Variable names

To allow HARP to perform automatic operations on variables, it imposes a strict naming convention for variables. This naming convention applies to the variable name itself and is therefore fully complementary to naming conventions that apply to the value of a variable attribute, such as standard_name (as specified by netCDF-CF).

Note that it is possible to use variables inside HARP products with names that do not follow the convention, but then these variables may not be handled correctly by operations that you perform on the product. The general rule is that if you have a quantity that can be represented by the naming convention below then you should use the HARP variable name for it.

HARP defines the following variable names:

Name	Prefixes	Postfixes	Quality	Vert	Lat/Lon	Spect	Comments
absolute_vorticity			X	X	X
absorbing_aerosol_index			X		X
aerosol_base_height			X		X
aerosol_base_pressure			X		X
aerosol_extinction_coefficient	surface		X	X	X	X
aerosol_height			X		X
aerosol_optical_depth	stratospheric, tropospheric		X	X	X	X	this is equal to ‘aerosol optical thickness’
aerosol_pressure			X		X
aerosol_top_height			X		X
aerosol_top_pressure			X		X
<aerosol_type>_aerosol_base_height			X		X
<aerosol_type>_aerosol_base_pressure			X		X
<aerosol_type>_aerosol_extinction_coefficient	surface		X	X	X	X
<aerosol_type>_aerosol_height			X		X
<aerosol_type>_aerosol_optical_depth	stratospheric, tropospheric		X	X	X	X	this is equal to ‘aerosol optical thickness’
<aerosol_type>_aerosol_pressure			X		X
<aerosol_type>_aerosol_top_height			X		X
<aerosol_type>_aerosol_top_pressure			X		X
altitude	sensor, surface		X	X	X
altitude_bounds			X	X	X
area			X				the size of an area defined by latitude/longitude bounds
backscatter_coefficient	surface		X	X	X	X
cloud_albedo			X		X
cloud_base_albedo			X		X
cloud_base_height			X		X
cloud_base_pressure			X		X
cloud_base_temperature			X		X
cloud_fraction			X		X
cloud_height			X		X
cloud_optical_depth			X		X		this is equal to ‘cloud optical thickness’
cloud_pressure			X		X
cloud_temperature			X		X
cloud_top_albedo			X		X
cloud_top_height			X		X
cloud_top_pressure			X		X
cloud_top_temperature			X		X
collocation_index							zero-based index as provided in the collocation result file
column_density	stratospheric, tropospheric	amf, apriori, avk, dfs	X	X	X		this is the mass density
column_number_density	stratospheric, tropospheric	amf, apriori, avk, dfs	X	X	X
count
datetime
datetime_length
datetime_start
datetime_stop
density			X	X	X		this is the mass density
extinction_coefficient	surface		X	X	X	X
frequency			X
frequency_irradiance			X			X
frequency_photon_irradiance			X			X
frequency_photon_radiance			X			X
frequency_photon_transmittance			X			X
frequency_radiance			X			X
frequency_transmittance			X			X
geoid_height			X		X
geopotential	surface		X	X	X
geopotential_height	surface		X	X	X
hlos_wind_velocity	surface		X	X	X		hlos means ‘horizontal line of sight’
index							zero-based index of the sample within the source product
integration_time				X	X	X	provides measurement specific integration time (at e.g. altitude or wavelength) compared to overal datetime_length; only use if integration time differs from datetime_length; integration_time longer than datetime_length that covers multiple datetime values means replication of measured value in time dimension
latitude	sensor		X		(lat)
latitude_bounds					(lat)
longitude	sensor		X		(lon)
longitude_bounds					(lon)
molar_mass			X	X	X		this is the molar mass of the total substance (it is defined by the relation between the variables ‘density’ and ‘number_density’)
month							category variable for month of year (‘January’, ..., ‘December’)
number_density	surface		X	X	X
optical_depth			X	X	X	X	this is equal to ‘optical thickness’
orbit_index							the absolute orbit number for data from polar orbiting satellites
pressure	surface		X	X	X
pressure_bounds			X	X	X
radiance			X			X
reflectance			X			X
relative_azimuth_angle			X				absolute difference between sensor and solar azimuth angles
relative_humidity			X	X	X
relative_vorticity			X	X	X
scan_direction_type
scan_subindex
scattering_angle			X
sensor_azimuth_angle			X
sensor_elevation_angle			X
sensor_name							used mainly for ground based networks to provide a unique sensor id
sensor_zenith_angle			X
site_name							used for data of a specific named geographical location
solar_azimuth_angle	sensor, surface, toa		X
solar_declination_angle
solar_elevation_angle	sensor, surface, toa		X
solar_hour_angle
solar_irradiance			X			X
solar_zenith_angle	sensor, surface, toa,		X
sun_normalized_radiance			X			X
surface_albedo			X		X	X
temperature	surface		X	X	X
tropopause_altitude			X		X		altitude of the troposphere/stratosphere boundary location
tropopause_pressure			X		X		pressure level of the troposphere/stratosphere boundary location
validity							validity flag for each time sample or whole product; only to be used if validity flag is for multiple variables combined
viewing_azimuth_angle			X
viewing_elevation_angle			X
viewing_zenith_angle			X
virtual_temperature			X	X	X
wavelength			X			X
wavelength_irradiance			X			X
wavelength_photon_irradiance			X			X
wavelength_photon_radiance			X			X
wavelength_photon_transmittance			X			X
wavelength_radiance			X			X
wavelength_transmittance			X			X
wavenumber			X			X
wavenumber_irradiance			X			X
wavenumber_photon_irradiance			X			X
wavenumber_photon_radiance			X			X
wavenumber_photon_transmittance			X			X
wavenumber_radiance			X			X
wavenumber_transmittance			X			X
wind_speed	surface		X	X	X
wind_direction	surface		X	X	X
year							integer value representing a year
<species>_column_density	stratospheric, tropospheric	amf, apriori, avk, dfs	X	X	X		this is the mass density
<species>_slant_column_density			X		X		this is the mass density
<pm>_column_density	stratospheric, tropospheric		X	X	X		this is the mass density
<species>_column_number_density	stratospheric, tropospheric	amf, apriori, avk, dfs	X	X	X
<species>_slant_column_number_density			X		X
<species>_column_mass_mixing_ratio	stratospheric, tropospheric		X		X
<species>_column_mass_mixing_ratio_dry_air	stratospheric, tropospheric		X		X
<species>_column_volume_mixing_ratio	stratospheric, tropospheric		X		X
<species>_column_volume_mixing_ratio_dry_air	stratospheric, tropospheric		X		X
<species>_density	surface		X	X	X		this is the mass density
<pm>_density	surface		X	X	X		this is the mass density
O3_effective_temperature			X		X
<species>_mass_mixing_ratio	surface	apriori, avk, dfs	X	X	X
<species>_mass_mixing_ratio_dry_air	surface	apriori, avk, dfs	X	X	X
<species>_number_density	surface	apriori, avk, dfs	X	X	X
<species>_partial_pressure	surface		X	X	X
<species>_partial_pressure_dry_air	surface		X	X	X
<species>_volume_mixing_ratio	surface	apriori, avk, dfs	X	X	X		this is equal to ‘number mixing ratio’
<species>_volume_mixing_ratio_dry_air	surface	apriori, avk, dfs	X	X	X

