Weather Events

Parameter Description
Precipitation
  • Intense precipitation associated with the warm conveyor belt.
  • Thunderstorms within the inner edge of the cloud head on the western side of the surface low. They can also develop within the dry slot.
Temperature
  • Substantial rise in surface temperature within the area of warm conveyor belt.
Wind (incl. gusts)
  • Strong winds within the area of the cloud head. In extreme cases winds can reach hurricane force.
  • Very strong gusts in the transition zone between dark and white as can be seen in the WV channels near the cold front and the cloud head.
  • Possibility of the development of a sting jet in the advanced and mature stages in the southern semicircle of the low and south of the decaying cloud spiral. (See below)
  • Sting jet cases may involve hurricane force winds with gusts up to 200 km/h.
Other relevant information
  • Very strong falls and rises in pressure. In the initial stage, falls ahead exceed the rises behind.
Fig. 32a: Weather events at the ground level Fig. 32b: Anatomy of an RC cyclone

The case of 8 - 9 January 2015 developed over the ocean, which means synoptic measurements are only available for the last phases and only from the British Isles and the North Sea. Although the cloud spiral is not directly over land, heavy showers and high wind speed can be seen in synoptic measurements over the Hebrides (NW Scotland).

Fig. 33: 9 January 2015, 00 UTC, Meteosat; IR 10.8. Synoptic observations.

10 meter wind speed maximum and its location, typical for a sting jet - discussed in the "Meteorological Physical Background" chapter - can be seen in the data from 9 January 2015, 00:00 UTC.

Fig. 34: 9 January 2015/00.00 UTC; Meteosat 10; IR 10.8 image, ECMWF 10 m wind

As already mentioned in the table above, sting jets may appear in the innermost part of the cloud spiral in the advanced and mature stages of a Rapid Cyclogenesis.

A sting jet is a mesoscale (no wider than about 100 km) zone of fast moving air descending from a height of 3-4 km to the surface south of the low center. The high winds last only for a few hours, but they can cause damage on the surface.

Sting jets may occur in connection with any intensive cyclogenesis. Their structure and development is not yet fully understood; both dynamical and thermodynamical approaches have been applied.

The next two images show the moist isentropes and isotachs in vertical cross sections from 8 January 2015 at 12:00 and 18:00 UTC. Both cross sections show the maximum of the upper level jet with the core between about 250 and 300 hPa. But during these 6 hours a second wind maximum has developed in the low layers with a core between 950 and 800 hPa.

Although it is not so easy to decide if this is a sting jet or some other wind speed maximum, these two figures do demonstrate the applicability of VCSs for a more detailed investigation of strong winds and the possibility of their damaging impact.

Fig. 35: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Black: moist isentropes; brown: isotachs; yellow: orientation of VCS line; yellow arrow: position of upper level jet core; blue line: area under the front from the surface up to 700 hPa.

Fig. 36: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Black: moist isentropes; brown: isotachs; yellow: orientation of VCS line; yellow arrow: position of upper level jet core; blue arrow: position of a low level jet (possibly a sting jet); blue line: area under the front from the surface up to 700 hPa.

Due to the strong winds that quite often occur during rapid cyclogenesis events, wind warnings for the relevant areas are issued by MeteoAlarm. The example below is from 31 January 2008 and shows a similar situation and cloud spiral as in the 8 January 2015 case. Also shown is a wind warning for Norway.

Fig. 37: 31 January 2008, 18:00 UTC - Meteosat 9 IR10.8 10.8 image. Weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation).

Fig. 38: 31 January 2008, 03:59 UTC - MeteoAlarm warning issued by the national meteorological services.