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

[1] We present an analysis of trajectory calculations in the tropical tropopause layer (TTL) based on European Centre for Medium-Range Weather Forecasts (ECMWF) analysis wind and temperature fields. Over 500,000 forward and backward trajectories were calculated for January/February and July/August 2001. We analyze the pathways between 340 K and 400 K potential temperature (θ) of those trajectories involved in troposphere-to-stratosphere transport (TST). Even though trajectory calculations in this region may suffer from deficiencies in the underlying vertical wind field, they incorporate not only slow radiative ascent but also effects of deep convection, zonal and meridional transport, and their regional variability. From the trajectory calculations we derive a mean residence time of air parcels in the TTL, which shows a maximum at θ ≈360 K of ∼13 days for a change in potential temperature of ±10 K. The analysis of trajectory pathways reveals that approximately 80% of the trajectories ascending into the stratosphere enter the TTL over the western Pacific. Upon further ascent, they typically travel ∼5000–10,000 km before they arrive at the location where they assume minimum water mixing ratios. These pathways show regional and seasonal patterns and are largely controlled by the upper level circulation of the Asian-Australian monsoon, the northern hemispherical subtropical jet and the equatorial easterly jet from South Asia to Africa. As a consequence of the interplay of these meteorological systems, about 70% of TST trajectories assume their minimum water mixing ratio over the western Pacific, which shows also a global minimum in tropopause temperatures. Average water mixing ratios of air after TST are χH2O = 1.6 ppmv for January/February and χH2O = 3.6 ppmv for July/August 2001. Mixing of stratospherically young air, which just underwent TST, with older air masses entering the lower tropical stratosphere sideways yields an estimate of χH2O = 2.3 ppmv for January/February and χH2O = 3.7 ppmv for July/August for air at θ = 400 K, which compares favorably with satellite observations. Our analysis emphasizes the importance of particular pathways for tropical TST, with the western Pacific being the dominant source of stratospheric air in general and being the place, in particular, where ∼70% of tropical TST assumes its final water mixing ratio.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JD004069

Tropical troposphere‐to‐stratosphere transport inferred from trajectory calculations

Case studies have shown that heavy precipitation events and rapid cyclogenesis in the extratropics can be fueled by moist and warm tropical air masses. Often the tropical moisture export (TME) occurs through a longitudinally confined region in the subtropics. Here a comprehensive climatological analysis of TME is constructed on the basis of seven-day forward trajectories started daily from the tropical lower troposphere using 6-hourly 40-yr ECMWF Re-Analysis (ERA-40) data from the 23-year period 1979–2001. The objective TME identification procedure retains only those trajectories that reach a water vapor flux of at least 100 g kg−1 m s−1 somewhere north of 35°N. The results show four distinct activity maxima with different seasonal behavior: (i) The “pineapple express,” which connects tropical moisture sources near Hawaii with precipitation near the North American west coast, has a marked activity maximum in boreal winter. (ii) TME over the west Pacific is largest in summer, partly related to the...

/pdf/a-lagrangian-climatology-of-tropical-moisture-exports-to-the-yxtu6y52wj.pdf

A Lagrangian Climatology of Tropical Moisture Exports to the Northern Hemispheric Extratropics

A novel approach is introduced to identify potential vorticity (PV) streamers and cutoffs as indicators of Rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces. The method is based on a contour searching algorithm that identifies the dynamical tropopause [2 potential vorticity units (PVU; PVU ≡ 1 × 10−6 K kg−1 m2 s−1) isoline] on isentropic surfaces. The contour is then analyzed to search for cutoffs and filament-like streamers. Whereas the identification of cutoffs is unambiguous, the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer. This technique has been applied to the PV distribution in the Northern Hemisphere on isentropes from 295 to 360 K during the time period from 1979 to 1993 using the 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15). The climatology reveals a pronou...

Identification and ERA-15 Climatology of Potential Vorticity Streamers and Cutoffs near the Extratropical Tropopause

Owing to the huge potential impact of precipitation extremes on society, it is important to better understand the mechanisms causing these events, and their variations with respect to a changing climate. In this study, the importance of a particular category of weather systems, namely cyclones, for the occurrence of regional-scale precipitation extremes is quantified globally using the ECMWF Interim reanalysis (ERA-Interim) dataset. Such an event-based climatological approach complements previous case studies, which established the physical relationship between cyclones and heavy precipitation. A high percentage of precipitation extremes is found to be directly related to cyclones. Regional hot spots are identified where this percentage of cyclone-induced precipitation extremes exceeds 80% (e.g., in the Mediterranean region, Newfoundland, near Japan, and over the South China Sea). The results suggest that in these regions changes of heavy precipitation with global warming are specifically sensitiv...

Quantifying the relevance of cyclones for precipitation extremes

[1] A new methodology is proposed to identify folds of the dynamical tropopause (taken as the 2 potential vorticity (PV) units (pvu) isosurface) from global analysis data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF). It consists of a three-dimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses. The method is applied to a 1-year period starting in March 2000. Seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics, with maximum frequencies of about 30%. Pronounced maxima are located in the subtropical bands that extend from 20°E to 120°E on both hemispheres during summer. Generally, subtropical folds are comparatively shallow. Deep folds occur most frequently in midlatitudes during winter (over eastern North America and the western North Atlantic), with maximum frequencies of about 1%. Detailed investigation of the relationship between individual folds and cross-tropopause exchange events reveals that, on the average, individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics. However, due to the much higher frequency of subtropical folds, the percentage of exchange events that are associated with tropopause folds is larger near 20°–40° latitude (50–70%) than further poleward (20–30% during winter and 10–20% during summer). This indicates that tropopause folds are the key feature for cross-tropopause exchange in the subtropics, whereas in the extratropics other tropopause structures are at least of equal importance.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002JD002587

Heini Wernli

Papers

Tropical troposphere‐to‐stratosphere transport inferred from trajectory calculations

A Lagrangian Climatology of Tropical Moisture Exports to the Northern Hemispheric Extratropics

Identification and ERA-15 Climatology of Potential Vorticity Streamers and Cutoffs near the Extratropical Tropopause

Quantifying the relevance of cyclones for precipitation extremes

Tropopause folds and cross-tropopause exchange: A global investigation based upon ECMWF analyses for the time period March 2000 to February 2001