Although the Polar Low is a small-scale system, the typical polar environment and main dynamic forcing mechanisms can be observed in a vertical cross section.

• Theta_W
  During the developing phase a baroclinc zone in the potential wet bulb temperature (ThetaW) pattern is visible. Upstream of the Polar Low a shallow layer of cold air is pouring out over the sea.
  This air is warmed up gradually by the relatively warm seawater, resulting in higher ThetaW values.
• Vorticity advection
  One of the main dynamical mechanisms in the initial or developing phase is Positive Vorticity Advection (PVA) caused by a moving upper level through. To trigger a Polar Low, a PVA maximum has to be superimposed upon the baroclinic zone.
• Potential vorticity
  Potential Vorticity (PV) is an important parameter in the develoment phase of a Polar Low. In this stage an upper level PV maximum is normally just upstream of a Polar Low.
  This PV maximum is situated at mid lvels of the troposphere but has transferred down from the stratosphere.
  A PV maximum can also be found in the gradient zone of the very stable layer of ThetaW near the surface over or near icefields.
• Vertical Motion (Omega)
  Upper level mechanisms result in negative omega. Negative omega above the Polar Low centre does not cover a deep layer because of a low tropopause and a stable upper layer.

Theta_W

Vorticity advection

Potential Vorticity

Vertical Motion (Omega)

MATURE PHASE

Vertical Motion (Omega)

In the mature phase, a deep layer of negative omega is surrounded by areas of positive omega. Although the ascending air motions are sometimes situated in an area of PVA and some warm advection, this negative omega can not be completely explained by these mechanisms alone. Convective instability is playing an important role too. The descending air motion to the rear of a Polar Low is often enhanced by cold air advection. Because of the relative small resolution of most operational models compared to the scale of a Polar Low, descending air motion at the centre of a Polar Low is difficult or impossible to see in a vertical cross section.