Investigating the Impact of Mixed Precision on Correctness for a Large Climate Code

Earth system models (ESMs) are computationally expensive and represent many complex processes on a wide range of scales from molecular to global. Certain ESM computations require high precision while others, such as atmospheric microphysics (e.g., precipitation) which are approximated by bulk properties, should not. As such, atmospheric microphysics models are prime candidates for conversion to single precision, which afford distinct computational and memory advantages over typical double precision numbers. However, care must be taken as indiscriminate type casting to single precision can result in numerical instability and divergent output when applied naively. In this work we relate our experiences attempting to improve the performance of the Morrison-Gettelman microphysics package (MG2) in a popular ESM by modifying it to compute in single precision without sacrificing correctness. We find that modification of the entire MG2 package to compute with single precision floats achieves a respectable performance increase but does not appear to be correct in terms of maintaining consistency with double-precision MG2. On the other hand, narrowing the scope of our conversion to a couple expensive subprograms yields more satisfying results in terms of correctness but with negligible overall performance improvement. We evaluate correctness with both an objective statistical tool and traditional approaches more familiar to climate scientists. While we are still working toward our ultimate goal of improving the performance of MG2 without negatively affecting model output, we believe that our experiences may be helpful to other groups pursuing similar goals.