Blocking Identification Based on the Kinematic Vorticity Number and Its Link to the Point Vortex Model

The point vortex model represents a theoretical simplified model which is able to describe the observed (quasi)stationarity of blocking. A point vortex configuration of zero total circulation with the anticyclone located polewards of one or two cyclones moves westward. If the velocity of this dipole/tripole is of the same magnitude as the general westerly flow of the midlatitudes, the vortex configuration becomes stationary. However, there is no unified definition of what a vortex and its properties are. Here, we will present a method based on the kinematic vorticity number Wk which in a first step identifies vortices in three-dimensional flow fields. Afterwards, the method is able to identify and differentiate between High-over-Low and Omega blocking patterns. The kinematic vorticity number Wk is given as the ratio of the local rotation vs. local strain rate. A vortex is then defined as simply-connected region of Wk larger than one, i.e. as a region of prevailing rotation. This novel method allows the analysis of blocking from the vortex point of view as interaction of different vortices with a mean flow or with other vortices. Moreover, we are now able to study the characteristics of the different blocking patterns seperately.