C2SM Newsletter vol. 29

News from MeteoSwiss

ETH Zurich and MeteoSwiss are working with Appenzeller Bahnen on an Innosuisse project to improve the Laseyerwind warnings to prevent trains from overturning. The IAC group for atmospheric dynamics develops an algorithm to predict wind peaks of the Laseyerwind in the narrow Schwende valley more precisely from measurement and model data (COSMO-1). On this basis, MeteoSwiss’ department Development of Forecasting develops a probabilistic, customer-specific wind warning system to implement a risk-based planning of the train operations.

Representing the Urban Heat Island Effect in Future Climates
A recently completed master thesis (conducted as collaborative project between MeteoSwiss, ETH Zurich and the University of Augsburg) investigates the urban heat island (UHI) effect in future climates by analyzing temperature data at both rural and urban sites in Switzerland and the northern Alpine Foreland. For rural sites, climate scenario data are available through CH2018. Due to lacking temperature projections for urban sites, statistical and physically-based methods are tested to transfer the CH2018 scenarios to the respective urban counterparts. The urban-rural temperature difference, i.e. the UHI, in future climates is then analyzed in terms of standard climate indices such as tropical nights, summer days and hot days. Results of urban and rural climate scenarios clearly indicate a strong increase in the frequency of heat indices until the end of the 21st century. Consistent with what is known from literature, the scenarios simulate distinct urban-rural temperature differences (UHI effect) mainly during nighttime, which becomes obvious when considering the frequency of tropical nights based on the daily minimum temperature (see Figure 1 for Zurich).
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News from Empa

Selected press articles

• TagesAnzeiger: „external pageSo heiss wird es in Zürich in 40 Jahrencall_made“ (Article by P. Vögeli und M. Brupbacher based on the CH2018 climate scenarios, July 2019
  
• Schaffhauser Fernsehen: external pageHüt im Gschpröch mit A. Fischer zum neuen Klimaanpassungsbericht des Kt. Schaffhausencall_made, July 2019
  
• TagesAnzeiger: „external pageDie letzten drei Sommer waren alles Jahrhundertereignissecall_made“ (Article by P. Vögeli und M. Brupbacher with interview by S. Bader), September 2019

Paper: Effects of land use and anthropogenic aerosol emissions in the Roman Empire

Did the Romans have an impact on climate already 2000 years ago? Using the global aerosol-climate model ECHAM-HAM-SALSA, we estimated the possible anthropogenic impact of land use as well as aerosol emissions from burning activities (fuel consumption, crop residue burning, and pasture burning). For the simulations, two existing land use scenarios were used and three novel aerosol emission scenarios were created. The results suggest that both land use and aerosol emissions could have had an impact on climate. While land use induces a regional warming in one of the two scenarios, the anthropogenic aerosol emissions enhance the cooling effect of the clouds for all three scenarios. The magnitude of these changes in temperature is uncertain due to e.g. the prescribed sea surface temperatures.

Effects of land use and anthropogenic aerosol emissions in the Roman Empire by Gilgen, A., S. Wilkenskjeld, J.O. Kaplan, T. Kühn and U. Lohmann, Climate of the Past, 2019, in press, https://doi.org/10.5194/cp-15-1885-2019

Papers: Diabatic processes in extratropical cyclones simulated with COSMO and IFS

Three publications present detailed results about different aspects of moist dynamics in extratropical cyclones. The first study investigates the occurrence of embedded convection in warm conveyor belts, simulated with kilometre-scale COSMO simulations of a North Atlantic cyclone (Oertel et al. 2019). Episodes of fast ascent along warm conveyor belt trajectories calculated online during the COSMO integration reveal that convection occurs embedded within the larger-scale slantwise ascent, leading to prominent surface precipitation maxima and a heterogeneous cloud texture, in agreement with satellite observations. Two other studies focus on non-conservative processes that modify the potential vorticity (PV) in extratropical cyclones near the surface (Attinger et al. 2019) and near the tropopause (Spreitzer et al. 2019). IFS simulations are performed with detailed output of temperature and momentum tendencies for individual parameterisations, allowing for a detailed Lagrangian budget analysis of the formation of positive and negative PV anomalies. The results show that, in addition to latent heating by condensation, below-cloud processes (e.g., snow sublimation and rain evaporation) as well as turbulence contribute essentially to the formation of important PV features.

