...
Defining input variables for computations
The input variables to compute derived quantities need to be specified when the dataset is loaded. The following example shows the "Add datset" dialog in Met.3D v1.7. The input variables need to be specified by a "/"-separated string using the mapping "CF standard name:NetCDF or GRIB input variable name".
For example, assume that you are loading ECMWF ensemble forecast data from GRIB files. The variables will be named "u", "v", etc. in the GRIB files. For ECMWF-GRIB files only, Met.3D appends "(ens)", "(an)", "(fc)" to indicate whether the data refers to ensemble forecasts, analyses or HRES forecasts. Hence, your mapping string could look like:
| Code Block |
|---|
eastward_wind:u (ens)/northward_wind:v (ens)/air_temperature:t (ens)/specific_humidity:q (ens)/surface_geopotential:z (ens)/surface_air_pressure:sp (ens)/surface_temperature:2t (ens)/lwe_thickness_of_precipitation_amount:tp (ens)/geopotential:z (ens)/atmosphere_relative_vorticity:vo (ens) |
If your input data is read from NetCDF files, you would use the NetCDF variable names directly. E.g., if you u-wind variable is called "u_wind" in the input file, the corresponding part in the mapping would be "eastward_wind:u_wind".
Available derived quantities
As of Met.3D v1.7, the following derived variables are implemented (all input variable names follow the CF standard names):
| Info | ||
|---|---|---|
| ||
The variables in the following list support all grid and vertical level types unless otherwise noted. |
- Horizontal wind speed ("wind_speed")
- Requires eastward_wind, northward_wind
- 3D wind velocity ("magnitude_of_air_velocity")
- Requires eastward_wind, northward_wind, upward_air_velocity
- Potential temperature ("air_potential_temperature")
- Requires air_temperature
- Equivalent potential temperature ("equivalent_potential_temperature")
- Requires air_temperature, specific_humidity
- Computed using equation (43) of Bolton (MWR, 1980), "The Computation of Equivalent Potential Temperature".
- Relative humidity ("relative_humidity")
- Requires air_temperature, specific_humidity
- Computed using saturation vapour pressure by Huang (JAMC, 2018), "A Simple Accurate Formula for Calculating Saturation VaporPressure of Water and Ice", DOI: 10.1175/JAMC-D-17-0334.1
- Potential vorticity ("ertel_potential_vorticity")
Requires eastward_wind, northward_wind, air_temperature, surface_air_pressure
- NOTE: Only available for regular lon-lat grids defined on hybrid sigma-pressure levels (e.g. ECMWF, ERA-5)
- Computed using the ETH Lagranto formulation
- Geopotential height ("geopotential_height")
- Requires air_temperature, specific_humidity, surface_geopotential, surface_air_pressure, surface_temperature
- Geopotential height ("geopotential_height_from_geopotential")
- Requires geopotential
- Simply divides input by 9.81m^2/s^2
- Dew point temperature ("dew_point_temperature")
- Requires specific_humidity
- Hourly total precipitation ("lwe_thickness_of_precipitation_amount_Nh")
- Requires lwe_thickness_of_precipitation_amount
- Computes N-hourly precipitation from accumulated total precipitation (i.e., difference in the accumulated amount over N hours)
- Vertically integrated horizontal moisture flux ("magnitude_of_vertically_integrated_horizontal_transport_of_moisture")
- Requires surface_air_pressure, eastward_wind, northward_wind, specific_humidity
- Air pressure ("air_pressure")
- Requires air_temperature
- Reconstructs 3D pressure field from grid level specification
Source code links
Computation of derived variables is implemented in the "derivedvars" source code subdirectory:
...