Deterministic prediction of stratospheric sudden warming events in the Global/Regional Integrated Model system (GRIMs)
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Deterministic prediction of stratospheric sudden warming events in the Global/Regional Integrated Model system (GRIMs) Kanghyun Song1 · Seok‑Woo Son1 · Andrew Charlton‑Perez2 Received: 11 February 2020 / Accepted: 30 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The boreal-winter stratospheric sudden warming (SSW) events and their prediction skills by an operational numerical weather prediction model are examined by applying the Global/Regional Integrated Model system (GRIMs) for 18 SSW events from 1980–2012. Based on the mean squared skill score of the 10-hPa geopotential height field, which considers the SSW spatial structure, most SSW events are predicted with a maximum forecast lead time of approximately 15 days. The vortex-displacement SSW events are slightly better predicted than the vortex-split SSW events, and the predictions are improved during El Niño or easterly quasi-biennial oscillation winters. However, the skill difference in vortex morphology and background state is statistically insignificant. The decomposition of model errors into zonal-mean and eddy errors reveals that the model errors mostly result from eddy components. In particular, eddy-amplitude errors, which originate from a misrepresentation of the planetary-scale wave amplitude (i.e., polar vortex strength), play an important role in determining the SSW prediction skill with a non-negligible contribution from eddy-phase errors. These errors are mostly caused by the misdetection of short-term wave activities immediately before the SSW onset. Furthermore, an improved SSW prediction through well-represented planetary-scale wave activities is related to an improved prediction in the troposphere on time scales of 10 days and longer. This result confirms that better representation of the stratosphere could lead to improved subseasonalto-seasonal predictions in the northern extratropics.
1 Introduction Stratospheric sudden warming (SSW) events are one of the most remarkable examples of subseasonal variability in the boreal-winter stratosphere. These events are driven by vertically-propagating planetary-scale waves from the upper troposphere, but also significantly modulate the troposphere through stratosphere-troposphere dynamical coupling (e.g., Baldwin and Dunkerton 2001). The downward influence is often maintained for up to two months in the northern extratropics. As such, SSW events have been regarded as an important source for subseasonal-to-seasonal (S2S) prediction in the northern extratropics (Vitart et al. 2017; Butler et al. 2019). Previous studies have shown that the prediction skill at the surface tends to increase when the model is * Seok‑Woo Son [email protected] 1
School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea
Department of Meteorology, University of Reading, Reading, UK
2
initialized during SSW events (Sigmond et al. 2013; Tripathi et al. 2015b). Tripathi et al. (2015a) recently reviewed the SSW prediction skill of various models
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