Benefits of Stochastic Weight Averaging in Developing Neural Network Radiation Scheme for Numerical Weather Prediction

Stochastic weight averaging (SWA) was applied to improve the radiation emulator based on a sequential neural network (SNN) in a numerical weather prediction model over Korea. While the SWA has advantages in terms of generalization such as the ensemble model, the computational cost is maintained at the same level as that of a single model. In this study, the performances of both emulators were evaluated under ideal and real case frameworks. Various sensitivity experiments using different sampling ratios, activation functions, hidden layers, and batch sizes were also conducted. The emulators showed a 60-fold speedup for the radiation processes and 84%-87% reduction of the total computation. In the ideal simulation, compared to the infrequent radiation scheme by 60 times, SNN improved forecast errors by 5.8%-14.1%, and SWA further increased these improvements by 18.2%-26.9%. In the real case simulation, SNN showed 8.8% and 4.7% improvements for longwave and shortwave (SW) fluxes compared to the infrequent method; however, these improvements decreased significantly after 5 days, resulting in 1.8% larger error for skin temperature. By contrast, SWA showed stable 1-week forecast features with 12.6%, 8.0%, and 4.4% improvements in longwave and SW fluxes, and skin temperature, respectively. Although the use of two hidden layers showed the best performance in this study, it was thought that the optimal number of hidden layers could differ depending on the given problem. Compared to temperature and precipitation observations, all experiments showed a variability of error within 1%, implying that the operational use of the developed emulators is possible.