precipitation and its extent beyond the Alpine crest are used to distinguish three deep-foehn subtypes (dry foehn, moist foehn, dimmer foehn). The main foehn types differ distinctively in the synoptic conditions over the Alps. During deep foehn, pronounced southwesterlies ahead of an upper-level trough induce upstream orographic precipitation. Shallow foehn, in turn, is associated with cross-Alpine temperature differences that provoke a gap flow. The gegenstrom-foehn type is also restricted to major gaps, but a strong westerly flow prevails above crest level. The deep-foehn subtypes primarily differ in terms of the upper-level trough. While a weaker trough and the influence of an upper-level ridge over the Mediterranean inhibit precipitation (dry foehn), a deeper trough which is closer to the Alps induces stronger crest-level winds and intense precipitation on the Alpine south side (dimmer foehn). The different foehn types are found to strongly affect the local characteristics at Altdorf, which are investigated using station measurements. Backward trajectories from Altdorf are calculated for each of the foehn hours and used to define three clusters of air parcels depending upon their upstream thermodynamic evolution. Trajectories in cluster 1 are diabatically heated and transported within a low-level easterly barrier jet in the Po Valley prior to their ascent to crest level. They constitute the main precipitating airstream and, hence, are of key importance for moist foehn and dimmer foehn. Cluster-2 and cluster-3 trajectories are subject to weak diabatic heating or even diabatic cooling. They originate from southerly to southwesterly regions and from either slightly below or above crest level. Accordingly, these air parcels are associated with little to no precipitation, and as such, they take a key role for dry foehn, shallow foehn and gegenstrom foehn. Furthermore, these three foehn types feature a pronounced stable layer over the Po Valley, which, to some extent, inhibits air parcels to ascend from lower levels. In summary, the study introduces a systematic classification of south foehn using state-of-the-art data sets. It concludes by setting the new classification into a historic context and revisiting the rich body of literature with respect to different Alpine south-foehn types. In particular, analogies to and discrepancies with the existing conceptual models of “Swiss foehn” and “Austrian foehn” are discussed.

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Section: Lagrangian Analysismentioning

confidence: 99%

Section: Impact Of Foehn Types In Altdorfmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data

Jansing¹,

Papritz²,

Dürr³

et al. 2022

Weather Clim. Dynam.

Self Cite

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Synoptic fingerprints of Zonda wind from a statistical prediction model

Otero,

Araneo

2023

Intl Journal of Climatology

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Zonda wind is a typical downslope windstorm over the eastern slopes of the Central Andes in Argentina, which produces extremely warm and dry conditions and has substantial socioeconomic impacts. In this study, we propose a new statistical model for Zonda prediction based on the “synoptic fingerprints” of atmospheric diagnostic variables from ERA5. The model combines principal component analysis (PCA) and logistic regression to establish a relationship between the observed occurrence and the PCA loading component of a predictor variable. This approach enables us to determine the probability of Zonda occurrence at selected stations and identify the synoptic structure features (fingerprints) associated with Zonda events. The obtained fields successfully discriminate between Zonda and non‐Zonda events, suggesting that the available information in the reanalysis data is sufficient for predicting the presence of Zonda. The synoptic fingerprints generated by the model reveal a cross‐barrier pressure gradient resulting from a negative geopotential height anomaly at low levels. The cross‐barrier flow remains unimpeded by the Andes, leading to forced vertical motions on the windward side, accompanied by cooling and precipitation. On the lee side, sinking motions, warming and drying are observed, further facilitated by favourable mid‐ and upper‐level conditions that establish the Zonda wind. The model performs comparably to previous research, with the best results achieved using low‐level variables as predictors. Key performance measures, including the area under the receiver operating curve (ROC) (AUC) of 0.9468 and a Brier score lower than 0.1, demonstrate the model's effectiveness. Using a 0.5 threshold, the accuracy, F1 score and correct alarm ratio (CAR) all exceed 88%, with a probability of detection (POD) higher than 90%. Studies on this type of downslope windstorm are scarce in South America, making this work a significant contribution to understanding synoptic‐scale atmospheric structures associated with Zonda occurrences.

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A station‐based evaluation of near‐surface south foehn evolution in COSMO‐1

Tian,

Duarte,

Schmidli

2023

Quart J Royal Meteoro Soc

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This study investigates the skill of the COSMO model (v5.7) at 1.1 km horizontal grid size in simulating the near‐surface foehn properties and evolution for five south foehn events and a 5‐year‐long climatology. A significant near‐surface cold bias is found during foehn, with an average bias of ‐3 K in the Rhine Valley in the five foehn cases, and of ‐1.8 K in the major northern foehn valleys in the 5‐year foehn climatology. The cold bias tends to be larger in the stronger and moister deep foehn events. Sensitivity experiments are carried out to examine the possible causes of the cold bias, including changes to the parameterization of the land‐atmosphere interaction, to the 1D turbulence parameterization, and to the horizontal grid spacing. Most sensitivity experiments have only a very minor impact on the cold bias, except for the model run with a horizontal grid spacing of 550 m. The 550 m COSMO run shows a reduced cold bias during foehn hours and also an improvement in the simulated foehn duration and northward foehn extent. By inspecting the vertical dimension, we found that the near‐surface cold bias downstream might partly originate upstream. A further contribution to the downstream cold bias is likely due to insufficient vertical mixing in the foehn flow. The latter is possibly enhanced in the 550 m model run, leading to a less stably stratified atmosphere in the lower few hundred meters of the atmosphere and a reduction of the reported model cold bias.This article is protected by copyright. All rights reserved.

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Thermodynamics and airstreams of a south foehn event in different Alpine valleys

Cited by 5 publications

References 43 publications

Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data

Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data

Synoptic fingerprints of Zonda wind from a statistical prediction model

A station‐based evaluation of near‐surface south foehn evolution in COSMO‐1

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