Weather and Climate Dynamics最新文献

{"title":"Can low-resolution CMIP6 ScenarioMIP models provide insight into future European post-tropical-cyclone risk?","authors":"Elliott M. Sainsbury, R. Schiemann, K. Hodges, A. Baker, L. Shaffrey, Kieran Bhatia, Stella Bourdin","doi":"10.5194/wcd-3-1359-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1359-2022","url":null,"abstract":"Abstract. Post-tropical cyclones (PTCs) can cause extensive damage across\u0000Europe through extreme winds and heavy precipitation. With increasing sea\u0000surface temperatures, tropical cyclones (TCs) may form and travel further\u0000poleward and eastward than observed historically. Recent work has\u0000suggested that the frequency of intense Europe-impacting PTCs may increase\u0000substantially in the future. Using an objective feature-tracking scheme and TC identification method, we\u0000track and identify the full life cycle of TCs in the North Atlantic in five\u0000CMIP6 climate models in the historical (1984–2014) period and in the future\u0000under the SSP5-85 scenario (2069–2099). These five models are selected based\u0000on their ability to simulate TC frequency similar to observed in the North\u0000Atlantic, although model deficiencies remain. We find no robust changes in Europe-impacting PTC frequency or intensity in\u0000the future. This is because two competing factors – a significant decrease\u0000in TC frequency of 30 %–60 % and an increase in the proportion of TCs\u0000reaching Europe – are approximately the same size. The projected increase\u0000in the proportion of TCs reaching Europe is largely driven by an increase in\u0000the likelihood of recurvature and is consistent with projected decreases in\u0000vertical wind shear and increases in potential intensity along the US East\u0000Coast in the future. The projected increased likelihood of recurvature is\u0000also associated with a shift in TC genesis away from the main development\u0000region, where model biases cause very few TCs to recurve. This study\u0000indicates that large uncertainties surround future Europe-impacting PTCs and\u0000provides a framework for evaluating PTCs in future generations of climate\u0000models.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127578019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Non-linear intensification of monsoon low-pressure systems by the BSISO","authors":"Kieran M. R. Hunt, A. Turner","doi":"10.5194/wcd-3-1341-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1341-2022","url":null,"abstract":"Abstract. More than half of the rainfall brought to the Indian subcontinent by the summer monsoon is associated with low-pressure systems (LPSs). Yet their relationship with the boreal summer intraseasonal oscillation (BSISO) – the dominant intraseasonal forcing on the monsoon – is only superficially understood. Using reanalysis data, we explore the relationship between the BSISO and LPS intensity, propagation and precipitation, and associated underlying mechanisms. The BSISO has a large impact on mean monsoon vorticity and rainfall as it moves northward – maximising both in phases 2–3 over southern India and phases 5–6 over northern India – but a much weaker relationship with total column water vapour. We present evidence that LPS genesis also preferentially follows these phases of the BSISO.\u0000We identify significant relationships between BSISO phase and LPS precipitation and propagation: for example, during BSISO phase 5, LPSs over northern India produce 51 % heavier rainfall and propagate northwestward 20 % more quickly.\u0000Using a combination of moisture flux linearisation and quasi-geostrophic theory, we show that these relationships are driven by changes to the underlying dynamics rather than the moisture content or thermodynamic structure of the monsoon. Using the example of LPSs over northern India during BSISO phase 5, we show that the vertical structure of anomalous vorticity can be split into contributions from the BSISO background circulation and the non-linear response of the LPS to anomalous BSISO circulation. Complementary hypotheses emerge about the source of this non-linear vorticity response: non-linear frictional convergence and secondary barotropic growth. We show that both are important. The BSISO imparts greater meridional shear on the background state, supporting LPS intensification. The BSISO background and non-linear LPS response both contribute significantly to anomalous boundary layer convergence, and we show through vortex budget arguments that the former supports additional LPS intensification in boundary layer, while the latter supports faster westward propagation. This work therefore yields important insights into the scale interactions controlling one of the dominant synoptic systems contributing to rainfall during the monsoon.