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Appearance in Satellite Data

Upper Level Lows are relatively long-lived (mostly from 2 to 10 days) phenomena. Their life cycle consists of three stages:

  • Upper level trough
  • Tear-off
  • Cut-off
  • Final stage

1. Upper level trough stage

There is a pronounced upper level trough behind a frontal zone.

  • In IR10.8 images white or light grey cloud band connected to the frontal zone on the leading side of the trough, some thin white cloud stripes due to Cloud Fibres on the rear side and possibly some convective white cells around the axis of the trough.
  • In VIS0.6 images white or light grey cloud band connected to the front on the leading side of the trough and possibly some convective white cells around the axis of the trough.
  • In WV6.2 images light grey bands on the leading and rear part of the trough.

2. Tear-off

The bottom of the upper trough is detached from the main stream resulting in a closed circulation.

  • In IR10.8 images white to light grey cyclonically curved cloud band on the leading side of the trough, possibly also on the rear side. Some white convective shells may occur between the bands.
  • In VIS0.6 images grey cyclonically curved cloud band on the leading side of the trough, possibly also on the rear side. Some white convective shells may occur between the bands.
  • In WV6.2 images grey cyclonically curved cloud band around the detaching low.

3. Cut-off stage

The Upper Level Low is separated from the main upper stream.

  • In IR10.8 and VIS0.6 images white to light grey cyclonically curved cloud bands on the leading and rear side of the low, later forming a spiral. Possibly some white convective cells within it.
  • In WV6.2 images round area or a spiral of grey, with some white cells on the leading side.
  • In the WV6.2-WV7.3;IR9.7i-IR10.8i;WV6.2 combination ("airmass") the descending stratosperic air in the centre of the Upper Level Low appears dark red, and the cold pool appears bluish
  • In WV6.2-WV7.3;IR3.9i-IR10.8i;NIR1.6i-VIS0.6 combination ("convective storms") convective clouds appear red; strong, growing cells are yellow.

4. Final stage

The Upper Level Low merges with the main stream, or dissolves slowly while being almost stationary.

  • In IR10.8 and VIS0.6 images there are light grey cyclonically curved stripes that merge with a white cloud band of a frontal zone.
  • In WV6.2 images there is a dark grey area that soon disappears under the light grey of the frontal zone.

1. The cloudiness belonging to the main stream is approaching
2. The ULL cloudiness merges with the frontal cloudiness

On the 14th of September 2005 at 06.00 UTC there is an elongated upper trough over Southern Spain and Gibraltar.

14 September 2005/06.00 UTC - Meteosat 8 IR 10.8 image
14 September 2005/06.00 UTC - Meteosat 8 WV 6.2 image

6 hours later tear-off occurs over Southern Spain.

14 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
14 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
14 September 2005/12.00 UTC - Meteosat 8 HIRVIS image

On the 15th of September 2005 at 12.00 UTC the separate Upper Level Low is clearly seen with core convection.

15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
15 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
15 September 2005/12.00 UTC - Meteosat 8 HIRVIS image
15 September 2005/12.00 UTC - Meteosat 8 Airmass RGB image
15 September 2005/12.00 UTC - Meteosat 8 Convection RGB image

On the 16th of September 2005 at 12.00 UTC the Upper Level Low is merging with an upper trough coming from the Atlantic.

16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
16 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
16 September 2005/12.00 UTC - Meteosat 8 HIRVIS image

Six hours later there is only a trough left.

16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
16 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image

The air below an Upper Level Low is potentially unstable, which leads to the so-called core convection and convective cloudiness. Over warm sea this convective development can be intensive. Contrary to this, the centre of an Upper Level Low over land is often overcast with low or middle level cloudiness with some convective cells embedded.

Other conceptual models that may look like an Upper Level Low in satellite images are Comma (see Comma ) and Polar Low (see Polar Low ). These can be separated from each other with the help of numerical fields, especially on the 500 hPa level.

Meteorological Physical Background

Upper Level Lows are closed cyclonically circulating eddies in the middle and upper troposphere. They are sometimes also called "cold drops", because the air within an Upper Level Low is colder than in its surroundings.

The development of a typical Upper Level Low goes through four stages, during which a bottom of an upper trough is detached from the main stream, until it finally fills up or merges with another trough:

  1. Upper level trough
  2. Tear-off
  3. Cut-off
  4. Final stage

1. Upper Level Trough stage

The prerequisite of the forming of the Upper Level Low are unstable waves within the main stream, where the temperature wave is behind the geopotential wave.

