Heini Wernli

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Wernli

First Name

Heini

ORCID

0000-0001-9674-4837

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03854 - Wernli, Johann Heinrich / Wernli, Johann Heinrich

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Publications 1 - 10 of 228

The LAGRANTO Lagrangian analysis tool – version 2.0
Sprenger, Michael; Wernli, Heini (2015)
Geoscientific Model Development
Lagrangian trajectories are widely used in the atmospheric sciences, for instance to identify flow structures in extratropical cyclones (e.g., warm conveyor belts) and long-range transport pathways of moisture and trace substances. Here a new version of the Lagrangian analysis tool LAGRANTO (Wernli and Davies, 1997) is introduced, which offers considerably enhanced functionalities. Trajectory starting positions can be defined easily and flexibly based on different geometrical and/or meteorological conditions, e.g., equidistantly spaced within a prescribed region and on a stack of pressure (or isentropic) levels. After the computation of the trajectories, a versatile selection of trajectories is offered based on single or combined criteria. These criteria are passed to LAGRANTO with a simple command language (e.g., "GT:PV:2" readily translates into a selection of all trajectories with potential vorticity, PV, greater than 2 PVU; 1 PVU = 10−6 K m2 kg−1 s−1). Full versions of this new version of LAGRANTO are available for global ECMWF and regional COSMO data, and core functionality is provided for the regional WRF and MetUM models and the global 20th Century Reanalysis data set. The paper first presents the intuitive application of LAGRANTO for the identification of a warm conveyor belt in the North Atlantic. A further case study then shows how LAGRANTO can be used to quasi-operationally diagnose stratosphere–troposphere exchange events. Whereas these examples rely on the ECMWF version, the COSMO version and input fields with 7 km horizontal resolution serve to resolve the rather complex flow structure associated with orographic blocking due to the Alps, as shown in a third example. A final example illustrates the tool's application in source–receptor analysis studies. The new distribution of LAGRANTO is publicly available and includes auxiliary tools, e.g., to visualize trajectories. A detailed user guide describes all LAGRANTO capabilities.
Influences of sources and weather dynamics on atmospheric deposition of Se species and other trace elements
Breuninger, Esther S.; Tolu, Julie; Thurnherr, Iris; et al. (2024)
Atmospheric Chemistry and Physics
Atmospheric deposition is an important source of the micronutrient selenium for terrestrial ecosystems and food chains. However, the factors determining the total concentrations and chemical forms (speciation) of selenium in atmospheric deposition remain poorly understood. Here, aerosol samples were collected weekly over 5 years at Pic du Midi Observatory (French Pyrenees), alongside highly temporally resolved samples of aerosols, precipitation, and cloud water taken during a 2-month campaign. Firstly, measurements of selenium, other elements, and water isotopes were combined with sophisticated modelling approaches (aerosol-chemistry-climate SOCOL-AERv2 model and air parcel backward trajectories and Lagrangian moisture source analyses). Aerosol selenium measurements agreed well with SOCOL-AERv2-predicted values, and interestingly, higher fluxes of selenium and other elements were associated with deep convective activity during thunderstorms, highlighting the importance of local cloud dynamics in high deposition fluxes. Our results further indicate the coupling of element and water cycles from source to cloud formation, with decoupling during precipitation due to below-cloud scavenging. Secondly, selenium speciation was investigated in relation to sulfur speciation, organic composition, and moisture sources. While in the 5-year aerosol series, selenite (SeIV) was linked to anthropogenic source factors, in wet deposition it was related to pH and Atlantic moisture sources. We also report an organic selenium fraction, tracing it back to a marine biogenic source in both aerosols and wet deposition. With a comprehensive set of observations and model diagnostics, our study underscores the role of weather system dynamics alongside source contributions in explaining the atmospheric supply of trace elements to surface environments.
Weather discoveries. New aspects of how clouds and the atmospheric circulation interact are currently being discovered, combining theory, numerical experiments and aircraft measurements
Wernli, Heini (2015)
Pan European Networks: Science & Technology
Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model
De Vries, Andries-Jan; Aemisegger, Franziska; Pfahl, Stephan; et al. (2022)
Atmospheric Chemistry and Physics