The supported aerosol types are:

Aerosol type	Description
sea_salt	sea salt
dust	dust
organic_matter	organic matter
black_carbon	black carbon
sulphate	sulphate

The supported PM (particulate matter) types are:

Name	Description
PM1	particulate matter with d < 1 um
PM2p5	particulate matter with d < 2.5 um
PM10	particulate matter with d < 10 um

The supported species are:

Name	Description	Aliases (not used by HARP)
dry_air	dry air
BrO	bromine oxide
BrO2	bromine dioxide
CCl2F2	dichlorodifluoromethane	freon-12, CFC-12, R-12, F12
CCl3F	trichlorofluoromethane	freon-11, CFC-11, R-11, F11
CCl4	tetrachloromethane
CF4	tetrafluoromethane	CFC-14, F14
CHClF2	chlorodifluoromethane	HCFC-22, R-22, F22
CH3Cl	chloromethane, methyl chloride	HCC-40, R-40
CH3CN	acetonitrile, methyl cyanide
CH3OH	methanol
CH4	methane
CO	carbon monoxide
COF2	carbonyl fluoride
COS	carbonyl sulfide	OCS
CO2	carbon dioxide
C2H2	acetylene	HCCH
C2H2O2	glyoxal	OCHCHO, CHOCHO
C2H3NO5	peroxyacetyl nitrate	PAN
C2H6	ethane
C3H8	propane
C5H8	isoprene
ClNO3	chlorine nitrate
ClO	chlorine monoxide
HCHO	formaldehyde	CH2O, H2CO
HCOOH	formic acid	HCO2H
HCN	hydrogen cyanide
HCl	hydrogen chloride
HF	hydrogen fluoride
HNO2	nitrous acid
HNO3	nitric acid
HNO4	peroxynitric acid
HOCl	hypochlorous acid
HO2	hydroperoxyl
H2O	water
H2O_161	water (H1/O16/H1 isotopes)
H2O_162	water (H1/O16/H2 isotopes)	HDO
H2O_171	water (H1/O17/H1 isotopes)
H2O_181	water (H1/O18/H1 isotopes)
H2O2	hydrogen peroxide
IO	hypoiodite
IWC	ice water content; H2O in ice state
LWC	liquid water content; H2O in liquid state
NH3	ammonia
NO	nitric oxide
NOCl	nitrosyl chloride
NO2	nitrogen dioxide
NO3	nitrate
N2	nitrogen gas
N2O	nitrous oxide	NOS
N2O5	dinitrogen pentoxide
OClO	chlorine dioxide	ClO2
OH	hydroxyl
O2	oxygen
O3	ozone
O3_666	ozone (O16/O16/O16 isotopes)
O3_667	ozone (O16/O16/O17 isotopes)
O3_668	ozone (O16/O16/O18 isotopes)
O3_686	ozone (O16/O18/O16 isotopes)
O4	tetraoxygen, oxozone
SF6	sulfur hexafluoride
SO2	sulfur dioxide