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"84 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133088611","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The impact of microphysical uncertainty conditional on initial and boundary condition uncertainty under varying synoptic control","authors":"T. Matsunobu, C. Keil, C. Barthlott","doi":"10.5194/wcd-3-1273-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1273-2022","url":null,"abstract":"Abstract. The relative impact of individual and combined uncertainties of cloud condensation nuclei (CCN) concentration and the shape parameter of the cloud\u0000droplet size distribution (CDSD) in the presence of initial and boundary condition uncertainty (IBC) on convection forecasts is quantified using the\u0000convection-permitting model ICON-D2 (ICOsahedral Non-hydrostatic). We performed 180-member ensemble simulations for five real case studies representing different synoptic forcing\u0000situations over Germany and inspected the precipitation variability on different spatial and temporal scales. During weak synoptic control, the\u0000relative impact of combined microphysical uncertainty on daily area-averaged precipitation accounts for about one-third of the variability caused by\u0000operational IBC uncertainty. The effect of combined microphysical perturbations exceeds the impact of individual CCN or CDSD perturbations and is\u0000twice as large during weak control. The combination of IBC and microphysical uncertainty affects the extremes of daily spatially averaged rainfall\u0000of individual members by extending the tails of the forecast distribution by 5 % in weakly forced conditions. The responses are relatively\u0000insensitive in strong forcing situations. Visual inspection and objective analysis of the spatial variability in hourly rainfall rates reveal that\u0000IBC and microphysical uncertainties alter the spatial variability in precipitation forecasts differently. Microphysical perturbations slightly shift\u0000convective cells but affect precipitation intensities, while IBC perturbations scramble the location of convection during weak control. Cloud and\u0000rainwater contents are more sensitive to microphysical uncertainty than precipitation and less dependent on synoptic control.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"15 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121047965","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The stratosphere: a review of the dynamics and variability","authors":"N. Butchart","doi":"10.5194/wcd-3-1237-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1237-2022","url":null,"abstract":"Abstract. Large-scale, intra-seasonal to inter-annual variability of the stratosphere is reviewed.\u0000Much of the variability is dynamical and induced by waves emanating from the troposphere.\u0000It is largely characterized by fluctuations in the strength of the polar vortex in winter\u0000and a quasi-biennial oscillation in the equatorial winds.\u0000Existing theories for the variability are generally formulated in terms of wave–mean-flow\u0000interactions, with refinements due, in part, to teleconnections between the tropics and\u0000extratropics.\u0000Climate and seasonal forecast models are able to reproduce much of the observed\u0000polar stratospheric variability and are increasingly successful in the tropics too.\u0000Compared to the troposphere the models display longer predictability timescales for variations within the stratosphere.\u0000Despite containing just ∼17 % of the atmosphere's mass, the stratosphere's variability\u0000exerts a powerful downward influence on the troposphere that can affect surface extremes.\u0000The stratosphere is therefore a useful source of additional skill for surface predictions.\u0000However, a complete dynamical explanation for the downward coupling is yet to be established.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"268 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128792839","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stratospheric downward wave reflection events modulate North American weather regimes and cold spells","authors":"G. Messori, M. Kretschmer, Simon H. Lee, Vivien Wendt","doi":"10.5194/wcd-3-1215-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1215-2022","url":null,"abstract":"Abstract. The Arctic stratospheric polar vortex is an important driver of mid-latitude winter cold spells. One proposed coupling mechanism between the\u0000stratospheric polar vortex and the troposphere is upward-propagating planetary waves being reflected downward by the polar vortex. However, while\u0000the wave reflection mechanism is well-documented, its role in favouring cold spells is still under-explored. Here, we analyse such stratospheric\u0000wave reflections and their impact on the tropospheric circulation and surface temperatures over North America in winter. We present a physically\u0000interpretable regional stratospheric wave reflection detection metric and identify the tropospheric circulation anomalies associated with prolonged\u0000periods of wave reflection, which we term reflection events. In particular, we characterise the tropospheric anomalies through the lens of\u0000North American weather regimes. Stratospheric reflection events show a systematic evolution from a Pacific Trough regime – associated on average\u0000with positive temperature anomalies and a near-complete absence of anomalously cold temperatures in North America – to an Alaskan Ridge regime,\u0000which favours low temperatures over much of the continent. The most striking feature of the stratospheric reflection events is thus a rapid,\u0000continental-scale decrease in temperatures. These emerge as continental-scale cold spells by the end of the reflection events. Stratospheric\u0000reflection events are thus highly relevant in a tropospheric predictability perspective.