Meteorological forcing for offline CABLE only

The meteorological forcing is a collection of meteorological variables that need to be read into CABLE's met arrays when CABLE is run in offline (uncoupled) mode.

The filename is set using the CABLE namelist variable filename%met. The global offline simulations use multiple met files, one for each meteorological variable. The data must be in NetCDF format with the correct units, broadly conforming to the ALMA standard.

An example input file can be found at the NCI THREDDS server.

Number of land points in the simulation

The number of points simulated by offline CABLE is not specified in the code but is determined using the number of land points found in the meteorological forcing file. CABLE will consider the number of patches to determine the number of simulated points according to the patchfrac variable.

Model configuration and grid

Site or regional/global meteorological input file(s) need to follow the ALMA convention. The grid in the input file(s) can either be:

an x-y grid with 3 dimensions (x,y,t) or 4 dimensions (x,y,z,t), or
a compressed land-only grid of 2 dimensions (land, t)

x-y grid

An x and a y dimension variable must be present, even if the simulation is only a single site/gridpoint. Additionally, single precision variables named latitude and longitude (or nav_lat and nav_lon if using ALMA formatting), both dependent on the x and y dimensions only, must be present.

If both sea and land points are present, an integer mask(x,y) variable must be provided. A value of 1 for the mask implies a land gridpoint, anything else is assumed to be ocean.

Land-only compressed grid

More information on this format is available here. In this grid, a single spatial dimension is used. An example of the NetCDF header from such a file is shown below:

dimensions:
    tstep = UNLIMITED ; // (1461 currently)
    land = 15238 ;
    y = 180 ;
    x = 360 ;
variables:
    float SWdown(tstep, land) ;
        SWdown:axis = "TYX" ;
        SWdown:units = "W/m^2" ;
        SWdown:long_name="Surface incident shortwave radiation" ;
        SWdown:associate = "time (nav_lat nav_lon)" ;
        SWdown:missing_value = 1.e+20f ;
    int land(land) ;
        land:compress = "y x" ;
    float nav_lat(y, x) ;
        nav_lat:units = "degrees_north" ;
        nav_lat:valid_min = -90.f ;
        nav_lat:valid_max = 90.f ;
        nav_lat:long_name = "Latitude" ;
    float nav_lon(y, x) ;
        nav_lon:units = "degrees_east" ;
        nav_lon:valid_min = -180.f ;
        nav_lon:valid_max = 180.f ;
        nav_lon:long_name = "Longitude" ;
    float time(tstep) ;
        time:units = "seconds since 1949-01-01 00:00:00" ;
        time:title = "Time" ;
        time:long_name = "Time axis" ;
        time:time_origin = " 1949-JAN-01 00:00:00" ;

Most of the variables will be structured as SWdown is above. The relationship between land and x and y inside the CABLE NetCDF driver is:

y = INT((landGrid(j)-1)/xdimsize)
x = landGrid(j) - y*xdimsize
y = y + 1

From (cable_input.F90)[https://cable.readthedocs.io/en/latest/api/sourcefile/cable_input.f90.html#ln-633], where landGrid is the land variable above.

Variables

Required variables

Name	Description	Units
`time`	time of each time step (double precision)	\( s \)
`SWdown`	Surface incident shortwave radiation	\( W \cdot m^{-2} \)
`Tair`	Surface temperature	\( K \)
`Qair`	Near surface specific humidity	\( kg \cdot kg^{-1} \)
`Rainf`	Rainfall rate	\( mm \cdot s^{-1} \)
`Wind` or `Wind_E` and `Wind_N`	Scalar wind speed	\( m \cdot s^{-1} \)

Optional variables

Name	Description	Units
`LWdown`	Surface incident longwave radiation (can be synthesised from `Tair`)	\( W \cdot m^{-2} \)
`PSurf`	Surface air pressure (can be estimated at a fixed value based on Tair and an "`elevation`" variable	\( Pa \)
`elevation`	Surface elevation (required if `PSurf` is not present)	\( m \)
`Snowf`	Snowfall (assumed to be included in `Rainf` if not present)	\( mm \cdot s^{-1} \)
`CO2air`	CO\(_2\) concentration (assumes a fixed value, determined in the `cable.nml` namelist file)	\( ppm \)
`patchfrac`	The fraction of each vegetation patch in each land grid point. CABLE counts the number of "active" patches (patches with non-zero `patchfrac`) as the number of points for simulation.	\( - \)

Site-specific parameters

Site-specific parameters recorded in the met file will overwrite the default values obtained during initialisation and have top priority over values specified in other files. For example, the default value of za (reference height or measurement height) is 40 m; it will be overwritten by the value read from the meteorological file. The site-specific parameters include:

Name	Description	Units
`hc`	Vegetation height	\( m \)
`za`	Reference height	\( m \)
`iveg`	Vegetation type	\( - \)
`frac4`	C4 fraction	\( - \)

Metadata

All variables must have a "units" string attribute and the data must be in the correct units specified in the tables above.

Details on time and CABLE's time stepping

Meteorological input data must be continuous in time and have regular intervals.

CABLE's running period is deduced from the time period of the meteorological forcing data. The time variables units attribute is of the form seconds since <reference_time>, where the reference time can be any date on or before the starting date (e.g. 2001-02-22 00:00:00). For example, the first value of the time variable may be 86400, in which case the actual start time might be 2001-02-22 00:00:00 + 86400 seconds; i.e. 2001-02-23 00:00:00.

The time values for regional or global simulations are assumed to be GMT, while the value for single site/grid cell simulations is assumed to be "local". Single site simulation time values will be read as GMT if and only if a coordinate attribute is present for the time variable and set to be "GMT". This time coordinate system will be reported in the log file.

CABLE’s time step size is calculated from the first two values of the time variable, and the run length of the simulation is decided by the length of this same variable.

Checking ranges

It is possible to check that the ranges of the meteorological variables are physically possible using check%ranges=.TRUE. in the cable.nml namelist variable.