Variables for which a prefix and/or postfix is provided can have any of the given prefixes and/or any of the given postfixes (separated by underscores). It is not allowed to provide more than one prefix or more than one postfix. Variables having an ‘X’ in the Quality column can have any of the following additional versions of the variable (where <variable> can include any of the allowed prefix and/or postfix combinations):

• <variable>_covariance
• <variable>_uncertainty
• <variable>_uncertainty_random
• <variable>_uncertainty_systematic
• <variable>_validity

Some examples of valid variable names are: tropospheric_O3_column_number_density, tropospheric_O3_column_number_density_apriori, O3_column_number_density_apriori, tropospheric_O3_column_number_density_uncertainty, O3_column_number_density_apriori_uncertainty.

The Vert, Lat/Lon, and Spec columns indicate whether a variable can be dependent on the vertical, latitude & longitude, and/or spectral dimensions (any variable can be dependent on the time dimension).

The ‘surface’ prefix should only be used when quantities are combined together with quantities that have a vertical dimension. If a product just contains surface quantities then don’t use a ‘surface’ prefix but just omit the vertical dimension and indicate the vertical level (i.e. location of the surface) using a ‘pressure’, ‘altitude’, and/or ‘geopotential_height’ variable.

All (horizontal) azimuth angles in HARP should follow the convention that 0 is North facing and the angle is increasing when moving Eastwards (i.e. clockwise). Wind direction follows the same rules as for azimuth angles (North = 0, East = 90 degrees), but the direction indicates where the wind is coming from.

In addition to the conventions above there can also be variables that describe a ‘difference’. These difference variables can only be used to describe differences of the same quantity between different datasets (‘x’ and ‘y’) and only for variables that have a unit. All difference variables in a single product should apply to the same datasets ‘x’ and ‘y’ (i.e. the difference variables should only reflect a single comparison of datasets; you should not combine one difference variable for ‘x-y’ and another for ‘x-z’ (even for different quantities) within the same product). A difference variable is indicated by a postfix. The ‘difference postfix’ can come before a ‘quality postfix’ if we are talking about the ‘quality of the difference’. If the ‘difference postfix’ comes after a ‘quality postfix’ then we are talking about the ‘difference of the quality quantity’. The supported differences are:

• <variable>_diff (\(x-y\))
• <variable>_diffrelx (\(\frac{x-y}{x}\))
• <variable>_diffrely (\(\frac{x-y}{y}\))
• <variable>_diffrelmin (\(\frac{x-y}{\min(x,y)}\))
• <variable>_diffrelmax (\(\frac{x-y}{\max(x,y)}\))
• <variable>_diffrelavg (\(\frac{2(x-y)}{x+y}\))
• <variable>_diffabs (\(|x-y|\))
• <variable>_diffabsrelx (\(\frac{|x-y|}{|x|}\))
• <variable>_diffabsrely (\(\frac{|x-y|}{|y|}\))
• <variable>_diffabsrelmin (\(\frac{|x-y|}{\min(|x|,|y|)}\))
• <variable>_diffabsrelmax (\(\frac{|x-y|}{\max(|x|,|y|)}\))
• <variable>_diffabsrelavg (\(\frac{2|x-y|}{|x+y|}\))

The postfix ‘avk’ is used for averaging kernels of atmospheric vertical profiles. An AVK that only depends once on the vertical dimension is a column averaging kernel, and an AVK that depends twice on the vertical dimension is a profile averaging kernel. The ‘amf’ postfix is used for air mass factors. The ‘dfs’ postfix is used for the ‘degree of freedom for signal’ for vertical profiles which equals the trace or diagonal of the two-dimensional AVK and provides information on the vertical resolution and information content of profiles.