Diagnosing atmospheric persistence for heatwaves and in extended range forecasts

Extreme events can cause severe disruption to society on many levels, and the ability to forecast these events represents a significant step towards the ability to reduce their impacts. Anomalously persistent atmospheric configurations are typically regarded to be strongly linked with temperature extremes in Europe, however, traditional methods of analysing atmospheric persistence lack a mathematically well-grounded definition. Furthermore, we are not aware of a metric which allows for quantification of instantaneous atmospheric persistence for forecasts for either an individual ensemble member or a deterministic forecast. We aim to help refine the definition of atmospheric persistence by presenting a mathematically well-grounded definition of persistence, which can potentially also be applied in a forecasting environment. We examine the link between the extremal index, an indicator for atmospheric persistence based on dynamical systems theory, and warm temperature extremes in several regions of Europe. We then consider the applicability of this technique to forecast data, in particular ECMWF extended range reforecast data, discussing its potential value as an additional forecast evaluation metric.