  • There is cold advection within the trough and warm advection on the ridge of the geopotential wave.
  • The vertical axis of the trough has a backward-oriented inclination with height.
  • The amplitudes of the waves increase; the wavelenght can decrease.

Cyan: 500 hPa geopotential height, green: 500 hPa temperature

2. Tear-off stage

  • The amplitudes of the waves increases further.
  • The isohypses form an inverse omega-shape and the cold air flows into the middle of this omega.
  • Often at the same time the ridge behind the main upper trough continues to move eastward quicklier than the trough, appearing to "fall forward".
  • In the end of this stage the cold bottom of the trough is detached from the main stream.

upper_level_low

3. Cut-off stage

  • The bottom of the upper trough is completely detached from the main stream forming a closed circulation.
  • If there is a strong forward-falling ridge behind, it may also separate from the main stream and form an upper level high (a counterpart for the upper level low). This happens in most of the ULL cases.
  • The cold core of the Upper level Low warms up slowly because of the diabatic warming of the sinking clod air.
  • If a cold Upper Level Low is situated over a warm surface, convection arises within the core. This occurs especially over the Atlantic Ocean (Canarian Isles) and over the Mediterranean in summertime.
  • Another location for convection is ahead of the low within the area of a thickness ridge.

upper_level_low

4. Final stage

Within an Upper Level Low there is convection, unless the surface is very cold. The air near the surface is warm and the circulation is slowed down by the friction. The convection brings warm air and friction upwards. Consequently, the Upper Level Low weakens slowly.

  • In most cases the Upper Level Low merges with the main stream before it has completely dissolved by the convection. Usually a large trough in the main stream approaches from the rear and catches the upper level low.
  • The Upper Level Low can also merge with another Upper Level Low.

upper_level_low

For an example see Key Parameters.

If the Upper Level Low is far from the main stream, it can dissolve solely by convection. This kind of development occurs mostly in southern areas; in Europe they can be found over the Mediterranean.

Upper Level Lows can be divided into two classes according to their size and lifetime:

  • small lows with a lifetime of 2-4 days
  • big lows with a lifetime of 5-14 days

Big lows are slightly more common than small ones.

Note that over land an Upper Level Low can also form when a surface low of an extratropical cyclone disappears due to friction. This is just a late stage of a cyclone development and the upper low fills up relatively quickly.

Key Parameters

  • Height contours 500 hPa:
    In the initial stage there is an upper trough. The bottom of the trough forms an inverse omega shape and is detached from the main stream forming a separate low. In the final stage the low weakens and merges with another trough or low.
  • Temperature 500 hPa:
    The air within the Upper Level Low is colder than in the surroundings. The isotherms shows a life cycle similar to that of the geopotential.
  • Height contours 1000 hPa:
    Usually loose field with no remarkable features. In some cases some weak cyclonic circulation may appear.
  • Equivalent thickness:
    There is a thickness ridge ahead of the low and a trough or a distinct minimum behind or in the centre of the low.
  • Thermal front parameter:
    There are two baroclinic zones:
    • A frontal-like cloud band ahead of the low
    • A Baroclinic Boundary (see Baroclinic Boundary ) behind the low.
  • Potential vorticity:
    Within the Upper Level Low there is colder air with lower tropopause than in its surroundings. Consequently, in the area of the low there is a local maximum of PV.

Height contours and temperature 500 hPa

1. Trough stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
14 September 2005/06.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

2. Tear-off stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
14 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

3. Cut-off stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

4. Final stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

Height countours at 1000 hPa

Magenta: height contours 500 hPa, cyan: height contours 1000 hPa
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: height contours 500 hPa, cyan: height contours 1000 hPa

Equivalent thickness and thermal frontal parameter

green: equivalent thickness, blue: front
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: equivalent thickness, blue: thermal frontal parameter

Potential vorticity

magenta: PV
15 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image; magenta: height of PV=1 unit

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    Often there is a conditionally unstable stratification in the lower and middle troposphere, with isentropic values decreasing with height. Otherwise isentropes show a shape of a ridge with the isolines far from each other in the area of the Upper Level Low.
  • Relative Vorticity:
    Due to the cyclonic circulation within the Upper Level Low there is a maximum of relative vorticity.
  • Temperature Advection:
    The air within an Upper Level Low is colder than in the surroundings. Therefore, there is distinct cold advection ahead of the Upper Level Low, and warm advection behind it.
  • Potential Vorticity:
    There is a maximum of potential vorticity above the centre of the Upper Level Low.

