Sources of sub-seasonal predictability for energy-industry-relevant weather events

Weather strongly affects the energy sector by driving both energy demand and supply. For instance, a two-week winter cold spell in Europe strongly enhances heating demand. Due to the large-scale deployment of renewables in the past decade in Europe, the importance of weather for the energy sector has increased even more. Variations in large-scale weather patterns impacting wind, precipitation and cloud cover critically affect energy supply and weather forecasts are hence important price drivers in energy markets. Of particular interest for many market participants is the time range from a few weeks to two months ahead, since large volumes are traded on these timescales, for which the weather evolution is however highly uncertain. Improved predictability on these timescales would therefore be particularly valuable for the energy sector. Motivated by the weather sensitivity of the energy industry, this thesis applies a "weather event perspective" to investigate predictors for anomalous weather conditions in Europe with a particularly high impact on the energy sector for up to two months ahead. In the first part of the thesis energy-relevant weather events are defned and the large-scale flow conditions in which they occur are discussed, using a novel weather regime defnition of 7 Atlantic-European weather regimes and the North Atlantic Oscillation (NAO).Weather events are multi-day periods with anomalously high or low temperatures, winds, or precipitation in one of five subregions of Europe. The role of the NAO is quite distinct for different weather events. Some event types preferably occur during weather regimes with a positive NAO, e.g., high wind and high temperature events in the northern subregions and low wind events in the southern subregions. Weather regimes with negative NAO favour fewer event types, i.e., only low temperature events in Central Europe and the Nordics. However, there are also a number of events that occur in weather regimes not related to the NAO, such as low wind events in northern and high wind events in southern Europe. The second part of the thesis investigates the implications of stratosphere-troposphere coupling for the weather events. Several event categories show statistically significant anomalous stratosphere-troposphere coupling (i.e., a stronger than normal stratospheric polar vortex and a positive NAO or a weaker than normal polar vortex and a negative NAO) up to about 40 days before the events. Strongest stratosphere-troposphere coupling is present before those wind events, which preferably occur during a strongly positive NAO. Stratosphere-troposphere coupling is also present for some temperature and precipitation events, but the signals are weaker than for the wind events. We also show that the stratospheric circulation modulates the 3 Atlantic-European weather regimes which strongly project onto the NAO, which corroborates the finding that mainly NAO related events occur during stratosphere-troposphere coupling. The third part of the thesis investigates the implications of the lower stratospheric circulation for month-ahead wind power and its prediction, using the novel dataset Renewables. ninja. We demonstrate that there exists a strong relationship between the lower stratospheric circulation and month-ahead wind electricity generation in different parts of Europe. This relationship exists due to episodes of stratosphere-troposphere coupling, which lead to prolonged periods of either the positive or the negative phase of the NAO. Since these persistent NAO periods are associated with strong surface wind anomalies, they have an important impact on wind electricity generation, in particular for Northern Europe. The state of the lower stratospheric circulation also determines the exact latitudinal position of these prolonged NAO patterns, with contrasting implications for wind electricity generation in specific countries. Using simple statistical forecasts we show that the observed relationship between the lower stratosphere and wind electricity generation can be used for skilful forecasts of month-ahead wind electricity generation. Particularly high forecast skill is found when the circulation in the lower stratosphere differs strongly from its climatological mean. Anomalous states of the lower stratospheric circulation therefore provide windows of sub-seasonal range predictability for wind power output in many European countries. In the last part of the thesis the state of low-frequency forcings other than the stratospheric circulation are analysed with respect to weather events. They include the Madden-Julian Oscillation (MJO), the Global Wind Oscillation (GWO), sea surface temperatures, Arctic sea ice and Eurasian snow cover. The signals are weaker than the ones found for the stratospheric circulation, but they exist and are mostly consistent with the findings of previous studies. Weather events occurring preferentially during NAO+ tend to be preceded by MJO phases 2 to 4, GWO phases 5 to 8, above normal Arctic sea ice, the warm phase of the Pacific decadal oscillation (PDO) and below normal Eurasian snow cover. In contrast, weather events occurring preferentially during NAO- are preceded by MJO phases 6 to 8, GWO phases 1 to 3, below normal Arctic sea ice, the cold phase of the PDO and above normal October Eurasian snow cover. We find one signal which is in clear contradiction to previous literature. Weather events preferentially occurring in NAO+ (NAO-) tend to occur during El Niño (La Niña), while the bulk of the literature so far suggested that El Niño favours NAO- dominated winters and vice versa