NOGAPS hindcast of the January 2009 Arctic major sudden stratospheric warming and the corresponding Arctic oscillation index

The Navy Operational Global Atmospheric Prediction System (NOGAPS) is used to hindcast the strong Arctic Major Sudden Stratospheric Warming (SSW) event occurred in late January 2009. A series of extended-range forecast experiments are performed using NOGAPS first without the aid of data assimilation in order to observe the genuine trend of the forecast model. To improve the simulation of the SSW event, a unified parameterization of orographic drag is designed by combining two parameterization schemes; one by Webster et al. and the other by Kim and Arakawa / Kim and Doyle. The essential features of the SSW event are successfully reproduced as verified by the UK Met Office analysis due to enhanced planetary wave activity induced by more comprehensive subgrid-scale orographic drag processes. The impact of the SSW on the surface weather is also investigated in view of the Arctic Oscillation (AO) Index calculated using 1000-hPa geopotential height. The trend of the AO Index is better simulated with upgraded orographic drag physics, demonstrating improved stratosphere-troposphere coupling in the model. The core experiments are then repeated with the data assimilation to validate the operational predictability of the improved model. The results will be compared with the non-data assimilation counterpart.