Multi-scale Incremental Analysis Update Scheme and Its Application to Typhoon Mangkhut (2018) Prediction

In the traditional incremental analysis update (IAU) process, all analysis increments are treated as constant forcing in a model’s prognostic equations over a certain time window. This approach effectively reduces high-frequency oscillations introduced by data assimilation. However, as different scales of increments have unique evolutionary speeds and life histories in a numerical model, the traditional IAU scheme cannot fully meet the requirements of short-term forecasting for the damping of high-frequency noise and may even cause systematic drifts. Therefore, a multi-scale IAU scheme is proposed in this paper. Analysis increments were divided into different scale parts using a spatial filtering technique. For each scale increment, the optimal relaxation time in the IAU scheme was determined by the skill of the forecasting results. Finally, different scales of analysis increments were added to the model integration during their optimal relaxation time. The multi-scale IAU scheme can effectively reduce the noise and further improve the balance between large-scale and small-scale increments in the model initialization stage. To evaluate its performance, several numerical experiments were conducted to simulate the path and intensity of Typhoon Mangkhut (2018) and showed that: (1) the multi-scale IAU scheme had an obvious effect on noise control at the initial stage of data assimilation; (2) the optimal relaxation time for large-scale and small-scale increments was estimated as 6 h and 3 h, respectively; (3) the forecast performance of the multi-scale IAU scheme in the prediction of Typhoon Mangkhut (2018) was better than that of the traditional IAU scheme. The results demonstrate the superiority of the multi-scale IAU scheme.