The concept of vorticity is of central importance in fluid mechanics and the change and variability of atmospheric flow is dominated by transient vortices of different time- and space scales. Of particular importance are the most in- tense vortices such as hurricanes, typhoons and tornadoes which are associated with extreme and hazardous weather events of great concern to society. In recent years the un- derstanding of these phenomena has grown due to increased and improved surveillance by satellites and aircraft as well as by numerical modelling and simulation, theoretical studies and...