Predictability of Stratospheric Sudden Warmings in the Beijing Climate Center Forecast System with Statistical Error Corrections

Abstract

AbstractPrevious studies have reported that the predictive limit of stratospheric sudden warming (SSW) events in the Beijing Climate Center forecast system (BCC_CSM) is shorter than 2 weeks. This study continues to analyze the general characteristics of this model in forecasting SSWs and carries out a trial of error corrections. The ratio of the ensemble members that forecast the zonal wind reversal with a 5‐day delay allowed (hit ratio) is higher for SSW events with a small decrease in the zonal mean zonal winds (moderate SSWs) than for events with a large decrease in the zonal mean zonal winds (radical SSWs) in hindcasts initialized around 1 (D‐7) and 2 (D‐14) weeks in advance. The underestimated cumulative eddy heat flux associated with weak wave activities accounts for the weaker‐than‐observed deceleration of westerlies. The preexisting extratropical wave patterns are satisfactorily forecast in D‐14 for most (9/12) cases, and the wave phase bias is reasonably small for those cases. After the climatology bias is deducted from the hindcasts, an increase in the hit ratio can be identified for moderate SSW events as the evolutions of zonal winds are improved. Following the error correction by remapping the zonal wind probability distribution function in the forecast system to the reanalysis, the SSW hit ratios increase in the D‐7 (43% to 57%) and/or D‐14 (11% to 21%) initializations. Based on the composite result, this error correction method robustly improves the hit ratio not only for both radical SSWs and moderate SSWs but also for both polar vortex split SSWs and polar vortex displacement SSWs. The best error correction method might propel the prediction limit of SSW events to around 2 weeks.