Overview
This document provides an overview of the runoff data of ERA5-Land, in the context of the larger ERA5-Land dataset. ERA5-Land is a replay of the land component of the ERA5 atmospheric global reanalysis using a finer spatial resolution and including a series of improvements making it more accurate for all types of land applications. ERA5-Land is produced by ECMWF framed within the Copernicus Climate Change Service (C3S) of the European Commission. The data covers a period from January 1950 to the present. It provides hourly data for many near-surface atmospheric and land-surface parameters.
Provider's contact information
ERA5-Land is produced by the Copernicus Climate Change Service (C3S) at ECMWF.
Copernicus User support (copernicus-support@ecmwf.int (external to C3S)).
Licensing
Licence: Copernicus (Licence agreement information can be found here or here).
Dataset citable as: Muñoz Sabater, J., 2019: ERA5-Land hourly data from 1981 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS). (Accessed on < DD-MMM-YYYY >), 10.24381/cds.e2161bac
Variable name and units:
Some water from rainfall, melting snow, or deep in the soil, stays stored in the soil. Otherwise, the water drains away, either over the surface (surface runoff), or under the ground (subsurface runoff) and the sum of these two is simply called 'runoff'. ERA5-Land offers data for the three variables as hourly and monthly subsets:
- Runoff in m
- Subsurface runoff in m
- Surface runoff in m
There three variables represent the total amount of water accumulated from the beginning of the forecast time to the end of the forecast step. The units of the three runoff variables are depth in metres. This is the depth the water would have if it were spread evenly over the grid box. More information about how runoff is calculated is given in the IFS Physical Processes documentation.
Those products can be found by selecting their name from the ‘Evaporation and Runoff’ category on the pages below:
- hourly data from 1950 to present,
- monthly averaged data from 1981 to present,
Spatial coverage and resolution:
ERA5-Land is a global land-surface dataset. The atmospheric data is available on a regular latitude-longitude grid at 0.1o x 0.1o resolution (converted from native reduced‐Gaussian grid resolution of approximately 9 km x 9 km), and on 4 surface layers. Oceans have been masked out with data available over landmasses and inland lakes.
Temporal coverage and resolution:
ERA5-Land data is available from 1950 to present at hourly time step. Monthly average over all hours or monthly averages by hour (24 values) for monthly data are also available from 1981 to present. The 1950 – 1980 back extension for monthly means is scheduled to be available in 2022.
ERA5-Land data updates are made synchronously with ERA5 updates, approximately 2-3 months behind real time.
Information about observations (number, homogeneity)
ERA5-Land is not directly influenced by observations, but rather, indirectly influenced through the ERA5 atmospheric forcings. ERA5’s data assimilation uses observations for all geophysical quantities from about 0.75 million observations per day in 1979 and about 24 million in 2018. The 2D-OI uses surface observations at 'screen level'. The online technical documentation provides tables with the satellite and in-situ observations used as input into ERA5.
Methodology
ERA5-Land is produced under a single simulation of the land component of the ERA5 climate reanalysis, without coupling to the atmospheric module of the ECMWF's Integrated Forecasting System (IFS) and without data assimilation. The low atmospheric forcing is provided by the ERA5 reanalysis, with additional lapse-rate correction. The core of ERA5-Land is the Tiled ECMWF Scheme for Surface Exchanges over Land incorporating land surface hydrology (H-TESSEL). Because it runs without data assimilation, it makes it computationally affordable for relatively quick updates. For example, if significant improvements of the land surface model are implemented, the whole or part of the dataset can be reprocessed in a relatively short period. Updates are possible in case improved auxiliary datasets are used as input for the production.
Production of ERA5-Land is not produced as a single continuous segment, but instead as three segments: Stream-1 (2001 onwards), Stream-2 (1981-2000), and Stream-3 (1950-1980). This is because it allows parallel production of data enabling sooner public access to the data, and because the atmospheric forcings used by ERA5-Land is derived from ERA5, thus needing corresponding completed ERA5 segment. Each stream is initialized with various meteorological fields from ERA5 (temperature, precipitation, humidity, radiation, etc.). While ERA5-Land does not assimilate observations directly, they are introduced via the ERA5 atmospheric forcings. These forcings are adjusted using ERA5 derived lapse rates before being integrated with the ECMWF Carbon Hydrology-Tiled ECMWF Scheme for Surface Exchanges over Land (CHTESSEL) land surface model. This is done in 24-hour cycles, generating hourly outputs and the evolution of the land surface state and water and energy fluxes.
In the land surface hydrology H-TESSEL scheme, each land grid box is divided into fractions (tiles), with up to six fractions (bare ground, low and high vegetation, intercepted water, shaded and exposed snow). Each fraction has its own properties defining separate heat and water fluxes used in an energy balance equation solved for the tile skin temperature. New infiltration and runoff schemes are introduced with a dependency on the soil texture and standard deviation of orography. The snowpack is treated taking into account its thermal insulation properties and a more realistic representation of density, the interception of liquid rain and a revised scheme for the albedo and metamorphism aging processes. A new formulation to represent inland water bodies both for resolved lakes and sub-grid coastal water in liquid and frozen state is introduced with a dedicated new water tile. A reference document is provided at ECMWF: A revised hydrology for the ECMWF model: Verification from field site to terrestrial water storage and impact in the Integrated Forecast System.
