Historical reanalyses that span more than a century are needed for a wide range of studies, from understanding large‐scale climate trends to diagnosing the impacts of individual historical extreme weather events. The Twentieth Century Reanalysis (20CR) Project is an effort to fill this need. It is supported by the National Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Research in Environmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and is facilitated by collaboration with the international Atmospheric Circulation Reconstructions over the Earth initiative. 20CR is the first ensemble of sub‐daily global atmospheric conditions spanning over 100 years. This provides a best estimate of the weather at any given place and time as well as an estimate of its confidence and uncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has several significant issues, including inaccurate estimates of confidence and a global sea level pressure bias in the mid‐19th century. These and other issues can reduce its effectiveness for studies at many spatial and temporal scales. Therefore, the 20CR system underwent a series of developments to generate a significant new version of the reanalysis. The version 3 system (NOAA‐CIRES‐DOE 20CRv3) uses upgraded data assimilation methods including an adaptive inflation algorithm; has a newer, higher‐resolution forecast model that specifies dry air mass; and assimilates a larger set of pressure observations. These changes have improved the ensemble‐based estimates of confidence, removed spin‐up effects in the precipitation fields, and diminished the sea‐level pressure bias. Other improvements include more accurate representations of storm intensity, smaller errors, and large‐scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects that have ameliorated issues in 20CRv2c. Despite the many improvements, some challenges remain, including a systematic bias in tropical precipitation and time‐varying biases in southern high‐latitude pressure fields.

/pdf/towards-a-more-reliable-historical-reanalysis-improvements-3osv1v8h3n.pdf

Towards a more reliable historical reanalysis: improvements for version 3 of the Twentieth Century Reanalysis system

Abstract In recent decades, the warming in the Arctic has been much faster than in the rest of the world, a phenomenon known as Arctic amplification. Numerous studies report that the Arctic is warming either twice, more than twice, or even three times as fast as the globe on average. Here we show, by using several observational datasets which cover the Arctic region, that during the last 43 years the Arctic has been warming nearly four times faster than the globe, which is a higher ratio than generally reported in literature. We compared the observed Arctic amplification ratio with the ratio simulated by state-of-the-art climate models, and found that the observed four-fold warming ratio over 1979–2021 is an extremely rare occasion in the climate model simulations. The observed and simulated amplification ratios are more consistent with each other if calculated over a longer period; however the comparison is obscured by observational uncertainties before 1979. Our results indicate that the recent four-fold Arctic warming ratio is either an extremely unlikely event, or the climate models systematically tend to underestimate the amplification. 

/pdf/the-arctic-has-warmed-nearly-four-times-faster-than-the-2f7lp97b.pdf

The Arctic has warmed nearly four times faster than the globe since 1979

. The European Centre for Medium-Range Weather Forecasts' (ECMWF's)
next-generation reanalysis ERA5 provides many improvements, but it
also confronts the community with a “big data” challenge. Data
storage requirements for ERA5 increase by a factor of ∼80 
compared with the ERA-Interim reanalysis, introduced a decade ago.
Considering the significant increase in resources required for
working with the new ERA5 data set, it is important to assess its
impact on Lagrangian transport simulations. To quantify the
differences between transport simulations using ERA5 and ERA-Interim
data, we analyzed comprehensive global sets of 10-day forward
trajectories for the free troposphere and the stratosphere for the
year 2017. The new ERA5 data have a considerable impact on the
simulations. Spatial transport deviations between ERA5 and
ERA-Interim trajectories are up to an order of magnitude larger than
those caused by parameterized diffusion and subgrid-scale wind
fluctuations after 1 day and still up to a factor of 2–3 larger
after 10 days. Depending on the height range, the spatial
differences between the trajectories map into deviations as large as
3 K in temperature, 30 % in specific humidity, 1.8 % in potential
temperature, and 50 % in potential vorticity after 1 day. Part of
the differences between ERA5 and ERA-Interim is attributed to the better
spatial and temporal resolution of the ERA5 reanalysis, which allows for
a better representation of convective updrafts, gravity waves,
tropical cyclones, and other meso- to synoptic-scale features of the
atmosphere. Another important finding is that ERA5 trajectories
exhibit significantly improved conservation of potential temperature
in the stratosphere, pointing to an improved consistency of ECMWF's
forecast model and observations that leads to smaller data
assimilation increments. We conducted a number of downsampling
experiments with the ERA5 data, in which we reduced the numbers of
meteorological time steps, vertical levels, and horizontal grid
points. Significant differences remain present in the transport
simulations, if we downsample the ERA5 data to a resolution similar
to ERA-Interim. This points to substantial changes of the forecast
model, observations, and assimilation system of ERA5 in addition to
improved resolution. A comparison of two Lagrangian trajectory
models allowed us to assess the readiness of the codes and workflows
to handle the comprehensive ERA5 data and to demonstrate the
consistency of the simulation results. Our results will help to
guide future Lagrangian transport studies attempting to navigate the
increased computational complexity and leverage the considerable
benefits and improvements of ECMWF's new ERA5 data set.

