Kathrin Wehrli

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Wehrli

First Name

Kathrin

ORCID

0000-0001-7692-6676

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Publications 1 - 8 of 8

Storylines of the 2018 Northern Hemisphere heatwave at pre-industrial and higher global warming levels
Wehrli, Kathrin; Hauser, Mathias; Seneviratne, Sonia I. (2020)
Earth System Dynamics
Extreme temperatures were experienced over a large part of the Northern Hemisphere during the 2018 boreal summer (hereafter referred to as “NH2018 event”), leading to major impacts on agriculture and society in the affected countries. Previous studies highlighted both the anomalous atmospheric circulation patterns during the event and the background warming due to human greenhouse gas emissions as main drivers of the event. In this study, we present Earth system model experiments investigating different storylines of the NH2018 event given the same atmospheric circulation and alternative background global warming for no human imprint, the 2018 conditions, and different mean global warming levels 1.5, 2, 3 and 4 ∘C. The results reveal that the human-induced background warming was a strong contributor to the intensity of the NH2018 event, and that resulting extremes under similar atmospheric circulation conditions at higher levels of global warming would reach dangerous levels. Compared to 9 % during the NH2018 event, about 13 % (34 %) of the inhabited or agricultural area in the investigated region would reach daily maximum temperatures over 40 ∘C under 2 ∘C (4 ∘C) of global warming and similar atmospheric circulation conditions.
The ExtremeX global climate model experiment: investigating thermodynamic and dynamic processes contributing to weather and climate extremes
Wehrli, Kathrin; Luo, Fei; Hauser, Mathias; et al. (2022)
Earth System Dynamics
The mechanisms leading to the occurrence of extreme weather and climate events are varied and complex. They generally encompass a combination of dynamic and thermodynamic processes, as well as drivers external to the climate system, such as anthropogenic greenhouse gas emissions and land use change. Here we present the ExtremeX multi-model intercomparison experiment, which was designed to investigate the contribution of dynamic and thermodynamic processes to recent weather and climate extremes. The numerical experiments are performed with three Earth system models: CESM, MIROC, and EC-Earth. They include control experiments with interactive atmosphere and land surface conditions, as well as experiments wherein the atmospheric circulation, soil moisture, or both are constrained using observation-based data. The temporal evolution and magnitude of temperature anomalies during heatwaves are well represented in the experiments with a constrained atmosphere. However, the magnitude of mean climatological biases in temperature and precipitation are not greatly reduced in any of the constrained experiments due to persistent or newly introduced biases. This highlights the importance of error compensations and tuning in the standard model versions. To show one possible application, ExtremeX is used to identify the main drivers of heatwaves and warm spells. The results reveal that both atmospheric circulation patterns and soil moisture conditions substantially contribute to the occurrence of these events. Soil moisture effects are particularly important in the tropics, the monsoon areas, and the Great Plains of the United States, whereas atmospheric circulation effects are major drivers in other midlatitude and high-latitude regions.
The Swiss Alpine zero degree line: Methods, past evolution and sensitivities
Scherrer, Simon C.; Gubler, Stefanie; Wehrli, Kathrin; et al. (2021)
International Journal of Climatology
The near-surface zero degree line (ZDL) is a key isotherm in mountain regions worldwide, but a detailed analysis of methods for the ZDL determination, their properties and applicability in a changing climate is missing. We here test different approaches to determine the near-surface ZDL on a monthly scale in the Swiss Alps. A non-linear profile yields more robust and more realistic ZDLs than a linear profile throughout the year and especially in the winter-half year when frequent inversions disqualify a linear assumption. In the period 1871-2019, the Swiss ZDL has risen significantly in every calendar month: In northern Switzerland, the monthly ZDL increases generally amount to 300-400 m with smaller values in April and September (200-250 m) and a larger value in October (almost 500 m). The largest increases of 600-700 m but also very large uncertainties (+/- 400 m, 95% confidence interval) are found in December and January. The increases have accelerated in the last decades, especially in spring and summer. The ZDL is currently increasing by about 160 in the summer-half year and by up to 340 +/- 45 m center dot degrees C-1 in winter months m per degrees C warming. In southern Switzerland, ZDL trends and temperature scalings are somewhat smaller, especially in winter. Sensitivity analyses using a simple shift of the non-linear temperature profile suggest that the winter ZDL-temperature scalings are at a record high today or will reach it in the near future, and are expected to decrease with a strong future warming. Nevertheless, the cumulative ZDL increase for strong warming is considerably larger in winter than in summer. Based on a few key criteria, we also present best practises to determine the ZDL in mountain regions worldwide. The outlined methods lay a foundation for the analysis of further isotherms and to study the future ZDL evolution based on climate scenario data. © John Wiley & Sons Ltd
Local and Remote Atmospheric Responses to Soil Moisture Anomalies in Australia
Martius, Olivia; Wehrli, Kathrin; Rohrer, Marco (2021)
Journal of Climate
