Key Parameters

The behaviour of the key parameters of the Shapiro-Keyser cyclone will be demonstrated here for the four development stages:

  1. wave stage
  2. intensification stage
  3. mature stage
  4. dissipation stage

Figure 1: The four developement stages of the Shapiro-Keyser cyclone.


The key parameters chosen here are displayed on pressure levels. The important key parameters are basically the same as those for the Norwegian cyclone type, so we will focus on those key parameters, which follow different behaviour in the Shapiro-Keyser cyclone type. These parameters are:

  • the geopotential height at 500 hPa and the mean sea level pressure
  • the equivalent potential temperature at 850 hPa
  • the temperature advection at 850 hPa
  • the humidity at different levels
  • the position of the jet and the jet streaks at 300 hPa

A case occurring from 02 to 04 March 2022 is chosen here to illustrate the Shapiro-Keyser cyclone development. It demonstrates the typical behaviour of the selected key parameters and reflects the idealised interaction of NWP parameters with the cloud configuration of a Shapiro-Keyser cyclone.

Figure 2: 2  – 4 March 2022, Airmass RGB
U.l. 09:00 UTC on 2 March 2022, Wave stage; u.r.: 18:00 UTC on 3 March 2022, Intensification stage;
l.l. 09:00 UTC on 4 March 2022, Mature stage and l.r. 21:00 UTC on 4 March 2022, Dissipation stage



Typical key parameter features for Shapiro-Keyser cyclogenesis

Geopotential height

  • Mean sea-level pressure (or pressure close to the surface):
    • At the early cyclogenesis stage, either a weak surface low or a distinct trough is observed. The surface pressure minimum deepens until the cyclone reaches the mature stage and then it starts to weaken.
  • 500 hPa geopotential height:
    • Shapiro-Keyser cyclone tend to develop in a high-index circulation. This means that the geopotential height in the upper troposphere has a strong zonal component at least during the early development stages.
    • In the upper-levels, two phenomena can be observed regarding the geopotential height: the transition from a trough to a closed circulation around a pressure minimum and the shift of the trough axis from west to east during the wave and intensification stages. During the mature stage, the upper-level pressure minimum is superimposed on the surface pressure minimum and moves eastward of the surface pressure minimum during the dissipation stage.
    • If we consider an axis that links the pressure minima at different levels, this axis is tilted westward until the cyclone reaches the mature stage (at which it is vertical) and is then tilted eastward in the dissipation phase.

Figure 3: Schematics of geopotential Height at 1000 hPa (black) and 500 hPa (cyan) superimposed on the Airmass RGB.

Figure 4: 2  – 4 March 2022, Airmass RGB, mean sea level pressure (black) and geopotential height at 500 hPa (cyan).
U.l. 09:00 UTC on 2 March 2022, Wave stage; u.r.: 18:00 UTC on 3 March 2022, Intensification stage;
l.l. 09:00 UTC on 4 March 2022, Mature stage; l.r. 21:00 UTC on 4 March 2022, Dissipation stage.

Equivalent potential temperature

  • Equivalent potential temperature at 850 hPa:
    • This parameter is suited for displaying air mass boundaries as it combines temperature and humidity. Both these quantities typically have high gradients at fronts and air mass boundaries. In the case of a Shapiro-Keyser cyclone, the parameter clearly shows how cold and dry air masses wrap around the warm core of the system until it is completely secluded from the warm sector.
    • The frontal zones (i.e., cold and warm fronts) are characterized by higher gradients in the equivalent potential temperature field. A cold frontal fracture is seen in the intensification and mature phases in the region where the cold front would normally join the warm front. In fact, a weaker but inactive frontal zone can be seen connecting the western end of the cold front to the northern tip of the warm front.

Figure 5: Schematics of the equivalent potential temperature at 850 hPa (red and blue) superimposed on the Airmass RGB.

Figure 6: 2  – 4 March 2022, Airmass RGB, equivalent potential temperature at 850 hPa (red and blue).
U.l. 09:00 UTC on 2 March 2022, Wave stage; u.r.: 18:00 UTC on 3 March 2022,
Intensification stage; l.l. 09:00 UTC on 4 March 2022, Mature stage; l.r. 21:00 UTC on 4 March 2022, Dissipation stage.

