Title: Precision of the ENDGame: Mixed-precision arithmetic in the iterative solver of the Unified Model,
Abstract: The Met Office's weather and climate simulation code the Unified Model is
used for both operational Numerical Weather Prediction and Climate modelling.
The computational performance of the model running on parallel supercomputers
is a key consideration. A Krylov sub-space solver is employed to solve the
equations of the dynamical core of the model, known as ENDGame. These describe
the evolution of the Earth's atmosphere. Typically, 64-bit precision is used
throughout weather and climate applications. This work presents a
mixed-precision implementation of the solver, the beneficial effect on run-time
and the impact on solver convergence. The complex interplay of errors arising
from accumulated round-off in floating-point arithmetic and other numerical
effects is discussed. A careful analysis is required, however, the
mixed-precision solver is now employed in the operational forecast to satisfy
run-time constraints without compromising the accuracy of the solution.