MITgcm Coupled 2.8 degree Atmosphere Ocean and Seaice Configuration - JMC Code L Baseline

These pages describe an intermediate complexity coupled configuration of MITgcm.

Current Configuration

The configuration described here is the ESMF JMC Code L Baseline configuration. This is a coupled atmosphere ocean simulation that uses MITgcm for both fluids. The isomorphic equation set that is used in both fluids is described in the Continuous Formulation section of the first chapter of the MITgcm release1 documentation. Additionally a thermodynamic ice model is included for sea-ice covered regions. Land surface processes are driven by climatologies of soil moisture, land temperature and albedo.

The fluid equations are stepped forward on a latitude longitude grid using the time stepping procedure outlined in the Time Stepping section of the second chapter of the MITgcm release1 documentation. The sections Spatial discretization of the dynamical eqautions and Tracer equations describe the form of the horizontal (latitude-longitude) and vertical (pressure in the atmosphere and height in the ocean) spatial gridding.

Computationally the system is configured as three separate executable programs that are derived from the MITgcm code base. Additional MITgcm packages are included in the coupled configuration to support sea-ice and surface boundary flux calculations and to support data transport between programs. The code structure for each program follows the outline give under the Browse Code tab of the MITgcm release1 web page. The approach to parallelism is described in the Software Architecture chapter of the MITgcm release1 documentation.

The directory structure for the contents of the downloaded source tree are described in a README file in that source tree. The layout and build procedures follow closely the arrangement described in the MITgcm documentation, however, for this configuration the code organization contains independent MITgcm code trees for each of the three separate executable components. The standard MITgcm genmake tool is used, but it is driven from a higher level build script described in the confiugration users guide.

Results

The downloaded configuration is set to simulate a ten-day period following a fifty year spin up. The figure below shows the monthly average of the ocean currents for the upper ocean (25m and 170m deep) at the start of the August immediatly prior to the fifty year pickup.


Velocity vectors for the top and third ocean levels (25m and 170m) after 49 years and 8 months of integration


Atmospheric winds for the same period at 950mb and 500mb are shown below.


Wind vectors from the lower and mid atmospheric levels (950mb and 500mb) after 49 years and 8 months of integration


Time to solution

The time to solution for a 17 processor run is measured as 120 seconds on the HP Alpha system at NASA Goddard ( halem.gsfc.nasa.gov ). This is the time for a ten-day simulation that starts from a spun-up fifty year state. The base time-step for this configuration is 450 seconds. In a ten day period all the elements of the coupled system are excercised many times so that the timing is representative of extended simulations. The time to solution has also been measured at other processor sizes in order to allow a scaling profile to be calculated.

Configuration Scaling

The latitude longitude configuration scaling has been tested to 33 processors on the HP Alpha at NASA Goddard (halem.gsfc.nasa.gov). The plot below shows the number of simulated years per day that the system currently achieves for different processor counts.

Scaling of latitude-longitude grid point scheme with processor count.

Throughput in years of coupled simulation per day for 2.8 degree couple configuration JMC Code L Baseline.


Future Development Plans

During the course of the ESMF project the coupled code that forms the basis for JMC Code L will evolve in all of the following areas These developments will affect both the scaling and the time to solution for the test problem in both its non-framework compliant and framework compliant forms.

 Authors: Stephanie Dutkiewicz, Jean-Michel Campin, Chris Hill.
                 Department of Earth, Atmospheric and Planetary Sciences, MIT