The Accumulation of Rounding Errors and Port Validation for Global Atmospheric Models

The growth of machine rounding-sized initial errors in the NCAR Community Climate Model (CCM2) is studied. Growth is faster than predictability error growth that results from turbulent flow. Rounding accumulation dominates the growth when temperature differences are below the order of 10-3 K. Discontinuous code branches are not a source of growth when the differences are less than 10-3 K. Arguments are presented to explain this observation. The fast growth of differences is caused by the physical parameterizations, as they respond to the evolving states produced by the dynamical flow. Based on a careful examination of this growth of small differences, a validation strategy for codes ported to new computing environments is offered and illustrated.