2 Feb 2015: 9-day and 15-day ensemble forecasts of the polar vortex
This site now displays an ensemble of 9-day and 15-day forecasts of Potential Vorticity (PV), interpolated to an isentropic level defined by a potential temperature of 485K - i.e. in the lower stratosphere where polar ozone depletion happens.
These mosaics of PV-maps are derived from a subset of the ECMWF Medium Range Ensemble Forecast. The primary intention is to allow a subjective evaluation of the probability of vortex splits during the polar Spring. Vortex splits are important because they mark the end of ozone hole events.
The mosaics are updated daily. They include the analysed PV once it becomes available, allowing for past dates to compare the actual vortex shape with the corresponding forecasts. Preliminary investigation indicates that the 9-day forecasts have a good skill at both poles while the 15-day forecasts have some skill above the South Pole and basically no skill above the North Pole. This conclusion may be premature though, because currently the detection of vortex splits is subjective and we have only one season to investigate above each pole (ECMWF started distribution of operational Medium-Range ensemble forecasts in December 2013).
More information can be found in a dedicated MACC-III deliverable (a short read: only three pages of text).