3 Creating and Running the Executable

The CLM2.1 model can be built to run in one of three modes. It can run as a stand alone executable where atmospheric forcing data is periodically read in (e.g., using the data in NCEPDATA). This will be referred to as offline mode. It can also be run as part of the Community Atmosphere Model (CAM) where communication between the atmospheric and land models occurs via subroutine calls. This will be referred to as cam mode. Finally, it can be run as a component in a system of geophysical models (CCSM). In this mode, the atmosphere, land, ocean and sea-ice models are run as separate executables that communicate with each other via the CCSM flux coupler. This will be referred to as ccsm mode.

Clm2.1 may be run serially (i.e., on a single processor), in parallel using the Message Passing Interface (MPI) for distributed memory tasks, in parallel using the Open Multi-Processing (OpenMP) directives for shared memory tasks, or finally in parallel using both MPI and OpenMP (hybrid parallelism). For example, the IBM SP consists of distributed memory nodes interconnected by a high performance network connection, and each node contains multiple shared memory processors. When run on the IBM SP, CLM2.1 uses OpenMP directives for parallelism on processors within a shared memory node and MPI routines for parallelism across distributed memory nodes to take full advantage of the capabilities of the hardware. The configurations supported on each architecture, along with the model run modes, are shown in Table 3.

Table 3: Supported Architectures

Architecture		Run Modes			Configurations
Hardware	OS	offline	cam	ccsm	serial	MPI	OpenMP	Hybrid
IBM SP	AIX	X	X	X	X	X	X	X
SGI	IRIX64	X	X	X	X	X	X
Intel	Linux	X	X		X	X	X
Compaq	OSF1	X	X		X	X	X	X
Sparc	SunOS	X	X		X	X	X

The method of building and running CLM2.1 depends on the selected mode as well as the target architecture. A general discussion of the various aspects of building and running CLM2.1 follows.