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Appearance in Satellite Data

Secondary Lows appear as a cloud vortex within the frontal cloud band. In this study 23 cases of Secondary Lows within an occluded front were investigated.

The formation of a Secondary Low can be seen in a series of satellite images:

  • Initially the cloudiness thickens, and the whole front seems to be undulating.
  • In the developement stage a cloud vortex appears, rotating cyclonically around the secondary low.
  • In the mature stage there are two separate cloud spirals curling around the lows.
  • Visible images are rarely of use, because Secondary Lows within occluded fronts develop mostly over the Northern Atlantic during the winter season, and the time of daylight is short.
  • In infrared images a white to light grey multilayered cloud spiral develops
  • In water vapour images a white to light grey cloud spiral develops around the Secondary Low. Additionally, there is usually a Dark Eye over it, and a Dark Stripe behind the Cold Front and forward of the Occluded Front.

On the 17th of April 2008 at 12.00 UTC there is an occluded front reaching from the Atlantic Ocean to France. The original low is located southwest of Ireland, the secondary has developed over the Bay of Biscay:

 

17 April 2008/12.00 UTC - Meteosat 9 HRVIS IR image
17 April 2008/12.00 UTC - Meteosat 9 WV image

The sequence of satellite images shows the development of a Secondary Low: on 17 April 2008 there is a long Warm Conveyor Belt Occlusion over the northern Atlantic. The original low is seen in the upper left corner of the images as a cloud vortex. The Occlusion point is approximately over the coast of France. The Secondary Low deepens over the Bay of Biscay. At 06.00 UTC there is some thick cloudiness, by 12.00 UTC a small vortex can be seen. The Secondary Low moves slowly eastwards as the vortex becomes more pronounced, while the original low is stationary. In this case the cloud band related to the Occluded Front splits in two parts.

Loop: 01/12.00 - 02/12.00 UTC three-hourly image
17 April 2008/12.00 UTC - Meteosat 9 air mass image
17 April 2008/00.00 UTC - Meteosat 9 IR image

 

Meteorological Physical Background

The formation of a Secondary Low within an Occluded Front has three prerequisites:

  • A distinct maximum of vorticity both shear and curvature components are usually present)
  • There is a jet streak on the cyclonic side of the occluded front and parallel to it
  • Wind velocities in the jet streak are at least 50 m/s

A Secondary Low often forms (in 65% of the cases) within a Neutral Occlusion, that is, the Occlusion is neither warm nor cold (compare Occlusion: Cold Conveyor Belt Type - Meteorological physical background ). In this type a strong jet runs over the Occlusion point. The low forms usually between the Occlusion point and the midpoint of the occluded Front.

Nearly all Secondary Lows form in systems over the sea during the winter season. This follows from the fact that the basic westerly flow is stronger during the winter, and the deepening of a low is easier over the relatively warm water.

Although the secondary lows are associated with strong jets, there seems to be no obvious connection to the conceptual model FI by Jet (compare Front Intensification by Jet Crossing ). The processes in the left exit region are involved in less than half of the cases. But if the left exit region is favourably situated in the vicinity of the occluded front, it can enhance the development, as in the case on 07 February 2002.

In the process of the secondary low development there is temporarily a back bent part in the occluded Front. The situation, however, is different from the conceptual model Back - Bent Occlusion (see Back - Bent Occlusion - Meteorological physical background ).

Three stages can be distinguished in the development of a Secondary Low within an occluded front:

  1. Initial stage A Secondary Low begins to deepen within the occluded front in the area of a local vorticity maximum. The part of the front around the low has a Wave - like structure for a short time (compare Wave ). This Wave does not amplify, however, but turns into an Occlusion spiral.
  2. Developing stage Whilst the original low fills up, the secondary deepens. At this stage the occluded front with the Secondary Low still moves forward.
  3. Mature stage The occluded front splits into two parts that spiral around their respective lows. The movement of the Secondary Low slows and it begins to fill up.

 

The case of 17 of April 2008:

Below, initial stage at 00.00 UTC: There is an area of a local vorticity maximum and thick frontal cloudiness west of France, where the Secondary Low begins to deepen.

17 April 2008/06.00 UTC - Meteosat 9 IR image; magenta: height contours 1000 hPa, cyan: relative vorticity 500 hPa

Below, developing stage at 12.00 UTC: A deep low pressure centre lies over the Bay of Biscay.

17 April 2008/12.00 UTC - Meteosat 9 IR image; magenta: height contours 1000 hPa, cyan: relative vorticity 500 hPa

Below, mature stage on 17 April 2008 at 18.00 UTC: The Secondary Low is over Brittany.

17 April 2008/18.00 UTC - Meteosat 9 IR image; magenta: height contours 1000 hPa, cyan: relative vorticity 500 hPa

 

Key Parameters

  • Height contours at 1000 hPa:
    On the surface there is an elongated low pressure area connected to a long occluded front. The original low centre is separated from the occluded front, whereas the secondary centre develops within the frontal cloud band.
  • Equivalent thickness at 500-850 hPa:
    There is the typical structure of an Occlusion, and usually the thermal structure of the Occlusion is neutral, see e.g. Occlusion: Warm Conveyor Belt Type (compare Occlusion: Warm Conveyor Belt Type ). In all types of Occlusions there is a thickness ridge along the occluded front.
  • Omega at 700 hPa:
    There is ascending motion related to the whole frontal system, but also a distinct maximum over the deepening Secondary Low.
  • 300 hPa Isotaches at 300 hPa:
    The core of the jet stream crosses the occlusion point, and the strength of the jet streak is at least 50 m/s.

In the images below the Secondary Low is marked by a red arrow.

 

Height contours at 1000 hPa and equivalent thickness at 500 - 850 hPa

17 April 2008/12.00 UTC - Meteosat 9 IR image; magenta: height contours 1000 hPa, green: relative topography 500 - 850 hPa

 

Omega at 700 hPa

17 April 2008/12.00 UTC - Meteosat 9 IR image; yellow: omega

 

Isotachs at 300 hPa

17 April 2008/12.00 UTC - Meteosat 9 IR image; yellow: isotachs 300hPa

 

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    The structure of an Occlusion is seen in the isentropes. As the Occlusions can be fairly old, however, the v-shaped structure with warmer isentropes above may not always be as clear as in younger systems.
  • Divergence:
    As in a typical deepening low, there is convergence at low levels and divergence at high levels over the secondary low.
  • Positive vorticity advection:
    There is a maximum on PVA ahead of the developing low in the lower troposphere.

17 April 2008/10.00 UTC - Meteosat 9 IR image; position of vertical cross section indicated
17 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thick: convergence, magenta thin: divergence
17 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), green: positive vorticity advection (PVA)

 

Weather Events

The Secondary Low Centre within an occluded front is accompanied by rain, showers and possibly thunder.

Parameter Description
Precipitation
  • Moderate to heavy precipitation and showers, some with thunder around the secondary low
Temperature
Wind (incl. gusts)
  • Cyclonically veering winds around the low
Other relevant information

17 April 2008/12.00 UTC - Meteosat IR image; green: weather events ww-code

 

References

General Meteorology and Basics

  • CARR W. M. (1999): International Marine's Weather Predicting Simplified: How to Read Weather Charts and Satellite Images; McGraw-Hill

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.