The ERA5 global reanalysis

Changes in Atmospheric Constituents and in Radiative Forcing

Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

https://dspace.stir.ac.uk/bitstream/1893/30981/1/intcal20_northern_hemisphere_radiocarbon_age_calibration_curve_055_cal_kbp.pdf

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 cal kBP)

This chapter assesses long-term projections of climate change for the end of the 21st century and beyond, where the forced signal depends on the scenario and is typically larger than the internal variability of the climate system. Changes are expressed with respect to a baseline period of 1986-2005, unless otherwise stated.

Chapter 12 - Long-term climate change: Projections, commitments and irreversibility

Hydrogen technologies have experienced cycles of excessive expectations followed by disillusion. Nonetheless, a growing body of evidence suggests these technologies form an attractive option for the deep decarbonisation of global energy systems, and that recent improvements in their cost and performance point towards economic viability as well. This paper is a comprehensive review of the potential role that hydrogen could play in the provision of electricity, heat, industry, transport and energy storage in a low-carbon energy system, and an assessment of the status of hydrogen in being able to fulfil that potential. The picture that emerges is one of qualified promise: hydrogen is well established in certain niches such as forklift trucks, while mainstream applications are now forthcoming. Hydrogen vehicles are available commercially in several countries, and 225 000 fuel cell home heating systems have been sold. This represents a step change from the situation of only five years ago. This review shows that challenges around cost and performance remain, and considerable improvements are still required for hydrogen to become truly competitive. But such competitiveness in the medium-term future no longer seems an unrealistic prospect, which fully justifies the growing interest and policy support for these technologies around the world.

https://pubs.rsc.org/en/content/articlepdf/2019/ee/c8ee01157e

The role of hydrogen and fuel cells in the global energy system

Examining continental and marine sources of selenium in rainfall

Abstract. Recent forest decline in Europe is strongly influenced by meteorological conditions imposed by seasonal variations of temperature, T2m, and precipitation, P, and can be monitored with forest greenness. This study quantitatively investigates anomalous characteristics of the three-year meteorological storyline preceding events of reduced forest greenness in Europe's temperate and Mediterranean biome in the phase space of seasonal-mean anomalies of (T2m, P). A specific focus is on the amplitude, persistence, and co-variability of these anomalies. A pragmatic approach based on remote sensing observations of normalized difference vegetation index NDVI serves to identify low forest NDVI events at the 50 km scale in Europe in June to August 2000–2020. An independent forest disturbance data set is used to qualitatively validate the identified more than 1'500 low NDVI events. These events occur in summers with particularly dry and hot conditions but their meteorological storylines feature significant anomalies during multiple seasons preceding the events, with clear differences between the two biomes. In the Mediterranean biome, the anomalously dry conditions persist over more than 1.5 y prior to the events, whereas T2m is anomalously warm only during the last 0.4 y. In contrast, in the temperate biome, T2m is anomalously large during most of the 2.5 y prior to the events and, most interestingly, the autumn/winter preceding the events is characterized by anomalously wet and warm conditions. These anomalies potentially induce a negative legacy on the following summer drought. The seasonal-mean anomalies of P are strongly determined by synoptic-scale weather systems, such that long dry periods are characterized by a deficit of cyclones and an excess of anticyclones. A final analysis investigates the peculiarities of low NDVI events that occur in two consecutive summers and the potential role of drought legacy effects. In the temperate biome, the second event summer of an event sequence has less hot and less dry anomalies than the first one and than during a single event. In summary, detailed investigations of the multi-annual meteorological storyline of low forest NDVI events provided clear evidence that anomalies of surface weather and synoptic-scale weather systems over time periods of up to 2.5 y can negatively impact European forest activity, with important differences between the temperate and Mediterranean biomes. 

Analysis of multi-seasonal meteorological storylines leading to reduced forest greenness in Europe in 2000–2020

The lowermost stratosphere (LMS) as part of the extratropical UTLS can be divided into dynamically and chemically distinct regions. A layer of mixed tropospheric and stratospheric tracer characteristics in the proximity of the extratropical tropopause: the extratropical tropopause transition layer (ExTL). This chemically distinct layer roughly coincides with a layer of strongly enhanced thermal stratification: the tropopause inversion layer (TIL) (Birner, 2006). The LMS above the ExTL, also named the free LMS (Bonisch et al., 2009), is less coupled to the local extratropical troposphere. Simultaneous in-situ measurements of CO2 and SF6 have been used to calculate mean transport time from the troposphere to the measurement location in the free LMS (Bonisch et al., 2009) which is on the order of months.

