Future summer warming pattern under climate change is affected by lapse-rate changes

Greenhouse-gas-driven global temperature change projections exhibit spatial variations, meaning that certain land areas will experience substantially enhanced or reduced surface warming. It is vital to understand enhanced regional warming anomalies as they locally increase heat-related risks to human health and ecosystems. We argue that tropospheric lapse-rate changes play a key role in shaping the future summer warming pattern around the globe in mid-latitudes and the tropics. We present multiple lines of evidence supporting this finding based on idealized simulations over Europe, as well as regional and global climate model ensembles. All simulations consistently show that the vertical distribution of tropospheric summer warming is different in regions characterized by enhanced or reduced surface warming. Enhanced warming is projected where lapse-rate changes are small, implying that the surface and the upper troposphere experience similar warming. On the other hand, strong lapse-rate changes cause a concentration of warming in the upper troposphere and reduced warming near the surface. The varying magnitude of lapse-rate changes is governed by the temperature dependence of the moist-adiabatic lapse rate and the available tropospheric humidity. We conclude that tropospheric temperature changes should be considered along with surface processes when assessing the causes of surface warming patterns.publishedVersio

The pseudo-global-warming (PGW) approach: Methodology, software package PGW4ERA5 v1.1, validation, and sensitivity analyses

The term “pseudo-global warming” (PGW) refers to a simulation strategy in regional climate modeling. The strategy consists of directly imposing large-scale changes in the climate system on a control regional climate simulation (usually representing current conditions) by modifying the boundary conditions. This differs from the traditional dynamic downscaling technique where output from a global climate model (GCM) is used to drive regional climate models (RCMs). The PGW climate changes are usually derived from a transient global climate model (GCM) simulation. The PGW approach offers several benefits, such as lowering computational requirements, flexibility in the simulation design, and avoiding biases from global climate models. However, implementing a PGW simulation is non-trivial, and care must be taken not to deteriorate the physics of the regional climate model when modifying the boundary conditions. To simplify the preparation of PGW simulations, we present a detailed description of the methodology and provide the companion software PGW4ERA5 facilitating the preparation of PGW simulations. In describing the methodology, particular attention is devoted to the adjustment of the pressure and geopotential fields. Such an adjustment is required when ensuring consistency between thermodynamical (temperature and humidity) changes on the one hand and dynamical changes on the other hand. It is demonstrated that this adjustment is important in the extratropics and highly essential in tropical and subtropical regions. We show that climate projections of PGW simulations prepared using the presented methodology are closely comparable to traditional dynamic downscaling for most climatological variables.publishedVersio

Future intensification of precipitation and wind gust associated thunderstorms over Lake Victoria

Severe thunderstorms affect more than 30 million people living along the shores of Lake Victoria (East Africa). Thousands of fishers lose their lives on the lake every year. While deadly waves are assumed to be initiated by severe wind gusts, knowledge about thunderstorms is restricted to precipitation or environmental proxies. Here we use a regional climate model run at convection-permitting resolution to simulate both precipitation and wind gusts over Lake Victoria for a historical 10-year period. In addition, a pseudo global warming simulation provides insight into the region’s future climate. In this simulation, ERA5’s initial and boundary conditions are perturbed with atmospheric changes between 1995–2025 and 2070–2100, projected by CMIP6’s ensemble mean. It was found that future decreases in both mean precipitation and wind gusts over Lake Victoria can be attributed to a weaker mean mesoscale circulation that reduces the trigger for over-lake nighttime convection and decreases the mean wind shear. However, an intensification of extremes is projected for both over-lake precipitation and wind gusts. The observed 7 %K−1 Clausius–Clapeyron extreme precipitation scaling is ascribed to increased water vapor content and a compensation of weaker mesoscale circulations and stronger thunderstorm dynamics. More frequent wind gust extremes result from higher wind shear conditions and more compound thunderstorms with both intense rainfall and severe wind gusts. Overall, our study emphasizes Lake Victoria’s modulating role in determining regional current and future extremes, in addition to changes expected from the Clausius–Clapeyron relation

COSMO-CLM regional climate simulations in the Coordinated Regional Climate Downscaling Experiment (CORDEX) framework: a review

