Journal Club: 01-11-2018

Article title: Does the lower stratosphere provide predictability for month-ahead wind electricity generation in Europe?

Published in the Quarterly Journal of the Royal Meteorological Society in 2017.

This paper’s work aims at assessing a potential relationship between the lower stratosphere and month-ahead wind electricity generation in Europe which could be used to improve wind electricity generation forecasts.

Geopotential height anomalies at 150 hPa (indicator of the state of the lower stratospheric circulation) were used, averaged over the domain of 60 N to the North Pole. These anomalies are grouped into percentile bins, which indicate different intensities of the polar-vortex. For wind energy generation, daily capacity factor (CF) anomalies are computed from a 31 day running mean. Geopotential height and wind speeds anomalies composites are analyzed to check on the relationship between different vortex events (strong and weak) over the stratosphere, and wind speed anomalies in the troposphere, as well the temporal evolution of both anomalies before and after the events. This enables the observation of the coupling of the flow in “high” and “low” altitudes.

Figure: Composite mean of geopotential height anomalies averaged over the polar cap north of 60N in units of standard deviations (with respect to the variability of from 60 days before until 60 days after strong vortex events in different bins (rows). Right column refers to weak polar events. The bold black line indicates the 150 hPa level. The numbers in brackets in the panel titles indicate the number of events that are included in the respective bins.

Although the strong polar vortex events can be start showing up to 40 days before the event (a), it is in the weak polar vortex events (e) that a clear downward propagation of the geopotencial height anomalies can be seen. These relationships are a results of persistent long-lived periods with NAO+ or NAO- conditions, and they offer potential for some wind generation predictability but, as shown in the work, this predictability varies greatly amongst different regions of Europe, with countries in northern Europe (Sweden, Denmark) achieving higher scores in the wind electricity generation forecasts, when comparing with countries in southern Europe (Italy, Spain). Moderate predictability is found for countries like Germany, UK and Poland. This is due to the geographical location of the mentioned countries in relation to the location of the strongest geopotential anomalies – highest predictability is found for locations consistently under the spatial range of those anomalies for the various bins.

Conclusions:

This work suggests that anomalous flow conditions in the lower stratosphere offer month-ahead predictability of wind electricity generation in Europe, specially for Northern Europe.