3. ERA5 data merger

Creates a single monthly .nc file with SUMMA-ready variables for further processing. Combines ERA5’s u and v wind components into a single directionless wind vector.

3.1. Description

We downloaded separate ERA5 data at pressure level 137 and the surface. We want all the forcing aggregated into a single file and to be in the right units for SUMMA (ERA5 units are already OK for use in SUMMA):

  • Precipitation as pptrate in [kg m**-2 s**-1]

  • Air pressure at measurement height (assumed to be equivalent to air pressure at the surface) as airpres in [Pa]

  • Downward shortwave radiation as SWRadAtm in [W m**-2]

  • Downward longwave radiation as LWRadAtm in [W m**-2]

  • Temperature at measurement height as airtemp in [K]

  • Specific humidity at measurement height as spechum in [g g**-1]

  • Wind speed at measurement height as ‘windspd’ in [m s**-1]

The ERA5 data are split out over two files, that each contain hourly data for a given combination of year and month. Files named ERA5_surface\_[yyyymm].nc contain variables at the surface:

  • precipitation (mtpr),

  • surface pressure (sp),

  • mean surface downward shortwave radiation (msdwswrf)

  • mean surface downward longwave radiation(msdwlwrf)

Files named ERA5_pressureLevel137\_[yyymm].nc contain variables at at the lowest pressure level of the atmospheric model (10m):

  • temperature (t),

  • specific humidity (q)

  • u-component of wind speed (u)

  • v-component of wind speed (v)

This script goes through the following steps:

  1. Convert longitude coordinates in pressureLevel file to range [-180,180]

  2. Checks

  • are lat/lon the same for both data sets?

  • are times the same for both datasets?

  1. Aggregate data into a single file ERA5_NA_[yyyymm].nc, keeping the relevant metadata in place

3.2. Assumptions not included in control_active.txt

Code assumes it operates on the same years that were downloaded, contained in field forcing_raw_time in the control file. To merge only a subset of these files, change the specification of the years variable.