Potential Vorticity Dynamics
Mid-latitude weather, waves and vortices
Climate variability and extreme events in mid-latitudes are connected to complex dynamical structures propagating around the whole globe.These structures build bridges in time and space connecting weather and climate and large distances as, for example, wet and dry periods in Europe and Central Asia. The potential vorticity (PV) is a combination of rotation and stratification, and allows to visualize these structures and their dynamics.
A simulation of an idealized Earth-type atmosphere is produced with PUMA (Portable University Model of the Atmosphere) at a horizontal resolution of T170 with 20 equidistant vertical layers. For demonstration, the absolute value of the potential vorticity (Units: PVU, i.e. Potential Vorticity Units) on the 315 K isentrope is shown in the movie.
The transition between tropospheric and stratospheric air in mid-latitudes is organized in a sharp PV front with wave- and vortex-like behaviour. The disturbances (green-yellow PV areas) formed during wave-breaking events can move rapidly along the front and trigger new breaking events downstream. The dynamical behaviour of the PV front in the upper troposphere and lower stratosphere has a direct influence on the weather at the surface. Traveling instabilities can explain the correlation of weather events often thousands of kilometers apart. The statistics of wave breaking and vortex formation yields a measure for the climate variability including extreme events.
Some typical PV structures are self-organizing at the turbulent PV front. Their impact on weather and climate is demonstrated below ...
Snapshots of typical potential vorticity structures
A potential vorticity front behaves like a non-linear wave: small wave-like disturbances form, grow and finally break up. Wave breaking occurs in two directions: in cyclonic direction, (planetary rotation), or in anti-cylonic direction (opposite to planetary rotation).
Mixing of different air masses is increased during wave-breaking events and disturbances are formed, which can propagate along the potential vorticity front. Weatherwise wave breaking is often connected with strong winds and precipitation.
Cut-off cyclones can form in the final phase of cyclonic wave-breaking. Their formation is characterized by stratospheric air being cut off and moving into the 'blue PV-sea' of the troposphere.
The vortices can be connected to strong precipitation. The movement of these vortices is slow, complex and, in general, difficult to predict.
Blocking and cut-off anticyclones can form in the final phase of anticyclonic wave-breaking (see filaments). Blocking is characterized by large and often circular intrusions of tropospheric low potential vorticity into the 'red sea' of stratospheric high potential vorticity. Such intrusion ends as a separated anti-cylcone moving slowly through the lower stratosphere before merging again with the troposphere. The processes of separation and merging are very abrupt and difficult to predict. Both processes are strong perturbations of the PV front.
A blocking event is a long living stable weather regime which, in summer, can be connected to droughts and heat waves.
Breaking events can form long filaments of high potential vorticity air reaching subtropical regions (see cut-off anticyclones). In some cases a filament can break up and form a cut-off cyclone.
The filaments are connected to moisture transport towards high latitudes and can induce heavy rainfall for example at the south side of the European Alps.