Uncertainty estimate: Currently, ERA5-Land variable uncertainty estimates are those corresponding to ERA5. ERA5 uncertainty estimate is sampled by a 10-member lower-resolution Ensemble Data Assimilation (EDA) which provides background-error estimates for the deterministic HRES 4D-Var Data Assimilation system. The analysis method is the same for each EDA member and follows that of the HRES. Each member (except the control) is run with different random perturbations added to the observations. Likewise, the model physical tendencies are perturbed in the short forecasts that link subsequent analysis windows. Ensemble mean and spread have been pre-computed for convenience. Such uncertainty estimates are closely related to the information content of the available observing system which has evolved considerably over time. They also indicate flow-dependent sensitive areas. To facilitate many climate applications, monthly-mean averages have been pre-calculated too, though monthly means are not available for the ensemble mean and spread.
The original plan was to apply the same methodology ERA5-Land was to provide an estimate of the uncertainty fields as was done for ERA5. However, the uncertainty was estimated to be extremely low, and would have assigned unrealistically high confidence to the ERA5-Land variables. As such, it is recommended to use the corresponding ERA5 uncertainty estimates for the time being until further studies are done.
Information about the technical and scientific quality
ERA5-Land represents one of the products of the latest global atmospheric reanalysis produced by Copernicus Climate Change Service at ECMWF. It is archived at a shorter (hourly) time step, has a fine spatial resolution, uses a more advanced assimilation system and includes more sources of data than previous versions (e.g., ERA-Interim-Land). It is accompanied by extensive technical documentation and two principal scientific documentation papers. A list of ‘known issues’ is maintained at the online documentation (https://confluence.ecmwf.int/display/CKB/ERA5-Land%3A+data+documentation). Validation against multiple in-situ datasets is presented in the reference paper Muñoz-Sabater et al., 2021.
Information on land surface model: The land surface model of the ERA5-Land was operational in 2018 with the IFS model cycle 45r1. While most of the changes from the IFS Cy41R2 used in ERA5 are primarily technical, there were a few improvements to various fields: 1) the parameterization of the soil thermal conductivity was updated to take the ice component of frozen soil into consideration, 2) conservation of the soil-water balance was fixed and improved, and 3) rain over snow is now accounted for and is not accumulated in snow pack. Furthermore, a bug exists in IFS Cy41R2, that affects potential evapotranspiration (PET) flux calculations over forests and deserts, has been corrected in ERA5-Land, and unlike ERA5, ERA5-Land PET is an available dataset. However, PET is now determined by assuming a vegetation type of crops and no soil moisture stress. These assumptions may not be always realistic, and therefore PET should be used cautiously. More information on the CHTESSEL land surface model used in ERA5-Land can be found in Muñoz-Sabater et al. (2021, preprint) and the ERA5 document previously prepared by the CCCS
Limitations and strengths for application in North Canada
ERA5-Land is a newer land surface reanalysis and there are few available scientific evaluations of the dataset dedicated specifically to northern Canada. However, it should be noted that in northern Canada, there are currently no sub-daily records over a long historical period for many weather stations. ERA5-Land aims to provide an improved description of the water and energy cycles at surface level meeting the growing requirement from land user communities to gain access to long-term higher-resolution datasets. The high resolution improved land surface hydrology scheme incorporates surface runoff and drainage with functional dependencies on orography and soil texture, respectively.
As for all gridded data, observed values of the various parameters at local scales can differ from the values provided by the gridded dataset, which represent a statistical summary of the area surrounding a grid point. Care should be taken when comparing model variables with observations. Observations are also often taken in different units, such as mm/day, rather than the accumulated metres produced here for runoff.
As for all reanalysis' data, changes in the amounts and types of observational data that are assimilated may have an adverse impact on trends or variability.
References to documents describing the methodology or/and the dataset
Muñoz Sabater, J., 2019: ERA5-Land hourly data from 1981 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS). (Accessed on < 25-Jun-2021 >), https://doi.org/10.24381/cds.e2161bac
Muñoz-Sabater, J., E. Dutra, A. Agustí-Panareda, C. Albergel, G. Arduini, G., Balsamo, S. Boussetta, M. Choulga, S. Harrigan, H. Hersbach, B. Martens, D. G. Miralles, M. Piles, N. J. Rodríguez-Fernández, E. Zsoter, C. Buontempo, and J.-N. Thépaut, 2021: ERA5-Land: A state-of-the-art global reanalysis dataset for land applications. Earth System Science Data, 13(9), 4349-4383. https://doi.org/10.5194/essd-2021-82
Hersbach, H., B .Bell, P. Berrisford, S. Hirahara, A. Horányi, J. Muñoz‐Sabater, J. Nicolas, C. Peubey, R. Radu, D. Schepers, A. Simmons, C. Soci, S. Abdalla, X. Abellan, G. Balsamo, P. Bechtold, G. Biavati, J. Bidlot, M. Bonavita, G. Chiara, P. Dahlgren, D. Dee, M. Diamantakis, R. Dragani, J. Flemming, R. Forbes, M. Fuentes, A. Geer, L. Haimberger, S. Healy, R.J. Hogan, E. Hólm, M. Janisková, S. Keeley, P. Laloyaux, P. Lopez, C. Lupu, G. Radnoti, P. Rosnay, I. Rozum, F. Vamborg, S. Villaume, and J.-N. Thépaut, 2020: The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146(730), 1999–2049. https://doi.org/10.1002/qj.3803.
Link to download the data and format of data:
Data Access: Copernicus | ECMWF (requires login)
ERA5-Land is available in GRIB and NetCDF formats
Link to download hourly and monthly data on Copernicus:
- hourly data from 1950 to present,
- monthly averaged data from 1981 to present
Publications including dataset evaluation or comparison with other data in Canada