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ERA5 data

Locations

NCI Repo: /g/data/rt52/era5

Concatenated data: /g/data/xv83/bxn599/ACS/data/era5/concatenated

For consistency with icclim data we use hourly t2m and mtpr to calculate min/max daily temperature and mean precipitation. These are found in the concatenated data directory /g/data/xv83/bxn599/ACS/data/era5/concatenated. The CDO commands used to calculate the daily min/max/mean are as follows:

tasmax/tasmin

cdo -P 14 -b F64 sellonlatbox,88.0,208.0,-54.0,14.0 t2m_era5_oper_sfc_19790101-19790131.nc t2m_era5_oper_sfc_19790101-19790131_cordex_aus.nc
cdo -P 14 -b F64 cat t2m_era5_oper_sfc_19790101-19790131_cordex_aus.nc ... t2m_era5_oper_sfc_19790101-20211231_cordex_aus.nc
cdo -P 14 -b F64 -daymax -chname,t2m,tasmax t2m_era5_oper_sfc_19790101-20211231_cordex_aus.nc tasmax_daily_era5_19790101-20211231.nc
cdo -P 14 -b F64 -daymin -chname,t2m,tasmin t2m_era5_oper_sfc_19790101-20211231_cordex_aus.nc tasmin_daily_era5_19790101-20211231.nc

pr

cdo -P 14 -b F64 -selyear,1979/2021 -daymean -shifttime,-1hour -chname,mtpr,pr mtpr_era5_oper_sfc_19790101-20220131_cordex_aus.nc pr_daily_era5_19790101-20211231.nc
cdo -P 14 -b F64 cat mtpr_era5_oper_sfc_19790101-19790131_cordex_aus.nc ... mtpr_era5_oper_sfc_19790101-20220131_cordex_aus.nc
cdo -P 14 -b F64 sellonlatbox,88.0,208.0,-54.0,14.0 mtpr_era5_oper_sfc_19790101-19790131.nc mtpr_era5_oper_sfc_19790101-19790131_cordex_aus.nc

Notes

mtpr data needs to be concatenated first before calculating the daily mean as it is an accumulation variable and the timestep needs to be shifted back one hour (see below for more info and here)

Instantaneous and accumulation variables

Some variables in ERA5 are instantaneous (i.e., valid at the specified timestep (validity time)) whereas others are an accumulation (e.g., for the previous hour or 3 hours). See ERA5 documentation for a list of instantaneous variables.

For quick reference:

  • 2t (2m air temperature) is instantaneous
  • 10u (10m u component of wind) and 10v (10m v component of wind) are instantaneous and are used to calculate si10 (daily wind speed, /g/data/xv83/users/bxn599/ACS/data/era5/raw/si10)
  • 10fg (10m wind gust since previous processing) is a maximum/minimum and therefore the timestep (validity time) is for the previous hour
    • reanalysis: the minimum or maximum values are in the hour (the processing period) ending at the validity date/time. ERA5 documentation
  • mtpr (mean total precipitation rate) is rates/fluxes and therefore the timestep (validity time) is for the previous hour
    • For the CDS time, or validity time, of 00 UTC, the mean rates/fluxes and accumulations are over the hour (3 hours for the EDA) ending at 00 UTC i.e. the mean or accumulation is during part of the previous day. ERA5 documentation

Other references:

Concatenation process

ERA5 stored is in short data format and the scale factor and offset can vary between files. Therefore, when concatenating ERA5 data with CDO it is best to use the -b F64 flag. This will convert the data to double and avoid any issues.

This figure shows the difference between data which has been concatenated without (temp_daysum) and with the -b F64 flag (temp_f64_daysum).

The difference between the two files only occurs for time steps [367:458].

These two files can be found in /g/data/xv83/users/bxn599/era5/hourly/tp/test

Concatenated data is calculated using the -b F64 flag on all steps.

Old data

Hourly rainfall downloaded from ECMWF: /g/data/xv83/users/bxn599/era5/hourly/tp

  • tp_20210101-20210530.nc is 4D and has an exp_ver dimension, this is removed in tp_20210101-20210530_remove_exp_ver.nc using the following CDO command: cdo --reduce_dim -sellevidx,1 tp_20210101-20210530.nc tp_20210101-20210530_remove_exp_ver.nc