Variables
Dimension Variables
Some special variables have already been mentioned before when describing dimensions:
time and time_bnds for the measurement and time dimension
metadata_time and metadata_time_bnds for the metadata time dimension
Additional dimensions like Wavelength, D (particle diameter), Tower inlet height, etc.
Data variables
Each data variable in EBAS is represented by three NetCDF variables:
Measurement variable: The main variable containing the measurement values. Dimensions: time, optional: Additional dimensions
Flag variable: an additional variable containing the flag information for the variable. Dimensions: same as measurements + Flag dimensions
Metadata variable: a variable for changing metadata over time
Measurement variable
This is the main variable, containing the measurement values for an EBAS variable. The name for measurement variables is described here.
The dimensions are:
:ref`dim_time`
optional: Additional dimensions
The data type is double
.
Variable Attributes
The variable attributes contain the full set of EBAS metadata (constant over time for the variable). But the most important attributes for understanding the content of the variable are:
cell_methods
andebas_statistics
give information about the statistical interpretation (e.g. arithmetic mean, percentile:15.87, …)
ebas_component
: Component name in EBAS
ebas_matrix
: Matrix name in EBAS
units
andebas_unit
: Unit of the measurements
ancillary_variables
: reference to the associated Flag variable and Metadata variable
Flag variable
For each measurement variable a related flag variable is added which contains the flag information for the measurements.
Flag variable are named like the measurement variable plus a postfix _qc
.
The dimensions are:
Same dimensions as the measurement variable
An additional flag dimension called
<variable name>__qc_flags
The data type is int
.
Metadata variable
This additional variable implements Time depenendent EBAS metadata.
It contains metadata which change over time. This variable is
provided for each data variable.
The metadata variables are named like the main variable with a postfix _ebasmetadata
.
The dimensions are:
metadata_time the length of the dimension is 1 for static metadata and n + 1 for each metadata change over time within the file
The data type for the metadata variables is string and it contains a set of all ebas metadata for the specific time range as json encoded string.
Distinguishing Measurement variables, Flag variables and Metadata variables
Only the measurement variable has EBAS metadtaa as variable attributes
Only the measurement variable has a variable attributes
_metadata_variable
with references to the associated flag variable and metadata variableThe name of flag variables always ends with
_qc
(same name as measurement variable +_qc
)The variable attribute
standard_name
is set to``status_flag``The name of metadata variables always ends with
_ebasmetadata
(same name as measurement variable +_ebasmetadata
)
Variable names
Flag variables and metadata variables always use the same name as the measurement variables
plus a specific postfix (_qc
and _ebasmetadata
respectively).
The name for measurement variables is not straight forward, and may vary in different files depending on other variables in the file and related name conflicts.
Generally, the variable name will be set to the
ebas_component
nameIf there are name conflicts (variables with an identical name) and one or more of the following metadata distinguish the variables with identical names:
matrix
unit
statistics
additional dimensions
The respective element will be added to the name of all conflicting variables.
Example 1:
A file contains three variables with component name aerosol_light_scattering_coefficient_amean
,
one with ebas_statistics=arithmetic mean
, one with ebas_statistics=percentile:15.87
and one with ebas_statistics=percentile:84.13
The variables can be destinguished by adding information about the ebas_statistics to their name. Solution: The three variables will be called:
aerosol_light_scattering_coefficient_amean
aerosol_light_scattering_coefficient_prec1587
and
aerosol_light_scattering_coefficient_prec8413
.
And each of those variables will be accompanied by their _qc
and _ebasmetadata
variable (total 9 variables).
Example 2:
A file contains ozone
measurement both in the EBAS default unit ug/m3
and in a converted variable in nmol/mol
. Both are present as ebas_statistics=arithmetic mean
, ebas_statistics=min
, ebas_statistics=max
and ebas_statistics=stddev
.
The variables can only be distinguished by adding information on both the unit and the ebas_statistics to their names. Solution: The eight variables will be called:
ozone_ug_per_m3_amean
ozone_ug_per_m3_min
ozone_ug_per_m3_max
ozone_ug_per_m3_stddev
ozone_nmol_per_mol_amean
ozone_nmol_per_mol_min
ozone_nmol_per_mol_max
and
ozone_nmol_per_mol_stddev
And each of those variables will be accompanied by their _qc
and _ebasmetadata
variable (total 24 variables).
Warning
Never rely on variable names. As shown above, they might vary from file to file.
In order to find measurement variables in a file, use the variable attributes!
For example, search for a variable with the following combination of attributes:
ebas_component=ozone
ebas_statistics=arithmetic mean
ebas_unit=nmol/mol