In the last decade, the Climate Limited-area Modeling Community (CLM-Community) has contributed to the Coordinated Regional Climate Downscaling Experiment (CORDEX) with an extensive set of regional climate simulations. Using several versions of the COSMO-CLM-Community model, ERA-Interim reanalysis and eight global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) were dynamically downscaled with horizontal grid spacings of 0.44∘ (∼ 50 km), 0.22∘ (∼ 25 km), and 0.11∘ (∼ 12 km) over the CORDEX domains Europe, South Asia, East Asia, Australasia, and Africa. This major effort resulted in 80 regional climate simulations publicly available through the Earth System Grid Federation (ESGF) web portals for use in impact studies and climate scenario assessments. Here we review the production of these simulations and assess their results in terms of mean near-surface temperature and precipitation to aid the future design of the COSMO-CLM model simulations. It is found that a domain-specific parameter tuning is beneficial, while increasing horizontal model resolution (from 50 to 25 or 12 km grid spacing) alone does not always improve the performance of the simulation. Moreover, the COSMO-CLM performance depends on the driving data. This is generally more important than the dependence on horizontal resolution, model version, and configuration. Our results emphasize the importance of performing regional climate projections in a coordinated way, where guidance from both the global (GCM) and regional (RCM) climate modeling communities is needed to increase the reliability of the GCM–RCM modeling chain

Disentangling Drivers and Patterns of European Climate Change

Combined observations of the earth’s atmosphere, oceans, and land surface show a clear change in the global climate system over the past 100 years. The planet’s continuously rising temperature, which is shown by these observations, is predominantly driven by persistent human greenhouse gas emissions. The global warming trend is expected to continue throughout the 21st century as long as greenhouse gases accumulate in the atmosphere. Increased global temperatures threaten humans, life, and ecosystems on the planet by potentially triggering illnesses such as heat strokes, food insecurity, biodiversity loss, and natural hazards. In regions where precipitation is decreasing together with increasing temperature, temperature-related hazards are amplified and additional hazards connected to water scarcity arise. Model simulations of the future climate show substantial regional variations in both temperature and precipitation changes. It is critical to gain further confidence in such projected spatial climate change patterns, especially for vulnerable regions, where there is a risk of amplified warming or strong changes in precipitation. To gain further confidence, there is a need to understand which changes in the climate system cause these spatial patterns. Scientific confidence in the projected patterns is essential for developing effective regional strategies to mitigate and adapt to climate change. The goal of this thesis is to determine the causes of spatial patterns of future temperature and precipitation changes in climate simulations. The focus is on prominent patterns of climate change in Europe, namely on the projected amplified summer warming pattern in the Mediterranean and the projected year-round precipitation decline in the same region. We find that the amplified Mediterranean summer warming is predominantly caused by spatially different lapse-rate changes. Lapse-rate changes are substantial throughout the continent but weakest in the Mediterranean. The influence of Hadley circulation changes on the European summer climate is found to be negligible. The Mediterranean summer precipitation decline can largely be explained by thermodynamic changes including warming contrasts between land and ocean, lapse-rate changes, and changes in atmospheric humidity. Circulation changes further contribute to the summer precipitation decline but are of secondary importance. The situation in winter is different, where circulation changes and connected changes in the atmospheric state are the main cause for the precipitation decline in the Mediterranean. Our results also indicate that lapse-rate changes are not exclusively important for the pattern of summer land temperature changes in Europe but also in many other mid-latitude and tropical regions on the northern and southern hemispheres during summer. The connection of lapse-rate changes to amplified summer warming patterns increases the confidence in the regional projections since lapse-rate changes are a consequence of the well-understood higher moisture-holding capacity of warmer air. The confidence in the Mediterranean precipitation decline should be considered larger in summer than in winter since the influence of uncertain circulation changes is smaller in summer compared to winter

Rotor formation in the Inn valley : a modeling study

The aim of this thesis is to determine which atmospheric conditions induce rotor formation in the surroundings of the city of Innsbruck, located in the Alpine Inn Valley, Austria. For this purpose, the impact of several idealized atmospheric conditions on the realistic terrain surrounding Innsbruck was investigated using numerical simulations. The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) was used. The employed atmospheric conditions are favorable for southerly foehn and are derived from upstream radiosonde measurements. The presence of a rotor was diagnosed via characteristics of the wind field and a turbulence threshold. The simulation results suggest, that southerly foehn in the Inn Valley can be accompanied by rotors under the condition that an inversion above the Alpine crest is present. The large scale wind direction can influence the probability for rotor formation. No rotors form if the upper level wind is south-westerly, since the mountain wave forcing is reduced. In contrast, south-easterly upper level wind is favorable for rotor formation, due to significant mountain wave activity. In the presence of an inversion, the strongest rotors develop in connection with trapped lee waves, developing due to strong vertical wind shear. Rotors developing in the surroundings of Innsbruck are spatially variable. The simulations show, that rotors are more likely to develop in the lee of peaks surrounding Innsbruck, where the mountain wave forcing is stronger, than over the city. High amplitude trapped lee waves leading to rotors over two kilometers in height, connected with extreme turbulence, can form close to Innsbruck airport.by Roman BrogliUniversity of Innsbruck, Masterarbeit, 2016Innsbruck, Univ., Masterarb., 2016(VLID)97384