Tropical ice clouds have an important influence on the Earth's radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales. These processes are not yet fully understood and lead to large uncertainties in climate projections. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform convection-permitting simulations with the regional Consortium for Small-Scale Modelling isotope-enabled (COSMOiso) model for the period from June to July 2016. First, we evaluate our model simulations using space-borne observations of mid-tropospheric water vapour isotopes, monthly station data of precipitation isotopes, and satellite-based precipitation estimates. Next, we explore the isotope signatures of tropical deep convection in atmospheric water vapour and ice based on a case study of a mesoscale convective system (MCS) and a statistical analysis of a 1-month period. The following five key processes related to tropical ice clouds can be distinguished based on isotope information: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdraughts that enriches the environmental vapour, and (5) the freezing of liquid water just above the 0 °C isotherm in convective updraughts. Importantly, we note large variations in the isotopic composition of water vapour in the upper troposphere and lower tropical tropopause layer, ranging from below −800 ‰ to over −400 ‰, which are strongly related to vertical motion and the moist processes that take place in convective updraughts and downdraughts. In convective updraughts, the vapour is depleted by the preferential condensation and deposition of heavy isotopes, whereas the non-fractionating sublimation of ice in convective downdraughts enriches the environmental vapour. An opposite vapour isotope signature emerges in thin-cirrus cloud regions where the direct transport of enriched (depleted) vapour prevails in large-scale ascent (descent). Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the West African monsoon water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.
Characteristics and dynamics of extreme winters in the Barents Sea in a changing climate
Hartmuth, Katharina; Wernli, Heini; Papritz, Lukas (2025)
Weather and Climate Dynamics
The Barents Sea is experiencing large declines in sea ice and increasing surface temperatures while at the same time it is a key region of weather variability in the Arctic. In this study, we identify extreme winter seasons in the Barents Sea, based on a multivariate method, as winters with large seasonal anomalies in one or several surface parameters encompassing surface temperature, precipitation, surface heat fluxes, and surface net radiation. The analyses are based on large-ensemble climate model data for historical (S2000) and end-of-century (S2100) projections following an RCP8.5 emission scenario. In the phase space of the considered seasonal-mean surface weather parameters, we find distinct clusters of extreme winters that are characterized by similar combinations of anomalies in these parameters. In particular, during extreme winters in S2000 simulations, anomalies in surface air temperature during extreme seasons tend to be spatially extended with their maximum amplitude over sea ice. This maximum shifts towards the continental land masses in a warmer climate (S2100), as the formation of intense warm or cold anomalies is damped by the increasing area of open ocean. Our results reveal that large anomalies in surface parameters during extreme seasons are characterized by distinct patterns of anomalous frequencies in cyclones, anticyclones, and cold air outbreaks because these weather systems are responsible for temperature and moisture advection, the formation or suppression of precipitation, and intense surface fluxes. We further show that anomalous surface boundary conditions at the beginning of a season - that is, sea ice concentration and sea surface temperatures - facilitate the formation of persistent anomalous surface conditions or further enhance atmospherically driven anomalies due to anomalous surface heat fluxes. However, a decrease in the variability of both sea ice and sea surface temperatures in S2100 indicates a decreasing importance of anomalous surface boundary conditions for the formation of future extreme winters in the Barents Sea, while the robust link shown for surface weather systems persists in a warmer climate.
Impact of North Atlantic evaporation hot spots on southern Alpine heavy precipitation events
Winschall, A.; Pfahl, S.; Sodemann, H.; et al. (2012)
Quarterly Journal of the Royal Meteorological Society
The key role of diabatic processes in modifying the upper-tropospheric wave guide
Grams, Christian M.; Wernli, Heini; Böttcher, Maxi; et al. (2011)
Quarterly Journal of the Royal Meteorological Society
How intense daily precipitation depends on temperature and the occurrence of specific weather systems – an investigation with ERA5 reanalyses in the extratropical Northern Hemisphere
Zschenderlein, Philipp; Wernli, Heini (2022)
Weather and Climate Dynamics
