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

Abstract

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.

Significant Contribution of Mesoscale Overturning to Tropical Mass and Energy Transport Revealed by the ERA5 Reanalysis

“…Data assimilation of satellite radiances in ERA5 over the cloudy area and advanced model physics parameterizations, such as the improved freezing rain physics, representation of radiation‐surface interactions, and parameterization of gravity wave drag, likely also contribute to a better estimate of the mesoscale deep atmospheric overturning over the convectively intense regions. The results are consistent with Hoffmann et al (), which indicted a better representation of mesoscale convective activity in ERA5 than ERA‐interim based on Lagrangian transport simulations.…”

An ERA5-Based Hourly Global Pressure and Temperature (HGPT) Model

“…ERA5 is the fifth generation of the ECMWF atmospheric reanalysis that has been operational at ECMWF since 2016; it covers the period from 1979 till the present (from 1950 by early 2020), and has several significant innovative features beyond that of the discontinued ERA-Interim reanalysis model. The major improvement is attributed to an increase in the horizontal grid spacing (from 79 to 31 km), in the number of model levels (from 60 to 137), and in the temporal resolution (from 6 to 1 h), which enables an improved atmospheric representation of convective systems, gravity waves, tropical cyclones, and other meso-to synoptic-scale atmospheric structures [26]. Another ERA5 improvement is the number of observations that are assimilated, which went from on average 0.75 million per day in 1979 to about 24 million per day in 2018, boosted mainly by the increase of satellite radiances throughout the period, and more recently, by the GNSS-Radio Occultation, scatterometer ocean vector wind and altimeter wave height data, ozone products, and ground-based radar observations [27].…”

Differences in tropical high clouds among reanalyses: origins and radiative impacts

“…This variance decreases with decreasing LWCRE; however, the fraction of negativeθ rad values in Q1 and Q2 (9%) is more than double that in CFSR. The broader distribution of diabatic heating rates in this reanalysis may be related to improved consistency between diabatic and kinematic vertical motion in the lower stratosphere in ERA5 relative to ERA-Interim (Hoffmann et al, 2019).…”