11 Jan 2022
11 Jan 2022
Status: a revised version of this preprint was accepted for the journal WCD.

Differences in the Sub-seasonal Predictability of Extreme Stratospheric Events

Rachel Wai-Ying Wu1, Zheng Wu1, and Daniela I. V. Domeisen1,2 Rachel Wai-Ying Wu et al.
  • 1ETH Zurich, Zurich, Switzerland
  • 2University of Lausanne, Lausanne, Switzerland

Abstract. Extreme stratospheric events such as sudden stratospheric warming and strong vortex events associated with an anomalously weak or strong polar vortex can have downward impacts on surface weather that can last for several weeks to months. Hence, successful predictions of these stratospheric events would be beneficial for extended range weather prediction. However, the predictability limit of extreme stratospheric events is most often limited to around 2 weeks or less. The predictability also strongly differs between events, and between event types. The reasons for the observed differences in the predictability, however, are not resolved. To better understand the predictability differences between events, we expand the definitions of extreme stratospheric events to wind deceleration and acceleration events, and conduct a systematic comparison of predictability between event types in the European Centre for Medium-Range Weather Forecasts (ECMWF) prediction system for the sub-seasonal predictions. We find that wind deceleration and acceleration events follow the same predictability behaviour, that is, events of stronger magnitude are less predictable in a close to linear relationship, to the same extent for both types of events. There are however deviations from this linear behaviour for very extreme events. The difficulties of the prediction system in predicting extremely strong anomalies can be traced to a poor predictability of extreme wave activity pulses in the lower stratosphere, which impacts the prediction of deceleration events, and interestingly, also acceleration events. Improvements in the understanding of the wave amplification that is associated with extremely strong wave activity pulses and accurately representing these processes in the model is expected to enhance the predictability of stratospheric extreme events and, by extension, their impacts on surface weather and climate.

Rachel Wai-Ying Wu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2021-84', Anonymous Referee #1, 05 Feb 2022
  • RC2: 'Comment on wcd-2021-84', Anonymous Referee #2, 22 Feb 2022
  • AC1: 'Comment on wcd-2021-84', Rachel Wai-Ying Wu, 22 Apr 2022
  • AC2: 'Comment on wcd-2021-84', Rachel Wai-Ying Wu, 22 Apr 2022

Rachel Wai-Ying Wu et al.

Rachel Wai-Ying Wu et al.


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Short summary
Accurate predictions of the stratospheric polar vortex can enhance surface weather predictability. Stratospheric events themselves are less predictable, with strong inter-event differences. We assess the predictability of stratospheric acceleration and deceleration events in a sub-seasonal prediction system, finding that the predictability of events is largely dependent on event magnitude, while extreme drivers of deceleration events are not fully represented in the model.