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"241 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124649689","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Impact of grid spacing, convective parameterization and cloud microphysics in ICON simulations of a warm conveyor belt","authors":"Anubhav Choudhary, A. Voigt","doi":"10.5194/wcd-3-1199-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1199-2022","url":null,"abstract":"Abstract. Warm conveyor belts are important features of extratropical cyclones and are characterized by active diabatic processes. Previous studies reported\u0000that simulations of extratropical cyclones can be strongly impacted by the horizontal grid spacing. Here, we study to what extent and in which\u0000manner simulations of warm conveyor belts are impacted by the grid spacing. To this end, we investigate the warm conveyor belt (WCB) of the North\u0000Atlantic cyclone Vladiana that occurred around 23 September 2016 and was observed as part of the North Atlantic Waveguide and Downstream Impact\u0000Experiment. We analyze a total of 18 limited-area simulations with the ICOsahedral Nonhydrostatic (ICON) model run over the North Atlantic that\u0000cover grid spacings from 80 to 2.5 km, including those of current coarse-resolution global climate models with parameterized convection, as\u0000well as those of future storm-resolving climate models with explicit convection. The simulations also test the sensitivity with respect to the\u0000representation of convection and cloud microphysics. As the grid spacing is decreased, the number of WCB trajectories increases systematically, WCB\u0000trajectories ascend faster and higher, and a new class of anticyclonic trajectories emerges that is absent at 80 km. We also diagnose the\u0000impact of grid spacing on the ascent velocity and vorticity of WCB air parcels and the diabatic heating that these parcels experience. Ascent\u0000velocity increases at all pressure levels by a factor of 3 between the 80 and 2.5 km simulations, and vorticity increases by a factor of 2\u0000in the lower and middle troposphere. We find a corresponding increase in diabatic heating as the grid spacing is decreased, arising mainly from\u0000cloud-associated phase changes in water. The treatment of convection has a much stronger impact than the treatment of cloud microphysics. When\u0000convection is resolved for grid spacings of 10, 5 and 2.5 km, the above changes to the WCB are amplified but become largely independent of\u0000the grid spacing. We find no clear connection across the different grid spacings between the strength of diabatic heating within the WCB and the\u0000deepening of cyclone Vladiana measured by its central pressure. An analysis of the pressure tendency equation shows that this is because diabatic\u0000heating plays a minor role in the deepening of Vladiana, which is dominated by temperature advection.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"409 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131989945","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Subseasonal precipitation forecasts of opportunity over central southwest Asia","authors":"Melissa Breeden, J. Albers, A. Hoell","doi":"10.5194/wcd-3-1183-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1183-2022","url":null,"abstract":"Abstract. Subseasonal forecasts of opportunity (SFOs) for precipitation over\u0000southwest Asia during January–March at lead times of 3–6 weeks are\u0000identified using elevated expected forecast skill from a linear inverse\u0000model (LIM), an empirical dynamical model that uses statistical\u0000relationships to infer the predictable dynamics of a system. The expected\u0000forecast skill from this LIM, which is based on the atmospheric circulation,\u0000tropical outgoing longwave radiation, and sea surface temperatures, captures\u0000the predictability associated with many relevant signals as opposed to just\u0000one. Two modes of variability, El Niño–Southern Oscillation (ENSO) and\u0000the Madden–Julian Oscillation (MJO), which themselves are predictable\u0000because of their slow variations, are related to southwest Asia\u0000precipitation SFOs. Strong El Niño events, as observed in 1983, 1998,\u0000and 2016, significantly increase the likelihood by up to 3-fold of an SFO 3–4 and 5–6 weeks in advance. Strong La Niña events, as observed in 1989, 1999, 2000, also significantly increase the likelihood of an SFO at those same lead times. High-amplitude MJO events in phases 2–4 and 6–8 of greater than one standardized departure also significantly increase the\u0000likelihood of an SFO 3–4 weeks in advance. Predictable atmospheric\u0000circulation patterns preceding anomalously wet periods indicate a role for\u0000enhanced tropical convection in the South Pacific convergence zone (SPCZ)\u0000region, while suppressed convection is observed preceding predictable dry\u0000periods. Anomalous heating in this region is found to distinguish wet and\u0000dry periods during both El Niño and La Niña conditions, although the atmospheric circulation response to the heating differs between each ENSO phase.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"47 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121991499","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Identification of high-wind features within extratropical cyclones using a probabilistic random forest – Part 1: Method and case studies","authors":"Leah Eisenstein, Benedikt Schulz, Ghulam A. Qadir, J. Pinto, P. Knippertz","doi":"10.5194/wcd-3-1157-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1157-2022","url":null,"abstract":"Abstract. Strong winds associated with extratropical cyclones are one of the most dangerous natural hazards in Europe.