Using Elliptical and Zonal Mean Metrics to Examine Variability in Model Skill during the 7 January 2013 Sudden Stratospheric Warming Event

The purpose of this study is to investigate the sources of variability in stratospheric forecast skill amongst several operational models initiated at various lead times prior to the 7 January 2013 sudden stratospheric warming (SSW). Early analyses of numerical weather prediction models have shown that stratospheric regime changes (e.g. strong and weak vortex events) are not skillfully predicted at medium-range lead times. This study extends upon a previous analysis by the Stratospheric Network for the Assessment of Predictability (SNAP), which concluded that a change in forecast lead time from 15 to 10 days increased model skill in predicting the 2013 SSW by roughly 50 percent. The cause of such variability in predictive skill remains an important question and will be investigated further in this study.

It is hypothesized that model error in forecasting thermodynamic processes linked to the SSW precursor tropospheric blocking in the N. Atlantic resulted in systematic biases and variability in dynamical forcing (e.g. upward wave flux) into the polar stratosphere in forecasts of the 7 January 2013 SSW event. Both medium-range and sub-seasonal forecast periods will be interrogated for model variability and initialized at several different lead times using the SNAP dataset, with six ensembles that contain model tops extending into the upper stratosphere. Both elliptical diagnostics of the polar vortex – its eccentricity, center longitude and latitude, and area encompassed by the vortex – and zonal mean metrics will be used to assess model errors and biases in the stratosphere. Ensemble skill will be categorized into low and high error composite groups according to two separate metrics: the strength of the 10-hPa zonal mean zonal wind and the center of the stratospheric vortex ellipse. These low and high error stratospheric forecasts will be analyzed in terms of how well they resolved the thermodynamic processes linked to precursor tropospheric blocking. Potential vorticity advection via the irrotational wind has been calculated to analyze the thermodynamic processes linked to tropospheric blocking. Preliminary results reveal that errors in the maintaining a blocking ridge in the N. Atlantic forced by a deepening cyclone off the northeast coast of the United States produced deviations in ensemble forecasted wave activity flux into the lower stratosphere one week prior to the 7 January 2013 SSW event.