Three sets of model experiments are performed with the Community Earth System Model to study the role of soil moisture anomalies as a boundary forcing for the formation of upper-level Rossby wave patterns during the Southern Hemisphere summer. In the experiments, soil moisture over Australia is set to 61 standard deviation (STD) of an ERA-Interim-derived soil moisture reconstruction for the years 2009–16 and 50 ensemble members are run. The local response is a positive heating anomaly in the dry simulations that results in a thermal low–like circulation anomaly with an anomalous surface low and upper-level anticyclone. Significant differences in convective rainfall over Australia are related to differences in convective instability and associated with changes in near-surface moisture and moisture advection patterns. A circum-hemispheric flow response is identified both in the upper-level flow and in the surface storm tracks that overall resembles a positive southern annular mode–like flow anomaly in the dry simulations. The structure of this atmospheric response strongly depends on the background flow. The results point to a modulation of the hemispheric flow response to the forcing over Australia by El Niño–Southern Oscillation. Significant changes of precipitation over the Maritime Continent and South Africa are found and significant differences in the frequency of surface cyclones are present all along the storm tracks.
Identifying Key Driving Processes of Major Recent Heat Waves
Wehrli, Kathrin; Guillod, Benoit P.; Hauser, Mathias; et al. (2019)
Journal of Geophysical Research: Atmospheres
Heat waves lead to major impacts on human health, food production, and ecosystems. To assess their predictability and how they are projected to change under global warming, it is crucial to improve our understanding of the underlying processes affecting their occurrence and intensity under present‐day climate conditions. Beside greenhouse gas forcing, processes in the different components of the climate system—in particular the land surface, atmospheric circulation, and the oceans—may play a key role in changing the odds for a particular event. This study aims to identify the role of the individual drivers for five heat waves (and, in some cases, of concurrent droughts) in the recent decade. Simulations are performed with the Community Earth System Model using nudging of horizontal atmospheric circulation and prescription of soil moisture. The fully constrained model accurately reproduces how anomalous an event was. Factorial experiments, which force the model toward observations for one or several key components at a time, allow us to identify how much of the observed temperature anomaly of each event can be attributed to each driver. Considering all analyzed events, atmospheric circulation and soil moisture play similarly important roles, each contributing between 20% and 70% to the events' anomalies. This highlights that the role of thermodynamics can be just as important as that of the dynamics for temperature extremes, a possibly underestimated feature. In addition, recent climate change amplified the events and contributed between 10% and 40% of the events' anomalies.
Summertime Rossby waves in climate models: Substantial biases in surface imprint associated with small biases in upper-level circulation
Luo, Fei; Selten, Frank; Wehrli, Kathrin; et al. (2022)
Weather and Climate Dynamics
In boreal summer, circumglobal Rossby waves can promote stagnating weather systems that favor extreme events like heat waves or droughts. Recent work showed that amplified Rossby wavenumber 5 and 7 show phase-locking behavior which can trigger simultaneous warm anomalies in different breadbasket regions in the Northern Hemisphere. These types of wave patterns thus pose a potential threat to human health and ecosystems. The representation of such persistent wave events in summer and their surface anomalies in general circulation models (GCMs) has not been systematically analyzed. Here we validate the representation of wavenumbers 1–10 in three state-of-the-art global climate models (EC-Earth, CESM, and MIROC), quantify their biases, and provide insights into the underlying physical reasons for the biases. To do so, the ExtremeX experiments output data were used, consisting of (1) historic simulations with a freely running atmosphere with prescribed ocean and experiments that additionally (2) nudge towards the observed upper-level horizontal winds, (3) prescribe soil moisture conditions, or (4) do both. The experiments are used to trace the sources of the model biases to either the large-scale atmospheric circulation or surface feedback processes. Focusing on wave 5 and wave 7, we show that while the wave's position and magnitude are generally well represented during high-amplitude (> 1.5 SD) episodes, the associated surface anomalies are substantially underestimated. Near-surface temperature, precipitation and mean sea level pressure are typically underestimated by a factor of 1.5 in terms of normalized standard deviations. The correlations and normalized standard deviations for surface anomalies do not improve if the soil moisture is prescribed. However, the surface biases are almost entirely removed when the upper-level atmospheric circulation is nudged. When both prescribing soil moisture and nudging the upper-level atmosphere, then the surface biases remain quite similar to the experiment with a nudged atmosphere only. We conclude that the near-surface biases in temperature and precipitation are in the first place related to biases in the upper-level circulation. Thus, relatively small biases in the models' representation of the upper-level waves can strongly affect associated temperature and precipitation anomalies.
Building design in a changing climate – Future Swiss reference years for building simulations
Wehrli, Kathrin; Sidler, Franz; Gubler, Stefanie; et al. (2024)
Climate Services
With global climate change, temperatures in Switzerland are projected to rise in the coming decades, according to the national climate scenarios CH2018. Associated with the mean temperature increase, heatwaves are expected to become longer, more frequent, and more intense. The changing climate will affect the indoor climate as well as heating and cooling needs. In building design, these climatic changes have to be planned for today in order to ensure a comfortable indoor climate in the future. In collaboration with practitioners, a reference climate data set for the future is created that specifically targets building designers and engineers. The data set consists of hourly weather data of one-year length based on the Swiss climate change scenarios CH2018. These future reference years are representative of two time periods in the future: one around 2030 and one around 2060. Climate change uncertainty is considered by using two emission scenarios (RCP2.6 and RCP8.5). Reference data for the future is provided not only for a typical year (called Design Reference Year, or DRY) but also for an above-average warm summer. The data is available at the sites of 45 measurement stations across Switzerland, including four stations inside major cities to take the urban heat island effect into account. The generated climate data set is applied to a building model to provide an application example. The results point out that the cooling needs will substantially increase, which is why an adaptation of the building design to the changing climate is vital.
