The Tear-Off Stage

During the tear-off stage, the upper-level pressure minimum moves away from the band of westerlies. The amplitude of the Rossby wave increases while at the same time the tops of the high-pressure ridges on both sides of the trough converge (see red W in Figure 2). As a consequence, the low-pressure bridge connecting the upper-level pressure minimum to the main pressure minimum gets narrower and narrower. Depending on the motion of the upper-level low, the low-pressure trough tilts either forward or backward. This is the beginning of the cut-off stage (see Figure 1).

Figure 1: Schematic of the tear-off process from left to right. Blue lines represent the geopotential height at around 500 hPa, red W indicates warm air and blue C cold polar air masses.

The SEVIRI water vapor image loop in Figure 2 shows the whole formation process: the deepening trough, the tear-off and cut-off stages until the ULL has reached its final evolution stage as a drop of cold air south of the polar front. In this winter case example, the cut-off low is located far in the south. Notice the strong gradient of the 500 hPa geopotential lines at the southern tip of the trough which becomes less pronounced as soon as the cold core forms. In the final stage the ULL shows a regular pressure gradient pattern around its core.



Figure 2: WV loop from 21 February 2021 at 00 UTC to 24 February 2021 at 00 UTC. The 500 hPa geopotential height is depicted in cyan isolines.

Let's have a look at the NWP parameters that best characterize the tear-off stage.

Temperature:

The polar air mass inside the core of the upper-level low is much colder than the air surrounding the cold core, and the tropopause is lowered above the core. This can best be seen in a vertical cross section trough the center of the ULL.


Figure 3: 16 February, 2021 at 12:00 UTC. Above: Position of the vertical cross section through the core of an ULL in the tear-off stage (orientation west to east). Below: Temperature (dotted blue lines) and equivalent potential temperature (black lines).

The temperature contrast between the center of the tear-off low and the surrounding warmer air masses is usually quite strong and is only marginally diminished by adiabatic processes that result from subsidence as long as the pressure minimum of the tear-off low continues to decrease.

Potential Vorticity (PV):

At the location of the ULL the height of the tropopause exhibits a minimum which is seen in the local PV values (see Figure 4). Descending air masses from higher levels create a dry spot that can be seen in water vapor imagery (black circular spot) and in the Airmass RGB (red circular area). Their water vapor signature is most distinct, especially in the phase when the upper-level pressure minimum deepens (i.e., during the tear-off stage).


Figure 4: 16 February, 2021 at 12:00 UTC. Above: Position of the vertical cross section though the core of an ULL in the tear-off stage (orientation west to east). Below: Potential Vorticity (magenta lines) and equivalent potential temperature (black lines).

The Jet:

During the tear-off stage, the highest wind speeds within the jet stream are found either on one or both sides of the elongated trough. When an upper-level low forms, the wind speed maximum is rarely found at the southern tip of the trough where the curvature is strongest.

Figure 5: Tear-off stage on 15 and 16 February 2021 at 12:00 UTC. Left: Geopotential height at 500 hPa (cyan). Right: Isotachs at 300 hPa (yellow).