5 Data Exchanged with the Coupler

Each component model exchanges data with the coupler only. Component models have no direct connection with each other - all data is routed through the coupler. Most data is in the form of 2D fields. This data is accompanied by certain timing and control information (arrays of scalar real or integer values), such as the current simulation data and time.

5.1 Units Convention

All data exchanged conforms to this units convention:

Sign convention:
     positive value <=> downward flux

Unit convention:
     temperature   ~ Kelvin
     salt          ~ g/kg 
     velocity      ~ m/s
     pressure      ~ N/m^2 = Pa
     humidity      ~ kg/kg
     air density   ~ kg/m^3
     momentum flux ~ N/m^2
     heat flux     ~ W/m^2
     water flux    ~ (kg/s)/m^2
     salt flux     ~ (kg/s)/m^2
     coordinates   ~ degrees north or east
     area          ~ radians^2
     domain mask   ~ 0 <=> an inactive grid cell

5.2 Time Invariant Data

This section provides a list of the time invariant data exchanged between the coupler and each component model. Generally this data is the "domain" data: coordinate arrays, domain mask, cell areas, etc. It is assumed that the domain of all models is represented by a 2D array (although not necessarily a latitude/longitude grid).

5.2.1 Data Sent to Coupler

     domain data
   * grid cell's center coordinates, zonal      (degrees north)
   * grid cell's center coordinates, meridional (degrees east)
   * grid cell's four vertex coordinates, zonal      (degrees north)
   * grid cell's four vertex coordinates, meridional (degrees east)
   * grid cell area (radians squared)
   * grid cell domain mask ( 0 <=> not in active domain)
   * ni,nj: the dimensions of the underlying 2D array data structure

     time coordination data
   * ncpl: number of times per day the component will communicate (exchange 
     data) with the coupler.

     other information
   * IC flag: indicates whether the coupler should use model IC's contained
     on the coupler's restart file or IC's in the initial message sent from
     the component model.

5.2.2 Data Received from Coupler

     time coordination data
   * date, seconds: the exact time the coupler will start the simulation from.

5.3 Time Variant Data

This section provides a list of the time-evolving data sent exchanged between the coupler and the data model. Generally this is state, flux, and diagnostic quantities.

Each data model provides the coupler with a set of output fields. Output fields from a model include output states (which can be used by another component to compute fluxes) and output fluxes (fluxes that were computed within the model and which need to be exchanged with another component model.

The coupler provides each component model with input fields. Input fields sent to a model include input states (the state variables of other models, which are needed to do a flux calculation) and input fluxes (a forcing fields computed by some other component).

Flux fields sent to or from the coupler are understood to apply over the communication interval beginning when the data was received and ending when the next message is received. The data models must insure that fluxes sent to the coupler are appropriate in this context.

5.3.1 Data Sent to Coupler

     states
   * surface temperature (Kelvin)
   * albedo: visible      , direct  
   * albedo: near-infrared, direct  
   * albedo: visible      , diffuse 
   * albedo: near-infrared, diffuse
   * snow depth (m)

     fluxes
   * zonal      surface stress  (N/m^2)
   * meridional surface stress  (N/m^2)
   * latent heat                (W/m^2)
   * sensible heat              (W/m^2)
   * longwave radiation, upward (W/m^2)
   * evaporation    ((kg/s)/m^2)
   * coastal runoff ((kg/s)/m^2)

     diagnostic quantities
   * 2 meter reference air temperature (Kelvin)

5.3.2 Data Received from Coupler

     states
   * atm layer height (m)
   * atm zonal      velocity (m/s)
   * atm meridional velocity (m/s)
   * atm potential temperature (Kelvin)
   * atm specific humidity (kg/kg)
   * atm pressure (Pa)
   * atm temperature (Kelvin)

     fluxes
   * precipitation: liquid, convective  ((kg/s)/m^2)
   * precipitation: liquid, large-scale ((kg/s)/m^2)
   * precipitation: frozen, convective  ((kg/s)/m^2)
   * precipitation: frozen, large-scale ((kg/s)/m^2)
   * longwave  radiation, downward (W/m^2)
   * shortwave radiation: downward, visible      , direct  (W/m^2)
   * shortwave radiation: downward, near-infrared, direct  (W/m^2)
   * shortwave radiation: downward, visible      , diffuse (W/m^2)
   * shortwave radiation: downward, near-infrared, diffuse (W/m^2)

5.3.3 How Output Fields are Derived

Data from the input data sequence is assumed to be monthly average data. This data is linearly interpolated in time to get instantaneous fields, and this data is sent to the coupler. All data sent to the coupler is taken directly from the input data sequence, which these exceptions:

1. It is assumed that the dlnd5 data files contain the 19 basins of runoff data that were used in the lsm land model found in CCSM 1.x, and that no new data files exist that were created using newer land models or using the newer 1x1 degree runoff grid scheme found in CCSM 2.0. Thus, by default, runoff sent to the coupler is a field consisting of all zeros. See the data_aroff namelist parameter for other options.

2. It is assumed that the 2 meter reference air temperature data is not in the input data files. The reference temperature field is set to 300.0 degrees K, and this field is sent to the coupler.