/pdf/timescales-of-transport-from-the-troposphere-into-the-4hzi1tluvj.pdf

Timescales of transport from the troposphere into the lowermost stratosphere

&amp;lt;p&amp;gt;Recent studies have emphasized the relevance of synoptic-scale atmospheric processes in driving &amp;lt;span&amp;gt;extreme events &amp;lt;/span&amp;gt;in the Arctic. In particular, warm extremes and episodes of anomalous sea ice melting &amp;lt;span&amp;gt;have&amp;lt;/span&amp;gt; been attributed &amp;amp;#8211; in part &amp;amp;#8211; to intrusions of warm and moist air from lower latitudes associated with &amp;lt;span&amp;gt;mid-latitude&amp;lt;/span&amp;gt; weather systems such as cyclones and blocks. &amp;lt;span&amp;gt;At the same time&amp;lt;/span&amp;gt;, also local processes such as subsidence in anticyclones and diabatic heating have been found to play an important role in some cases. &amp;lt;span&amp;gt;Accordingly&amp;lt;/span&amp;gt;, the relative importance of remote vs. local processes remains &amp;lt;span&amp;gt;a matter of debate&amp;lt;/span&amp;gt;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Here, we focus on&amp;lt;span&amp;gt; the atmospheric components of the &amp;lt;/span&amp;gt;surface energy budget (SEB) over Arctic sea ice. Specifically, we present a novel methodology to identify and track &amp;lt;span&amp;gt;extreme (&amp;gt; 95&amp;lt;/span&amp;gt;&amp;lt;sup&amp;gt;&amp;lt;span&amp;gt;th&amp;lt;/span&amp;gt;&amp;lt;/sup&amp;gt;&amp;lt;span&amp;gt; percentile) anomalies of the SEB&amp;lt;/span&amp;gt; in space and time in ERA5 reanalysis data (extended winters 1979 &amp;amp;#8211; 2020). The resulting 142 so-called SEB life-cycle events are predominantly &amp;lt;span&amp;gt;caused &amp;lt;/span&amp;gt;by enhanced downward longwave radiation and turbulent heat fluxes. &amp;lt;span&amp;gt;A&amp;lt;/span&amp;gt; synoptic analysis reveals that most of these events &amp;lt;span&amp;gt;occur in narrow gateways in the Atlantic and Pacific sectors and are associated with&amp;lt;/span&amp;gt; a poleward deflection of the storm tracks and with blocks over northern Eurasia and Alaska.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;Kinematic backward trajectories from the tropospheric column collocated with the events show that mid- and upper tropospheric air has predominantly been transported poleward from mid-latitudes, thereby ascending along slanted isentropes. Lower tropospheric air, in contrast, is largely of Arctic origin and subject to substantial (diabatic) air mass transformations (warming and moistening) during the days preceding the events. Despite the differences in origin of the air at different altitudes, the entire tropospheric column shows a pronounced warm anomaly (~ 10 K) building up over 2-4 days prior to the events. A decomposition of the temperature anomalies emphasizes the relevance of horizontal advection and latent heating for the generation of the mid- and upper tropospheric temperature anomalies, whereas subsidence and heating by surface heat fluxes over ocean are important for the transformation of the originally cold Arctic air at lower altitudes. &amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;Thus, this work emphasizes that a rich chain of processes is required for the generation of the most extreme Arctic SEB anomalies, resulting from a combination of air mass injections from (remote) lower latitudes and local air mass transformations in the Arctic.&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt; 

Local versus remote origin of wintertime extreme surface energy budget anomalies in the Arctic

&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;Extratropical cyclones strongly interact with the midlatitude waveguide and can thus actively influence onset, maintenance, and decay of large-scale weather regimes on &amp;lt;/span&amp;gt;&amp;lt;span&amp;gt;sub-seasonal&amp;lt;/span&amp;gt;&amp;lt;span&amp;gt; timescales (10 &amp;amp;#8211; 60 days).&amp;lt;/span&amp;gt;&amp;lt;span&amp;gt; For instance, individual cyclones over the Pacific-North American region have shown to be able to trigger positive or negative phases of the North Atlantic Oscillation, depending on the type of Rossby wave breaking they are associated with. Likewise, strongly diabatically driven cyclones have shown to be relevant for ridge amplification and thus blocking onset and maintenance. Biases in cyclone activity and characteristics in sub-seasonal numerical weather prediction models might therefore hinder exploiting the potential large-scale predictability on these timescales. We thus, for the first time, identify and track extratropical cyclones in 21 winters (2000 &amp;amp;#8211; 2020) of sub-seasonal ensemble hindcasts from the European Centre for Medium-Range Weather Forecasts. This quasi-Lagrangian, object-oriented approach allows us to validate various cyclone life cycle characteristics such as the deepening rate, location of genesis, maximum intensity, and decay, propagation direction and speed, and size, age and lifetime. Overall, the hindcasts reproduce the climatology of cyclone activity and characteristics remarkably well up to 6 weeks lead time, both over the North Atlantic and the North Pacific. However, the hindcasts tend to underestimate the frequency of the strongly intensifying subset of North Atlantic cyclones. This underestimation is likely linked to an underestimation of cyclogenesis frequency along the southeastern U.S. coast. As strongly intensifying cyclones are particularly relevant for the reorganization of the large-scale flow downstream, this bias might influence sub-seasonal forecast skill and deserves further attention.&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt; 

Heini Wernli

Papers

Examining continental and marine sources of selenium in rainfall

Analysis of multi-seasonal meteorological storylines leading to reduced forest greenness in Europe in 2000–2020

Timescales of transport from the troposphere into the lowermost stratosphere

Local versus remote origin of wintertime extreme surface energy budget anomalies in the Arctic

Validation of extratropical cyclone characteristics in sub-seasonal ECMWF forecasts