Mixed Seasonal and Subset Fourier Model with Seasonal Harmonics

In this work, Box-Jenkins seasonal model was fitted to a temperature series and the assumption of model adequacy was found to be violated. Subset Fourier series with seasonal harmonics was introduced and added to the pure seasonal component that was found to be inadequate. This combination resulted in a mixed seasonal and subset Fourier model with seasonal harmonics. The mixed model was fitted to the data and was subjected to diagnostic checks. The tests revealed that the model was adequate. Comparative study was also carried out and the results showed that the mixed model performed better than the pure seasonal and the subset Fourier model.