Extratropical cyclone classification and its use in climate studies

Extratropical cyclones have long been known to be important for midlatitude weather. It is therefore important that our current state-of-the-art climate models are able to realistically represent these features, in order that we can have confidence in how they are projected to change in a warming climate. Despite the observation that these cyclones are extremely variable in their structure and features, there have, over the years, been numerous attempts to classify or group them. Such classifications can provide insight into the different cloud structures, airflows, and dynamical forcing mechanisms within the different cyclone types. This review collects and details as many classification techniques as possible, and may therefore act as a reference guide to classifications. These classifications offer the opportunity to improve the way extratropical cyclone evaluation in climate models is currently done by giving more insight into the dynamical and physical processes that occur in climate models (rather than just evaluating the mean state over a broad region as is often done). Examples of where these ideas have been used, or could be used, are reviewed. Finally, the potential impacts of future climate changes on extratropical cyclones are detailed. The ways in which the classification techniques could improve our understanding of future changes in extratropical cyclones and their impacts are given.