# Monitoring and Forecasting the Impact of the 2018 Summer Heatwave on Vegetation
## 2 Material and Methods
### ECMWF Atmospheric Forcing  
LDAS-Monde is driven by near-surface meteorological fields from both ECMWFs' reanalysis, ERA5, released in 2018, as well as its high resolution operational high resolution weather analysis and forecasts (HRES). ERA5 underlying model and data assimilation system are very similar to that of the operational weather forecast. ERA5 production cycle (IFS Cycle 41r2) is still close to that of the HRES (IFS Cycle 41r2 to 43r3 from 2016 and 45r1 from June 2018, more information at [[https://www.ecmwf.int/en/forecasts/documentation-and-support/changes-ecmwf-model](https://www.ecmwf.int/en/forecasts/documentation-and-support/changes-ecmwf-model)]([https://www.ecmwf.int/en/forecasts/documentation-and-support/changes-ecmwf-model](https://www.ecmwf.int/en/forecasts/documentation-and-support/changes-ecmwf-model)), last access January 2019). The main difference between the two is the horizontal resolution with 31 km in ERA5 and 9 km in HRES. Another difference is the data assimilation time window which is from 21:00 UTC to 09:00 UTC in ERA5 and from 21:00 UTC to 03:00 UTC in HRES, as it allows more observations to be assimilated in ERA5. The shorter time window in HRES is due to ECMWF operational constraints to deliver timely forecasts.  
The ERA5 forcing data includes the lowest model level (about 10-meters above ground level) air temperature, wind speed, specific humidity and pressure and the downwelling fluxes of shortwave and longwave radiation as well as precipitation partitioned in solid and liquid phases. ERA5 forcing data is extracted from the forecasts initialized daily at 00:00 UTC and 12:00 UTC using the hourly forecasts from +1 to +12 h. HRES forcing data is extracted from the forecasts initialized at 00:00 UTC and 12:00 UTC also using the forecasts from +1 h to 12 h. The same downwelling fluxes as in ERA5 are used but for HRES 2-meter temperature and dewpoint temperature, 10-meters wind-speed are extracted. Specific humidity is then calculated from 2-meter temperature and dewpoint temperature. HRES also has the lowest model level data archived, but due to data storage and access constraints it was more efficient to extract the 2-meter temperatures and 10-meter wind speed. Despite the difference in extracting the near-surface fields, lowest model level and 2-meter temperature and 10-meters winds are very similar, and this is not expected to impact substantially the results. In ERA5 and HRES, the +1 h to +12 h hourly forecasts were concatenated to generate continuous time series. Atmospheric forcing is interpolated from the native grids of ERA5 and HRES to regular grids of 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) and 0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\), respectively. The grid to grid interpolation was performed via a bilinear interpolation using the four neighbors in the source grid fitting latitude and longitude linearly. From the forecast initialized at 00:00 UTC, HRES is also available up to 10-days ahead. HRES forecast step frequency is hourly up to time step 90, 3-hourly from time-step 93 to 144 and 6-hourly from time-step 150 to 240 (i.e., 10 days). While the original 3-hourly time steps are used up to day 6 (time step 144), the 6-hourly time steps from day 6 to 10 are interpolated to 3-hourly frequency.