资源环境科技发展态势分析平台

The observational impacts of satellite data assimilation on extended-range forecasts of sudden stratospheric warmings (SSWs) are investigated by conducting ensemble forecast experiments. We use two Japanese novel reanalysis products: the Japanese 55-year reanalysis (JRA-55) and its subset that assimilates conventional observations only (JRA-55C). A comparative examination on the reproducibility for SSWs between the two ensemble forecasts reveals that the impact of satellite observations is significant for forecasts starting 5 days before the SSW onset, with 20% less accuracy in the JRA-55C forecasts. Moreover, some of forecasts of vortex-splitting SSWs show a sudden appearance of deep difference, which lasts over a fewmonths in the lower stratosphere and significantly affects the surface climate. These results highlight an important role of mesospheric and upper stratospheric circulations on the onset and development of SSWs.

Plain Language Summary Satellite observations are valuable for producing initial conditions for numerical weather prediction (NWP) systems, especially over the upper stratosphere that typical upper-air observations cannot cover. However, many NWP models suffer from biases associated with unresolved processes. This study explores how the NWP system benefited from satellite data in forecasting the breakdown events of stratospheric polar vortexes/sudden stratospheric warmings (SSWs) by making many forecasts from typical initial conditions, both with and without satellite data. Due to unresolved bias over the upper stratosphere, some forecasts from no-satellite initial conditions miss the onset of SSWs and subsequent anomalous tropospheric conditions. Thus, the deteriorated grasp of the upper atmosphere in the absence of satellite observations degrades the deterministic predictability of extreme stratospheric events and following downward-propagating signals.