Precipitation and surface temperature are two of the most important variables that describe our weather and climate. Several previous studies investigated aspects of their relationship, for instance the climatological dependence of daily precipitation on daily mean temperature, P(T). However, the role of specific weather systems in shaping this relationship has not been analysed yet. This study therefore identifies the weather systems (WSs) that are associated with intense precipitation days as a function of T, focusing on the question of how this relationship, symbolically expressed as P(T, WS), varies regionally across the Northern Hemisphere and between seasons. To this end, we first quantify if intense precipitation occurs on climatologically warmer or on colder days. In winter, over most continental and ocean regions, intense precipitation falls on warmer days apart from the Mediterranean area and regions in the lee of the Rocky Mountains, where intense precipitation is favoured on colder days. In summer, only at high latitudes is intense precipitation favoured on warmer days, whereas continental areas experience intense precipitation on colder days. For selected regions in Europe and North America, we then identify the weather systems that occur preferentially on days with intense precipitation (referred to as wet days). In winter, cyclones are slightly dominant on colder wet days, whereas warm conveyor belts and atmospheric rivers occur preferentially on warmer wet days. In summer, the overall influence of atmospheric rivers increases, and the occurrence of weather systems depends less on wet day temperature. Wet days in the lee of the Rocky Mountains are influenced by most likely convective systems in anticyclones. Finally, we investigate P(T, WS) during the wettest and driest season in central Europe and the central United States (US). In qualitative agreement with the results from the first part of this study, the wettest winter is warmer than normal in central Europe but colder in the central US, and the wettest summer is colder in both regions. The opposite holds for the driest winter and summer, respectively. During these anomalous seasons, both the frequency and the precipitation efficiency of weather systems change in central Europe, while the wettest and driest seasons in central US mainly arise from a modified precipitation efficiency. Our results show that the precipitation–temperature–weather system relationship strongly depends on the region and that (extreme) seasonal precipitation is influenced by the frequency and precipitation efficiency of the different weather systems. This regional variability is reflected in the relative importance of weather system frequency and efficiency anomalies for the formation of anomalously wet and dry seasons. Interestingly, in some regions and seasons, the precipitation efficiency of weather systems is increased during anomalously cold seasons.
THORPEX Research and the Science of Prediction
Parsons, D. B.; Beland, M.; Burridge, D.; et al. (2017)
Bulletin of the American Meteorological Society
Detection, tracking and event localization of jet stream features in 4-D atmospheric data
Limbach, Sebastian; Schömer, Elmar; Wernli, Heini (2012)
Geoscientific Model Development
We introduce a novel algorithm for the efficient detection and tracking of features in spatiotemporal atmospheric data, as well as for the precise localization of the occurring genesis, lysis, merging and splitting events. The algorithm works on data given on a four-dimensional structured grid. Feature selection and clustering are based on adjustable local and global criteria, feature tracking is predominantly based on spatial overlaps of the feature's full volumes. The resulting 3-D features and the identified correspondences between features of consecutive time steps are represented as the nodes and edges of a directed acyclic graph, the event graph. Merging and splitting events appear in the event graph as nodes with multiple incoming or outgoing edges, respectively. The precise localization of the splitting events is based on a search for all grid points inside the initial 3-D feature that have a similar distance to two successive 3-D features of the next time step. The merging event is localized analogously, operating backward in time. As a first application of our method we present a climatology of upper-tropospheric jet streams and their events, based on four-dimensional wind speed data from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. We compare our results with a climatology from a previous study, investigate the statistical distribution of the merging and splitting events, and illustrate the meteorological significance of the jet splitting events with a case study. A brief outlook is given on additional potential applications of the 4-D data segmentation technique.

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Heini Wernli

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