Extreme Weather and Climate Events:
What are they and where do they come from?

David B. Stephenson 

Reading University


There are many different and rather confusing definitions of extreme event in weather and climate research. Much current interest is motivated by the high-impacts caused by such events and how such events are likely to evolve in the future under climate change. However, to develop confidence in such studies, it is necessary to develop deeper physical and statistical understanding of how such events arise.  Rather than focus on impacts, this talk will present taxonomy of extreme weather and climate events based on fundamental understanding of key underlying physical processes. Extreme events arise by several different dynamical processes such as:    *Fast growth caused by unstable positive feedback (e.g. baroclinic storms, convective precipitation, etc.)    *Survival of an event into a new spatial region or time period (e.g. transition of a tropical cyclone into mid-latitudes)
   *Conjunction of several non-rare events (e.g. freak waves)
   *Intermittency in space/time (e.g. precipitation totals, enstrophy).
   *Persistence of weather conditions leading to a climate extreme (e.g. drought)
   *Clustering/recurrence of weather events (e.g. unusually stormy wet season).
   *Other unknown processes related to natural variability that are perhaps best described as random variations.  Extreme events are therefore able to provide unique insights into these processes in both the real world and in numerical model simulations. Examples of some of these processes will be given together with a discussion of how to statistically diagnose such processes.