Causes of future Mediterranean precipitation decline depend on the season

Future mean precipitation in the Mediterranean is projected to decrease year-round in response to global warming, threatening to aggravate water stress in the region, which can cause social and economic difficulties. We investigate possible causes of the Mediterranean drying in regional climate simulations. To test the influence of multiple large-scale drivers on the drying, we sequentially add them to the simulations. We find that the causes of the Mediterranean drying depend on the season. The summer drying results from the land-ocean warming contrast, and from lapse-rate and other thermodynamic changes, but only weakly depends on circulation changes. In contrast, to reproduce the simulated Mediterranean winter drying, additional changes in the circulation and atmospheric state have to be represented in the simulations. Since land-ocean contrast, thermodynamic and lapse-rate changes are more robust in climate simulations than circulation changes, the uncertainty associated with the projected drying should be considered smaller in summer than in winter.ISSN:1748-9326ISSN:1748-931

Future summer warming pattern under climate change is affected by lapse-rate changes

Greenhouse-gas-driven global temperature change projections exhibit spatial variations, meaning that certain land areas will experience substantially enhanced or reduced surface warming. It is vital to understand enhanced regional warming anomalies as they locally increase heat-related risks to human health and ecosystems. We argue that tropospheric lapse-rate changes play a key role in shaping the future summer warming pattern around the globe in mid-latitudes and the tropics. We present multiple lines of evidence supporting this finding based on idealized simulations over Europe, as well as regional and global climate model ensembles. All simulations consistently show that the vertical distribution of tropospheric summer warming is different in regions characterized by enhanced or reduced surface warming. Enhanced warming is projected where lapse-rate changes are small, implying that the surface and the upper troposphere experience similar warming. On the other hand, strong lapse-rate changes cause a concentration of warming in the upper troposphere and reduced warming near the surface. The varying magnitude of lapse-rate changes is governed by the temperature dependence of the moist-adiabatic lapse rate and the available tropospheric humidity. We conclude that tropospheric temperature changes should be considered along with surface processes when assessing the causes of surface warming patterns.ISSN:2698-4016ISSN:2698-400

Causes of future Mediterranean precipitation decline depend on the season

Future mean precipitation in the Mediterranean is projected to decrease year-round in response to global warming, threatening to aggravate water stress in the region, which can cause social and economic difficulties. We investigate possible causes of the Mediterranean drying in regional climate simulations. To test the influence of multiple large-scale drivers on the drying, we sequentially add them to the simulations. We find that the causes of the Mediterranean drying depend on the season. The summer drying results from the land-ocean warming contrast, and from lapse-rate and other thermodynamic changes, but only weakly depends on circulation changes. In contrast, to reproduce the simulated Mediterranean winter drying, additional changes in the circulation and atmospheric state have to be represented in the simulations. Since land-ocean contrast, thermodynamic and lapse-rate changes are more robust in climate simulations than circulation changes, the uncertainty associated with the projected drying should be considered smaller in summer than in winter

The pseudo-global-warming (PGW) approach: Methodology, software package PGW4ERA5 v1.1, validation, and sensitivity analyses

The term "pseudo-global warming"(PGW) refers to a simulation strategy in regional climate modeling. The strategy consists of directly imposing large-scale changes in the climate system on a control regional climate simulation (usually representing current conditions) by modifying the boundary conditions. This differs from the traditional dynamic downscaling technique where output from a global climate model (GCM) is used to drive regional climate models (RCMs). The PGW climate changes are usually derived from a transient global climate model (GCM) simulation. The PGW approach offers several benefits, such as lowering computational requirements, flexibility in the simulation design, and avoiding biases from global climate models. However, implementing a PGW simulation is non-trivial, and care must be taken not to deteriorate the physics of the regional climate model when modifying the boundary conditions. To simplify the preparation of PGW simulations, we present a detailed description of the methodology and provide the companion software PGW4ERA5 facilitating the preparation of PGW simulations. In describing the methodology, particular attention is devoted to the adjustment of the pressure and geopotential fields. Such an adjustment is required when ensuring consistency between thermodynamical (temperature and humidity) changes on the one hand and dynamical changes on the other hand. It is demonstrated that this adjustment is important in the extratropics and highly essential in tropical and subtropical regions. We show that climate projections of PGW simulations prepared using the presented methodology are closely comparable to traditional dynamic downscaling for most climatological variables.ISSN:1991-9603ISSN:1991-959