On the sensitivity of heat waves to physical drivers and climate change
Wehrli, Kathrin (2020)
Extreme events have always been a threat to societies and ecosystems but they are natural to the climate system as a result of its internal variability. A specific extreme event is caused by the unique interplay of a number of different driving factors. The atmosphere, the ocean and the land surface are major drivers of climate variability and participate in feedback processes that may amplify or reduce the risk of an extreme event. As all of these natural drivers work together to create a specific extreme event, it is difficult to disentangle the individual contributions. Furthermore, by burning fossil fuels and converting land, humans have started to affect the climate system. At present, this has led to a global warming of about 1°C above pre-industrial levels. This change in the background global climate is altering natural processes that can be relevant for extreme events. However, the impact of this climatic change is particularly difficult to detect for extreme events as they are rare. Nonetheless, given the devastating impacts of extreme events, it is essential to study them to provide a basis for mitigation and adaptation planning. Understanding the underlying processes and involved uncertainties helps to provide reliable short-term forecasts for extreme events and projections for the future. This thesis focuses on heat waves and, in some cases, concurrent droughts. A main goal is to disentangle the role of the drivers involved. Global climate model simulations are conducted to separate the contributions from thermodynamic and dynamic processes by forcing parts of the climate system toward observations. Atmospheric circulation constitutes the dynamic part of the climate system and can be controlled by constraining its variability using atmospheric nudging. The thermodynamics can be controlled by constraining for example the land surface conditions, which is done here using soil moisture prescription. In the first part of this thesis, an evaluation of climatological model biases is provided. Global climate models often show consistent biases in the surface climate, for example a hot and dry bias in the Northern Hemisphere midlatitudes during summer, which affects the representation of hot extremes. To identify the origin of such biases, atmospheric nudging is used to control the large-scale atmospheric circulation in a global climate model. The results show that biases related to the atmospheric circulation are often of minor relevance for many regions of the world. This highlights that a large part of the biases is not related to circulation, but originates from incomplete or erroneous representation of the thermodynamics. The focus shifts to heat waves in the second part of the thesis. The driving processes that contributed to five heat waves occurring between 2010-2016 are studied individually. By nudging the atmosphere toward observed circulation and prescribing soil moisture, the role of three key physical drivers is quantified: (1) atmospheric circulation, (2) land surface conditions, and (3) sea surface temperatures. In addition, the role of recent climate change since the mid-1990s is evaluated. The contribution of the natural and anthropogenic drivers to daily maximum temperatures can be assessed quantitatively using conditional event attribution. The results show that the ocean conditions play a minor role for most of the investigated events, while the atmospheric circulation and land surface can each contribute up to 70% of the events' anomalies. Recent climate change amplified all of the investigated events. It played the largest role for the 2015 European heat wave, contributing about 40% to the temperature anomaly. The third part of this thesis combines climate change scenario information with atmospheric nudging to create storylines for a recent heat wave. Storylines are a tool to create analogues of an event for alternative levels of global background warming. Here these analogues are created by nudging the large-scale atmospheric circulation toward the same observed circulation for each storyline. The method is applied to the 2018 Northern Hemisphere heat wave, which affected a large fraction of populated and agriculturally used land. In an alternative world without global warming, the impact of this event would have been strongly reduced. Looking into a possible future with a higher level of global warming the event puts an increasing fraction of land and population at risk: the area experiencing extreme temperatures increases from 9% in 2018 to 13% (34%) at 2°C (4°C) global warming. This thesis contributes to the process understanding of heat waves in present-day and possible alternative future climates. The results demonstrate that thermodynamic processes related to the land surface can be as important as dynamic processes for driving heat waves. This highlights the value of assessing the separate contributions of these processes and points to the need for a joint effort in improving their representation in global climate models. The results further reveal an influence of recent climate change on aggravating heat waves and provide an outlook to future events. The presented scientific methods and concepts were proven useful in this thesis and will help to shed light on the role of thermodynamics and atmospheric dynamics in future studies on heat waves and other extreme events.

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