15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

Relative vorticity

15 September 2005/12.00 UTC - Vertical cross section; black: isentropes (Thetae), blue: relative vorticity

Temperature advection

15 September 2005/12.00 UTC - Vertical cross section; black: isentropes (Thetae), red thin: temperature advection - CA, red thick: temperature advection - WA

Potential vorticity

15 September 2005/12.00 UTC - Vertical cross section; black: isentropes (Thetae), dark green: potential vorticity

Weather Events

  • The frontal cloud band on the leading edge of an Upper Level Low is usually thick enough to produce precipitation. In some cases there is also frontal cloudiness on the rear edge. Within the cloud bands there are embedded cbs, and therefore the precipitation is showery.
  • When the Upper Level Low is over land, there is a low cloud layer in the centre.
  • Over cold surface there is no convection, and therefore no showers occur.

Over Sea

Parameter Description
Precipitation
  • Moderate to heavy showers.
  • In coastal areas high amounts of precipitation possible.
  • In winter season snow and snow showers are possible
  • Also hail and thunderstorms can occur
Temperature
  • No changes on the surface
Wind (incl. gusts)
  • Strong gusts around Cbs
Other relevant information
  • Risk of moderate to severe icing and turbulence.
  • Risk of flooding in coastal areas and further inland

On the 15th of September 2005 at 15.00 UTC there is an Upper Level Low partly over southern Portugal.

15 September 2005/15.00 UTC - Meteosat IR 10.8 image, weather events (green: rain and showers, cyan: snow, yellow: fog and mist, black: no precipitation)

Over Land

Parameter Description
Precipitation In summer season:
  • In cloud bands long lasted moderate to heavy showery rain
  • In the centre overcast low layered cloudiness with rain or drizzle, sometimes showers
  • Possibly thunderstorms and hail
In winter season:
  • In cloud bands showery snow or rain
  • In centre overcast low layered cloudiness possibly with snow, rain or drizzle
Temperature
  • No changes on the surface
Wind (incl. gusts)
  • Strong gusts around Cbs
Other relevant information
  • Risk of moderate to severe icing and turbulence.
  • Risk of flooding.

An Upper Level Low over Poland on the 8th of June 2005 at 12.00 UTC.

08 June 2005/12.00 UTC - Meteosat IR 10.8 image, weather events (green: rain and showers, cyan: snow, yellow: fog and mist, black: no precipitation)

References

General Meteorology and Basics

  • BOHR P., KATHE G., KNORR M., KURZ M. and LANGE K. - D. (1987): Allgemeine Meteorologie - Leitfäden für die Ausbildung im Deutschen Wetterdienst, 3. Auflage, Selbstverlag des Deutschen Wetterdienstes
  • CONWAY B. J., GERARD L., LABROUSSE J., LILJAS E., SENESI S., SUNDE J. and ZWATZ-MEISE V. (1996): COST78 Meteorology - Nowcasting, a survey of current knowledge, techniques and practice; Phase 1 report; Office for official publications of the European Communities
  • HOSKINS B. J., MCINTYRE M. E. and ROBERTSON A. W. (1985): On the use and significance of isentropic potential vorticity maps; Quart. J. R. Meteor. Soc., Vol. 111, p. 877 - 946
  • HOSKINS B. J. (1991): Towards a PV - Theta view of the general circulation; Tellus, Vol. 43 AB, p. 27 - 35
  • KOISTINEN J.: Lectures on Synoptic Meteorology in the University of Helsinki, unpublished paper
  • KURZ M. (1990): Synoptische Meteorologie - Leitfäden für die Ausbildung im Deutschen Wetterdienst; 2. Auflage, Selbstverlag des Deutschen Wetterdienstes

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LILLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press

Specific Satellite Meteorology

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  • KEYSER D. and SHAPIRO M. A. (1986): A review of the structure and dynamics of upper level frontal zones; Mon. Wea. Rev., Vol. 114, p. 452 - 499
  • LILJEQUIST G. H. and CEHAK K. K. (1984): Allgemeine Meterorologie, 3. Auflage, Braunschweig Vieweg
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  • RASMUSSEN E. (1987): A subsynoptic vortex over the Mediterranean with some resemblance to polar lows; Tellus, Vol. 39A, p. 408 - 425
  • REED R. J. (1990): Advances in knowledge and understanding of extratropical cyclones during the past quarter century: an overview; in Extratropical Cyclones, The Erik Palmen Memorial Volume, Ed. Chester Newton and Eero O Holopainen, p. 27 - 45
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  • SMITH R. K. and ULRICH W. (1993): Vortex motion in relation to the absolute vorticity gradient of the vortex environment; Quart. J. Roy. Meteor. Soc, Vol. 119, p. 207 - 215