Temperature advection

  • Temperature advection at 850 hPa:
    • During the wave stage, we find similar behaviour to that for the Norwegian cyclone type: the cold front is associated with cold air advection (CA) at all-levels on their rear side and so is the warm front with warm air advection (WA).
    • The intensification phase is characterized by a dipole of WA and CA, where WA is found at the surface low and CA behind the cold front. This strong dipole at low to middle levels is typical for rapid cyclogenesis.
    • During the mature stage, WA prevails behind the warm front that circles around the surface low, while CA coincides with the region where cold air penetrates the area between the cold front and the cyclone centre.
    • Finally, weak WA and CA maxima are visible around the cyclone core during the dissipation stage. They are collocated with either the warm front or the dry intrusion, both of which wrap around the warm core.

Figure 7: Schematics of the temperature advection at 850 hPa (red, positive and blue, negative) superimposed on the Airmass RGB.

Figure 8: 2  – 4 March 2022, Airmass RGB, temperature advection at 850 hPa (red and blue).
U.l. 09:00 UTC on 2 March 2022, Wave stage; u.r.: 18:00 UTC on 3 March 2022, Intensification stage;
l.l. 09:00 UTC on 4 March 2022, Mature stage; l.r. 21:00 UTC on 4 March 2022, Dissipation stage.

Humidity

  • Relative humidity at levels 300 and 700 hPa:
    • The warm core separation process is clearly reflected in the humidity fields at all levels. A downward directed stream of dry air is found at the rear side of the cold front. This stream is identical to the dry intrusion conveyor belt.
    • This downward propagation of dry air masses is most distinctive during the intensification and mature stages. It is first noticeable at higher levels during the intensification stage and later, at lower levels, during the mature and dissipation stages.
    • The downward progression of dry air masses is responsible for the transformation of the upper-level cold front into a cold front that again reaches down to the ground. Due to diabatic heating processes during the downward motion, the temperature gradient at the rebuilt cold front is not very strong. The cold front is characterized mainly by a humidity gradient.

Figure 9: Schematics of relative humidity at 700 and 300 hPa (blue) during the intensification stage superimposed on the Airmass RGB.

Figure 10: WV6.2 µm  – 3 March 2022 at 18:00 UTC, intensification stage. Relative humidity at levels 300 and 700 hPa.
NB: The WV6.2 µm image is not representative for 700 hPa humidity.

Figure 11: Schematics of relative humidity at 700 and 300 hPa (blue) during the mature stage superimposed on the Airmass RGB.

Figure 12: WV6.2 µm  – 4 March 2022 at 09:00 UTC, mature stage. Relative humidity at levels 300 and 700 hPa.
NB: The WV6.2 µm image is not representative for 700 hPa humidity.

Wind speed and vorticity advection at jet level

  • Isotachs and cyclonic (positive) vorticity advection (CVA) at 300 hPa:
    • During the wave stage of a Shapiro-Keyser system, we usually find two jet streaks. They are aligned in such a way that the left exit region of one jet streak is superimposed on the right entrance region of the other. This arrangement is called "jet streak coupling", with the effect that the two CVA maxima collocate, for which reason their impact on surface convergence is increased
    • Jet streak coupling is still present during the intensification stage, even though the angle between the jet streaks becomes narrower.
    • At the end of the intensification and during the mature stage, the axes of the jets become nearly perpendicular to each other.
    • With decreasing meridional temperature gradient, the jets become weaker in the dissipation phase.
    • During the later stages, the jet streaks are usually no longer coupled, nor is there an interaction between upper-level divergence and the surface pressure minimum.

Figure 13: Schematics of the jet streak at 300 hPa (yellow) superimposed on the Airmass RGB. Left exit and right entrance regions are indicated by red circles.

Figure 14: 2  – 4 March 2022, WV 6.2 µm, isotachs (yellow), CVA maxima at 300 hPa (red) and jet axis (blue arrows). CVA maxima in the left exit or in the right entrance region are marked by orange circles.
U.l. 09:00 UTC on 2 March 2022, Wave stage; u.r.: 18:00 UTC on 3 March 2022, Intensification stage;
l.l. 09:00 UTC on 4 March 2022, Mature stage; l.r. 21:00 UTC on 4 March 2022, Dissipation stage.