Attinger, R., E. Spreitzer, M. Boettcher, R. Forbes, H. Wernli, and H. Joos, 2019. Quantifying the role of individual diabatic processes for the formation of PV anomalies in a North Pacific cyclone. Quart. J. Roy. Meteorol. Soc., 145, 2454–2476. doi.org/10.1002/qj.3573

Oertel, A., M. Boettcher, H. Joos, M. Sprenger, H. Konow, M. Hagen, and H. Wernli, 2019. Convective activity in an extratropical cyclone and its warm conveyor belt – a case-study combining observations and a convection-permitting model simulation. Quart. J. Roy. Meteorol. Soc., 145, 1406–1426. doi.org/10.1002/qj.3500

Spreitzer, E., R. Attinger, M. Boettcher, R. Forbes, H. Wernli, and H. Joos, 2019. Modification of potential vorticity near the tropopause by nonconservative processes in the ECMWF model. J. Atmos. Sci., 76, 1709–1726. doi.org/10.1175/JAS-D-18-0295.1

Paper: Projections of Alpine Snow-Cover in a High-Resolution Climate Simulation

Besides allowing for an improved representation of precipitation and cloud cover, increased horizontal resolution in regional climate models also allows for a better representation of snow cover. The major cause for this improvement is the more accurate capture of topography, which influences quantities like snow depth and snow cover duration via the temperature lapse rate. In this study, we compare modelled snow cover for the Greater Alpine Region in a set of decadal-long COSMO simulations with different horizontal resolutions (50, 12 and 2.2 km). The performance of the simulations is evaluated with a gridded snow water equivalent (SWE) dataset for Switzerland. Only the 2.2 km simulation is able to capture elevations above 3000 m a.s.l., where substantial parts of the cryosphere (e.g. glaciers and permafrost) are located. The model evaluation indicates that the coarser-scale simulations (12 and 50 km) underestimate spatially averaged SWE throughout the winter, whereas the 2.2 km simulation reveals an excellent agreement with observations. Scenario simulations for the RCP8.5 and driven by the GCM MPI-ESM-LR reveal a distinctive decrease of SWE over the Alps. Highest relative changes in SWE are found at lower elevations (< 2000 m a.s.l.) but pronounced changes are also evident at high altitudes (> 3000 m a.s.l.), where SWE is projected to decrease between ~20-80% depending on the season and elevation.

Lüthi, S., Ban, N., Kotlarski, S., Steger, C. R., Jonas, T. and Schär, C., 2019: Projections of Alpine Snow-Cover in a High-Resolution Climate Simulation, Atmosphere, 10, 463, doi.org/10.3390/atmos10080463

Papers: The Mediterranean climate change hot spot and it’s causes

The Mediterranean region is a hot spot for future climate change, mainly due to the expected strong summer warming as well as the year-round precipitation decline. In our papers, we aim to identify the responsible physical drivers behind the regional temperature and precipitation changes. We find that the strong summer warming is due to large-scale changes in the vertical structure of the atmosphere (increases and spatial variations in lapse-rate), while – contrary to expectations – the northward shift and expansion of the Hadley cell has a negligible influence. The causes for the precipitation decline depend on the season. In summer, changes in thermodynamics, lapse-rates, and land-ocean temperature contrast are important and circulation changes play a secondary role. In winter, changes in circulation are the primary cause for the projected Mediterranean precipitation decline.

Brogli, R., Kröner, N., Sørland, S. L., Lüthi, D., & Schär, C. (2019). The Role of Hadley Circulation and Lapse-Rate Changes for the Future European Summer Climate. J. Climate, 32, 385–404. 10.1175/JCLI-D-18-0431.1

Brogli, R., Sørland, S. L., Kröner, N., & Schär, C. (2019). Causes of future Mediterranean precipitation decline depend on the season. Environ. Res. Lett., 14, 114017. 10.1088/1748-9326/ab4438

Paper: Stratospheric ozone trends for 1985-2018: sensitivity to recent large variability

Ozone in the stratosphere acts as a protective shield against ultraviolet radiation that may harm the biosphere, and leads to cataracts, skin damage, and skin cancer in humans. The ozone layer was damaged last century due to anthropogenic emissions of ozone depleting substances. A global effort, through the Montreal Protocol, halted emissions and the ozone layer is no longer clearly thinning. However, there is still no evidence of a statistically significant increase in total column ozone since 1998. In previous work, we showed that lower stratospheric ozone had continued to decline over 1998–2016, counter to modelling expectations, and is likely responsible for the non-detection of a recovery. While surprising, many questions remained as to the robustness of this result, related to natural atmospheric variability, confidence in the observations, and the efficacy of trend analysis approaches. In this paper, we extend the data series, and address many of the aforementioned concerns. Ultimately, we demonstrate that the lower stratospheric ozone trends remain in place and ozone remains lower in 2018 than in 1998. Further, we show that tropical stratospheric ozone (30° S–30° N) shows highly probable decreases in both the lower stratosphere and in the integrated stratospheric ozone layer. The paper reinforces the need to understand both the drivers and why chemistry climate models may not be reproducing these changes.