\u0000These high winds are mostly associated with five mesoscale dynamical features: the warm (conveyor belt) jet (WJ); the cold (conveyor belt) jet (CJ); cold frontal convection (CFC); strong cold-sector winds (CS); and, at least in some storms, the sting jet (SJ).\u0000The timing within the cyclone's life cycle, the location relative to the cyclone core and some further characteristics differ between these features and, hence, likely also the associated forecast errors.\u0000Here, we present a novel objective identification approach for these high-wind features using a probabilistic random forest (RF) based on each feature’s most important characteristics in near-surface wind, rainfall, pressure and temperature evolution. As the CJ and SJ are difficult to distinguish in near-surface observations alone, these two features are considered together here. A strength of the identification method is that it works flexibly and is independent of local characteristics and horizontal gradients; thus, it can be applied to irregularly spaced surface observations and to gridded analyses and forecasts of different resolution in a consistent way.\u0000As a reference for the RF, we subjectively identify the four storm features (WJ, CS, CFC, and CJ and SJ) in 12 winter storm cases between 2015 and 2020 in both hourly surface observations and high-resolution reanalyses of the German Consortium for Small-scale Modeling (COSMO) model over Europe, using an interactive data analysis and visualisation tool. The RF is then trained on station observations only. The RF learns physically consistent relations and reveals the mean sea level pressure (tendency), potential temperature, precipitation amount and wind direction to be most important for the distinction between the features.\u0000From the RF, we get probabilities of each feature occurring at the single stations, which can be interpolated into areal information using Kriging.\u0000The results show a reliable identification for all features, especially for the WJ and CFC. We find difficulties in the distinction of the CJ and CS in extreme cases, as the features have rather similar meteorological characteristics.\u0000Mostly consistent results in observations and reanalysis data suggest that the novel approach can be applied to other data sets without the need for adaptation.\u0000Our new software RAMEFI (RAndom-forest-based MEsoscale wind Feature Identification) is made publicly available for straightforward use by the atmospheric community and enables a wide range of applications, such as working towards a climatology of these features for multi-decadal time periods (see Part 2 of this paper; Eisenstein et al., 2022d), analysing forecast errors in high-resolution COSMO ensemble forecasts and developing feature-dependent post-processing procedures.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129501121","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Recurrent Rossby waves and south-eastern Australian heatwaves","authors":"S. M. Ali, M. Röthlisberger, Tess Parker, K. Kornhuber, O. Martius","doi":"10.5194/wcd-3-1139-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1139-2022","url":null,"abstract":"Abstract. In the Northern Hemisphere, recurrence of transient synoptic-scale Rossby\u0000wave packets in the same phase over periods of days to weeks, termed RRWPs,\u0000may repeatedly create similar surface weather conditions. This recurrence\u0000can lead to persistent surface anomalies. Here, we first demonstrate the\u0000significance of RRWPs for persistent hot spells in the Southern Hemisphere\u0000(SH) using the ERA-Interim (ERA-I) reanalysis dataset and then examine the role of RRWPs\u0000and blocks for heatwaves over south-eastern Australia (SEA). A Weibull regression analysis shows that RRWPs are statistically associated\u0000with a significant increase in the duration of hot spells over several\u0000regions in the SH, including SEA. Two case studies of heatwaves in SEA in\u0000the summers of 2004 and 2009 illustrate the role of RRWPs in forming\u0000recurrent ridges (anticyclonic potential vorticity – PV – anomalies), aiding in\u0000the persistence of the heatwaves. Then, using a weather-station-based\u0000dataset to identify SEA heatwaves, we find that SEA heatwaves are more\u0000frequent than climatology during days with extreme RRWPs activity over SEA\u0000(high RSEA). On days with both high RSEA and heatwaves, circumglobal zonal wavenumber 4 and 5 (WN4, WN5) anomaly patterns are present in the composite\u0000mean of the upper-level PV field, with an anticyclonic PV anomaly over SEA.\u0000The Fourier decomposition of the PV and meridional wind velocity fields\u0000further reveals that the WN4 and WN5 components in the suitable phase aids\u0000in forming the ridge over SEA for days with high RSEA. In addition, we\u0000find anomalous blocking over the Indian and the South Pacific oceans during\u0000SEA heatwaves, which may help to modulate the phase of RRWPs.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"105 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116476202","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data","authors":"Lukas Jansing, L. Papritz, B. Dürr, Daniel Gerstgrasser, M. Sprenger","doi":"10.5194/wcd-3-1113-2022","DOIUrl":"https://doi.org/10.5194/wcd-3-1113-2022","url":null,"abstract":"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 stat","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"28 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125629502","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}