4.2      Implementation of incremental analysis updates (IAU) in 1-hr updated cycling forecasting system.

Chen, Min, Institute of Urban Meteorology, CMA, Beijing, China, and Xiang-yu Huang, Centre for Climate Research Singapore, Singapore

Rather than the widely used DFI technique, an incremental analysis updates (IAU) scheme is successfully implemented into a WRF/WRFDA-based 1-hr updated cycling forecast system with the goal to reduce the imbalance introduced by the high-frequency intermittent data assimilation. With the application of IAU, the analysis increments are smoothly introduced into the integration with time-weighted optimal digital filter coefficients during the [-1h,1h] time window centered at the analysis time.

As only a tiny part of analysis increments of all prognostic variables is incorporated at each integration step, it makes the model away from the great shock at the initial time due to the intermittent data assimilation. Therefore, IAU leads to reasonable and desirable results in terms of the initial noise and hydrometeor spin-up for the 1-hr updated cycling system. In addition, the precipitation forecasts of the continuous cycles are much more stable, and precipitation forecast skills are significantly improved (around 10+% for CSI scores). On the performance of data assimilation, another advantage of IAU can be found that the better balanced 1-st guess generated by the previous cycle forecasts potentially give rise to more data can be assimilated and faster minimization.