CAM and CLM: A Special Relationship

The Community Land Model (CLM2) may be run

in offline mode,
coupled with the Community Atmosphere Model (CAM2) directly, or
coupled with CCSM component models through the flux coupler (cpl5, soon to be replaced by cpl6).

When CAM2 is run in offline studies, CLM2 is usually run with it (see 2 above). Some land surface studies employing CLM2 use CAM2 in this same mode.

When run in this mode on parallel computers, the models run as a single executable, and share the same processors. Data (surface fluxes and states) are exchanged between the land and the atmosphere using the Message Passing Interface (MPI).

In CLM2.0 (or more correctly CAM2.0.1) these data exchanges occurred by way of a single MPI process; i.e., all surface data (including surface data over the oceans) were sent from the atmosphere physics to a single process which subsequently redistributed the land data to the appropriate MPI processes.

As model resolutions and processor counts increase, this bottleneck could consume a significant amount of run time.

Needed was a method of exchanging only the land surface fluxes and states directly between the appropriate MPI processes running CAM and CLM.