Abstract. 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.