CHIRPS Precipitation, Copernicus DEM, (Ag)ERA5 Meteorological Data, GEOS-5 Meteorological Data, IMERG Precipitation, Landsat satellites, MODIS sensors, MSG satellites, Sentinel-2 satellites, VIIRS sensors and WorldCover Land Cover

ERA5 and AgERA5

ERA5 is the fifth generation of ECMWF atmospheric reanalyses of the global climate. It provides hourly estimates of a large number of atmospheric, land and oceanic climate variables. The data cover the Earth on a 30km grid and resolve the atmosphere using 137 levels from the surface up to a height of 80km. ERA5 combines vast amounts of historical observations into global estimates using advanced modelling and data assimilation systems. ERA5 is produced using 4D-Var data assimilation and model forecasts in CY41R2 of the ECMWF Integrated Forecast System (IFS), with 137 hybrid sigma/pressure (model) levels in the vertical and the top level at 0.01 hPa.

AgERA5 makes the ERA5 data available to users in the agricultural domain. It includes agro-meteorological variables such as daily mean, minimum and maximum temperature, precipitation, humidity and incoming solar radiation. All land areas are covered at a spatial resolution of 10 by 10 km from 1979 until now.

Purpose of data

ERA5 and AgERA5 data is used to create the Weather data for the final run and historical processing:
- Daily AgERA5 data is used as input for the ET model. The temperature, dewpoint temperature and windspeed are directly acquired. The relative humidity is derived from actual vapor pressure and saturated vapor pressure calculated using the dewpoint temperature and air temperature.
- Hourly ERA5 data is used as input for the soil moisture model. The temperature and mean sea level pressure are directly acquired. The relative humidity is derived from actual vapor pressure and saturated vapor pressure calculated using the dewpoint temperature and air temperature. Wind speed is calculated as the mean of the square root of the sum of eastward (u) and northward (v) wind speed components squared.

Approach

The full ERA5 and AgERA5 datasets are held in the ECMWF data archive (MARS) and a pertinent sub-set of these data, interpolated to a regular latitude/longitude grid, has been copied to the C3S Climate Data Store (CDS) disks. ERA5 and AgERA5 data on the CDS disks can be downloaded either from the relevant CDS download page or using the CDS API. The datasets used are temperature at 2m height, dewpoint temperature at 2m height, mean sea level pressure, wind speed in the east-west and north-south direction. Precipitation and solar radiation are back up datasets in case GEOS-5 and IMERG fail.. ERA5 and AgERA5 data is available from January 1979.

Challenges

The biggest challenge, and the reason why the ERA5 and AgERA5 datasets are used only for the final run and for the historical processing, is the latency of the datasets. ERA 5 has a latency of 5 days, while AgERA5 has a latency of 8 days. Furthermore, a potential challenge is that the final release ERA5 data is about two months after the month in question. In the event that serious flaws are detected in ERA5T (initial release data), the latter could be different to the final ERA5 data. Based on experience with the production of ERA5 so far (and ERA-Interim in the past), the expectation is that such an event would not occur often. So far, it has only occurred once: for the months September, October, November and at least a part of December 2021, the final ERA5 product is and will be different from ERA5T due to correction of the assimilation of incorrect snow observations in central Asia.

A water mask has been applied to AgERA5 data, resulting in missing pixels in coastal areas and no data for some islands. To include the coastline, a 1-2 pixel no data fill is applied on AgERA5 data. The missing islands are filled with ERA5 data.

Alternative sources

• MERRA for the period spanning 1979 to February 2016
• GEOS-5 from February 2014