Abstract
This study investigates false alarms of a major sudden stratospheric warming (MSSW) by real-time subseasonal-to-seasonal forecast data of the European Centre for Medium-Range Weather Forecasts system for the 2017/2018 Northern Hemisphere winter season. The analysis reveals two false alarm cases in the season, one in early December and the other in early February. Each case is characterized by ensembles of which a considerable part of the members (MSSW members) show an MSSW, that is, reversal of the zonal mean zonal wind in the extratropical stratosphere on similar calendar dates. Ensemble forecasts that are initialized earlier or later basically lack an MSSW, demonstrating clear intraseasonal variability in the frequency of forecasted MSSWs. For each false alarm case, the MSSW member mean field shows equatorward displacement of the polar vortex around the onset date. For both cases, the MSSW members accompany stronger wave activity in the lower stratosphere than other non-MSSW members and reanalysis data. They are further associated with higher geopotential height than the non-MSSW members, in the upper troposphere over northeastern Canada and Greenland before the first case, and lower height over northeastern Eurasia before the second case. These are located over the ridge and trough, respectively, of the climatological planetary wave of zonal wave number one, and are consistent with the increased wave activity.
Highlights
It is widely accepted that knowledge of the stratospheric state contributes to enhanced tropospheric predictability over intraseasonal and longer time scales as stratospheric variability influences the troposphere over various time scales [1,2]
The forecasted major sudden stratospheric warming (MSSW) in the second cluster are different from the real MSSW. They have different horizontal patterns of the polar vortex from the mid-February MSSW, as shown later. Such variability of the presence or absence of forecasted MSSWs has been relatively unrecognized, as previous studies were mostly interested in the predictability of real MSSWs, that is, how forecasts are improved with decreasing lead time to the MSSWs that occurred in reality
Especially MSSW predictability, has drawn a lot of attention, previous studies mostly focused on MSSWs that occurred in reality [1]
Summary
It is widely accepted that knowledge of the stratospheric state contributes to enhanced tropospheric predictability over intraseasonal and longer time scales as stratospheric variability influences the troposphere over various time scales [1,2]. When forecasts are initialized before the predictable time limit of a target MSSW, which depends on MSSWs and systems, they tend to miss or underestimate zonal wind deceleration to the target MSSW Such large stratospheric forecast errors occur when the forecasts are unsuccessful in representing enhanced planetary wave activity in the lower stratosphere and troposphere [8,10,21,22]. This study seeks to take a different approach, that is, conducts a case study of false alarms of a MSSW for the NH winter season of 2017/2018 in a S2S prediction system, that is, the European Centre for Medium-Range Weather Forecasts (ECMWF) system. Student’s t-distribution with a confidence level of 95% (two-sided test)
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