General comments on the conventions

General comments on the conventions

Variable names

The short variable names given in the tables have been selected to be unique and easy to understand. In order to easily analyze results from multiple models it is necessary that all output is returned with the same variable names. However, we recognize that to change the name of output variables in an existing model may be very difficult as it changes habits of the users. The program "ncrename" of the NCO package can be used to rename variable in the netCDF output file before it is send to the inter-comparison centre. In any case the quality screening program will prompt the user to provide alias names for variables when the standard ALMA variables are not encountered. The aliased variables will then be automatically replaced with the ALMA standard in the netCDF file to be returned for analysis.

Variable description

This column provides the long name of the variable which describes properly the content. This description should be included in the netCDF files as it is much more explicit than just the name of the variable. The long name depends much less than the variable names on personal preferences.

netCDF attribute for variable description : long_name

Choice of units

Much thought and discussion has gone into the choice of a standard set of units which will be logically defined and easy to analyze. We have decided that it is most appropriate to use SI units in all cases. Since in some cases the choice of SI units is not intuitive a program is provided which can be used to transfer between units.

netCDF attribute for variable units : units

Sign Convention

For the convenience of the user, it has been decided to allow the choice of sign convention between two options. Each scheme must consistently follow one of these conventions. For the 'traditional approach', all variables are positive in their 'dominant' direction, i.e. precipitation, and radiation positive towards the surface; evaporation, sensible heat and runoff are positive away from the surface. For the 'mathematical approach', the sign is chosen so water and energy fluxes will balance, i.e. precipitation, and radiation are positive towards the surface; evaporation, sensible heat and runoff are negative away from the surface. A netCDF global attribute is used to specify in the file the sign convention used. The quality check software will use this attribute to evaluate the data. The unit conversion program also allows for the sign change of variables.

netCDF global attribute for sign convention : SurfSgn_convention

Priority of Variables

This column indicates the level of importance associated with each variable. Variables considered "Mandatory" must be included for all LSS inter-comparison projects. It is encouraged that "Recommended" variables be included for most inter-comparisons. Those variables marked "Optional" refer to properties which may only be significant in some climates or for some applications and therefore may not be used for all inter-comparison projects. Each project within GLASS may choose to make any of the recommended or optional output variables mandatory for its inter-comparison. This depends on the type of analysis which is intended during the project. The mandatory variables on the other hand should always be provided as they allow to check the general behavior of the scheme.

Average surface temperature and Surface Radiative Temperature

There is a small but important distinction to be made between these two variables. The average surface temperature is the mean of all temperatures resulting from the energy balances performed by the schemes within the grid-box. This averaging process should be representative of the modeled interactions of the surface with the lower variables of the atmosphere. It thus needs to take into account the assumptions made in the land-surface scheme on the fractions of vegetation present in the grid-box as well as the way the relation between the ground and vegetation temperatures are considered for the calculation of the turbulent fluxes to the atmosphere.

The radiative temperature, on the other hand, is the average of all temperatures used by the model to compute the outgoing long-wave flux of the grid-box. Obviously this averaging process needs to take into account the fact that the outgoing long-wave flux has to be conserved. It is thus different from the one for the surface temperature as the T4 variables needs to be averaged. Other differences between both temperatures may arise from the numerical implementation of the energy balance. It is not given that the same temperature is used to compute the turbulent flux and the long-wave radiation. Thus the distinction between both temperatures is needed but very simple model may not show any difference between both values.

Surface Runoff

This variable should include the fast component of runoff response. This includes any surface runoff generated as infiltration excess runoff or saturation excess runoff (or the parameterized equivalent). In addition it should include any subsurface lateral quick-flow, which based on modelers experience is best routed as a fast storm-flow component, rather than a drainage or base-flow component.


This variable represents streamflow which leaves the channel network and returns to the grid cell interface. The sign will always be opposite to that of surface runoff and the recharge quantity should not be subtracted from the surface runoff variable when recharge is specified.

Subsurface Runoff

This variable should include the slow drainage component of runoff response. This includes gravity drainage and lateral base-flow.

Change in Storage

These variables represent the accumulated change in storage from the beginning of each archiving time step to the end. These are the quantities which are required to resolve the water balance at any time scale. They differ from the storage terms (SurfStor, SoilMoist, etc.) which represent the average storage for each time step.

Layer Average Values (SoilMoist and SoilTemp)

These variables should contain the average soil temperature for each simulated soil layer. The thickness and number of layers may not be identical for all schemes for all applications. In the case of a large number of simulated layers, participants may choose to combine results for some layers. For models which do not explicitly track soil layer boundaries it is left to the discretion of the modeler to define boundaries, or to return only one depth-averaged quantity.

Liquid Moisture Content

This variable should contain the average non-frozen moisture content for each simulated soil layer, as described above. If soil freezing is not represented, this quanity will be equal to SoilMoist.

Total Soil Wetness

To be calculated as the total simulated soil moisture (minus the soil moisture at wilting point) divided by the maximum allowable soil moisture (minus the soil moisture at wilting point). This will give an indication of the degree of saturation of the soil and allow calculation of the maximum available soil moisture, in conjunction with the total simulated soil moisture provided in table O.2.


Sea-ice refers to all ice, whether formed on oceans, seas or lakes. The sea-ice fraction is given relative to the area of the entire grid cell, even if only a portion of the grid cell contains oceans or lakes.

Rank of variable (3D Variables)

Although the atmospheric state variables needed to force land-surface schemes are two dimensional, it was chosen to save them in arrays of rank 3. This allows to specify the vertical dimension even if its size is equal to one. On this axis the hight of the atmospheric variables will be specified. There are two advantages to including the vertical axis : First the height of the variable does not need to be provided outside of the forcing data file and it can evolve over time, secondly if for future land-surface scheme inter-comparisons data on more than one atmospheric level will be provided the format does not need to change.

Expected range for the values

Variables describing the carbon cycle at the surface

The total Net assimilation of carbon by the vegetation. This variable is given by the mean gross assimilation minus the dark respiration. It should be averaged over all vegetation types within a grid cell.
Net primary production must be equal to GPP - AutoResp. Averaged over all vegetation types within a grid cell. l
Net Ecosystem Exchange sums all carbon fluxes exchanged between the surface and the atmosphere. It represents at least AutoResp + HeteroResp - GPP. But outgoing NEE can also contain others fluxes like for instance CO2 from fires. By convention it is negative when the outgoing flux (GPP) is greater than the incoming flux. This variable should also be averaged over all vegetation types within a grid cell
The total autotrophic respiration includes maintenance respiration and growth respiration. It can include others terms for very advanced models which for instance calculate respiration from ion uptake. This variable must be positive in the traditional sign convention and be a deficit for the surface.
The total flux from decomposition of organic matter. This include fluxes from soil and litter. It must also be positive in the traditional sign convention and be a deficit for the surface.
Total soil and litter carbon content integrated over the entire soil profile. This term must contain only carbon from dead material (not from roots for instance).
Total carbon content of living biomass. This include above and below ground biomass (e.g leaves, fine roots, coarse roots, heathwood, sapwood etc...). This variable needs to be averaged over all vegetation types within a grid cell.

Last modified: Wed Feb 20 12:36:42 CET 2002