Computing the ecRad radiation scheme with half-precision arithmetic

Numerical simulations of weather and climate models are conventionally carried out using double-precision floating-point numbers throughout the vast majority of the code. At the same time, the urgent need of high-resolution forecasts given limited computational resources encourages development of much more efficient numerical codes. A number of recent studies has suggested the use of reduced numerical precision, including half-precision floating-point numbers increasingly supported by hardware, as a promising avenue. In this paper, the possibility of using half-precision calculations in the radiation scheme ecRad operationally used in the ECMWF’s Integrated Forecasting System (IFS). By deliberately mixing half-, single- and double-precision variables, we develop a mixed-precision version of the Tripleclouds solver, the most computationally demanding part of the radiation scheme, where reduced-precision calculations are emulated by a Fortran software rpe. By employing two tools that estimate the dynamic range of model parameters and identify problematic areas of the model code using ensemble statistics, the code variables were assigned particular precision levels. It is demonstrated that heating rates computed by the mixed-precision code are reasonably close to those produced by the double-precision code. Moreover, it is shown that using the mixed-precision ecRad in OpenIFS has a very limited impact on the accuracy of a medium-range forecast in comparison to the original double-precision configuration. These results imply that mixed-precision arithmetic could successfully be used to accelerate the radiation scheme ecRad and, possibly, other parametrization schemes used in weather and climate models without harming the forecast accuracy.