/pdf/from-era-interim-to-era5-the-considerable-impact-of-ecmwf-s-1j0cdct9ka.pdf

From ERA-Interim to ERA5: the considerable impact of ECMWF's next-generation reanalysis on Lagrangian transport simulations

. Major, sudden midwinter stratospheric warmings (SSWs) are large and rapid temperature increases in the winter polar stratosphere are associated with a complete reversal of the climatological westerly winds (i.e., the polar vortex). These extreme events can have substantial impacts on winter surface climate, including increased frequency of cold air outbreaks over North America and Eurasia and anomalous warming over Greenland and eastern Canada. Here we present a SSW Compendium (SSWC), a new database that documents the evolution of the stratosphere, troposphere, and surface conditions 60 days prior to and after SSWs for the period 1958–2014. The SSWC comprises data from six different reanalysis products: MERRA2 (1980–2014), JRA-55 (1958–2014), ERA-interim (1979–2014), ERA-40 (1958–2002), NOAA20CRv2c (1958–2011), and NCEP-NCAR I (1958–2014). Global gridded daily anomaly fields, full fields, and derived products are provided for each SSW event. The compendium will allow users to examine the structure and evolution of individual SSWs, and the variability among events and among reanalysis products. The SSWC is archived and maintained by NOAA's National Centers for Environmental Information (NCEI, doi:10.7289/V5NS0RWP ).

/pdf/a-sudden-stratospheric-warming-compendium-zfd61k68et.pdf

A sudden stratospheric warming compendium

What global reanalysis best represents near surface winds

The climate research community uses atmospheric reanalysis data sets to understand a wide range of processes and variability in the atmosphere, yet different reanalyses may give very different results for the same diagnostics. The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets using a variety of key diagnostics. The objectives of this project are to identify differences among reanalyses and understand their underlying causes, to provide guidance on appropriate usage of various reanalysis products in scientific studies, particularly those of relevance to SPARC, and to contribute to future improvements in the reanalysis products by establishing collaborative links between reanalysis centres and data users. The project focuses predominantly on differences among reanalyses, although studies that include operational analyses and studies comparing reanalyses with observations are also included when appropriate. The emphasis is on diagnostics of the upper troposphere, stratosphere, and lower mesosphere. This paper summarizes the motivation and goals of the S-RIP activity and extensively reviews key technical aspects of the reanalysis data sets that are the focus of this activity. The special issue "The SPARC Reanalysis Intercomparison Project (S-RIP)" in this journal serves to collect research with relevance to the S-RIP in preparation for the publication of the planned two (interim and full) S-RIP reports.

/pdf/introduction-to-the-sparc-reanalysis-intercomparison-project-3n6ohhfj8d.pdf

Cheng Zhi Zou

Papers

Introduction to the SPARC Reanalysis Intercomparison Project (S-RIP) and overview of the reanalysis systems