4.2.6.  DATA_VARIABLES

The global attribute DATA_VARIABLES lists all data variables reported in the current data file. The list is a succession of fields in the DATA_VARIABLES entry; in other words, the entry consists of one field per variable. Generally, a field consists of the variable name, the variable modeand the variable descriptor, separated by underscores. 

Type: STRING

Entry: Arbitrary number of semicolon-separated fields (each field constructed according to the format below)

Format: Variable name[_Variable mode][_Variable descriptor]

Example 1: DATA_VARIABLES = DATETIME; LATITUDE; LONGITUDE; ALTITUDE.GPH;  O3.PARTIAL.PRESSURE_INSITU; PRESSURE_INSITU; TEMPERATURE_INSITU;HUMIDITY.RELATIVE_INSITU; INTERNAL.BOX.TEMPERATURE_INSITU

Example 2: DATA_VARIABLES = DATETIME;LATITUDE.INSTRUMENT;  LONGITUDE.INSTRUMENT; ALTITUDE.INSTRUMENT; ALTITUDE; H2O.COLUMN_ABSORPTION.SOLAR where ALTITUDEis an independent parameter and H2O.COLUMN_ABSORPTION.SOLAR is dependent on DATETIME and ALTITUDE.

The variables DATETIME, ALTITUDE (.INSTRUMENT) or any other vertical coordinate, LATITUDE (.INSTRUMENT) and LONGITUDE(.INSTRUMENT) variables are always mode-lessand descriptor-less. All other variables can have an optional modeand an optional descriptoras appropriate.

With some exceptions (cf. Section 4.2.6.4) variable names first refer to a geophysical entity followed by a physical property.

4.2.6.1.  Variable name

The variable nameis a basic identification of the data parameter described in the dataset, e.g., the geophysical entity that is reported. The name includes the chemical or physical identification of the reported entity. A typical example of a variable name is the column amount of ozone: O3.COLUMN

4.2.6.2.  Variable mode

The variable mode provides the information on the measurement/calculation method that can lead to differences when comparing to results obtained with other methods to observe/calculate the same quantity. It is optionalin the construction of every field in the DATA_VARIABLES entry. 

Examples related to ozone observations compliant with Table 7.2 are:

O3.COLUMN.SLANT_ABSORPTION.SOLAR

O3.COLUMN_ABSORPTION.SOLAR

4.2.6.3.  Variable descriptor

The variable descriptoris added only to construct variables that provide additional information about a primary variable or that describe some particular property of that variable. For instance for the ozone examples above, we may add additional variables as follow:

O3.COLUMN.SLANT_ABSORPTION.SOLAR_UNCERTAINTY.STDEV

O3.COLUMN_ABSORPTION.SOLAR_UNCERTAINTY.STDEV

The descriptor is not intended to distinguish subsets of a dataset. Such dataset distinctions should be made by

providing additional dependent or independent parameters, as outlined in the following examples:

Example 1:

The ozone columns obtained by SAOZ measurements are traditionally distinguished in two

subsets: measurements at dawn and measurements at dusk. The solar azimuth angle is the

relevant parameter distinguishing these measurements and should be provided together with

every measurement of the ozone column. 

Example 2:

Irradiance/radiance measurements are often performed at specific wavelengths. Wavelength

should therefore be an independent parameter if values at more than one wavelength are

reported.

Example 3:

For an instrument capable of measuring in alternating modes, for example zenith sky  vs.direct

sun, the data taken in these modes should be reported as distinct variables constructed with

different variable modes.

O3.COLUMN.SLANT_ZENITH

O3.COLUMN.SLANT_ABSORPTION.SOLAR

4.2.6.4.  Special variables

PRESSURE, TEMPERATURE, SURFACE.TEMPERATURE, DENSITY, and HUMIDITY:

Special rules apply for variable names referring to air. The variable name to be used does not explicitly carry the AIR keyword, but implicitly refers to air, e.g. TEMPERATUREinstead of AIR.TEMPERATURE. Note, SURFACE.TEMPERATURErefers the the air temperature above the surface. In case another medium than  AIRis measured, the medium or identifier will be on the first position, e.g. INTERNAL.BOX.TEMPERATURE, ICE.TEMPERATURE, or LAND.SURFACE.TEMPERATURE. 

The same approach is valid for gases mixed in air, e.g. CH4.MIXING.RATIO. The only variable still carrying the AIRkeyword is AIR.MASS. DATETIME, LATITUDE, LONGITUDE, ALTITUDE[.GPH], PRESSURE, DEPTH: 

These variable names refer to effective geolocation (mid-time/point) of a reported parameter.

4.2.6.5.  Geolocation

Every data file mustcontain a specification of geolocation in three or four dimensions(cf. Section 2.3), specifying when and where the reported data is sampled. In other words, the  DATETIME variable, as well as latitude and longitude are mandatorywith a vertical geolocation variable only mandatory if the measurement has been taken in the corresponding dimension.The vertical geolocation should be expressed as ALTITUDE or DEPTH. If ALTITUDE is not available, acceptable substitutes are PRESSUREand ALTITUDE.GPH 

(geopotential height), etc. If no altitude parameter is measured, the altitude of the instrument position should be given, if it makes sense for the observation, e.g. ALTITUDE.INSTRUMENT.

The geolocation variables must comply with stringent naming criteria. Acceptable combinations of geolocation

variables are:

• DATETIME;LATITUDE;LONGITUDE;[ALTITUDE|ALTITUDE.GPH|PRESSURE]

• DATETIME;LATITUDE;LONGITUDE;DEPTH

• DATETIME;LATITUDE.INSTRUMENT;LONGITUDE.INSTRUMENT;

ALTITUDE.INSTRUMENT;[LATITUDE;LONGITUDE;

[ALTITUDE|ALTITUDE.GPH|PRESSURE]]