The influence of extratropical vortices on a global scale is mainly
characterised by their size and by the magnitude of their circulation.
However, the determination of these properties is still a great challenge
since a vortex has no clear delimitations but is part of the flow field
itself. In this work, we introduce a kinematic vortex size determination
method based on the kinematic vorticity number Wk to atmospheric flows. Wk
relates the local rate-of-rotation to the local rate-of-deformation at every
point in the field and a vortex core is identified as a simply connected
region where the rotation prevails over the deformation. Additionally,
considering the sign of vorticity in the extended Wk-method allows to identify
highs and lows in different vertical layers of the atmosphere and to study
vertical as well as horizontal vortex interactions. We will test the Wk-method
in different idealised 2-D (superposition of two lows/low and jet) and real
3-D flow situations (winter storm affecting Europe) and compare the results
with traditional methods based on the pressure and the vorticity fields. In
comparison to these traditional methods, the Wk-method is able to extract
vortex core sizes even in shear-dominated regions that occur frequently in the
upper troposphere. Furthermore, statistics of the size and circulation
distributions of cyclones will be given. Since the Wk-method identifies vortex
cores, the identified radii are subsynoptic with a broad peak around 300-500km
at the 1000 hPa level. However, the total circulating area is not only
restricted to the core. In general, circulations are in the order of 107m2/s
with only a few cyclones in the order of 108m2/s
