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Appearance in Satellite Data

As already mentioned in the general remarks the cloud band of the Occlusion described in this chapter develops in the area of a Wave (see Wave ) but shows, in contrast to the Occlusion of the Warm Conveyor Belt type (see Occlusion: Warm Conveyor Belt Type ), clearly different cloud layers.

In a fully developed stage the satellite image shows two synoptic scale cloud bands:

  • a multilayered frontal cloud band of a Cold Front
  • a lower cloud spiral which seems to penetrate from below the higher cloud band
  • both cloud bands seem to be uncoupled
  • In the VIS image the cloud spiral is white indicating deep cloudiness with high albedo.
  • In the IR image the grey shades of a cold conveyor belt cloud band are spoken rather grey but with some increased white areas superimposed. The following points can be summarized:
    • At the transition between the cloud band of the Cold and Warm Front (oriented south-west to north-east) and the cloudiness of the cold conveyor belt type, a distinct gradient from white to (dark) grey can be observed. After some distance from the transition zone often cloud tops gradually become higher (whiter) followed again by a decrease of cloud tops within the innermost part of the cloud spiral. Such a cloud structure can be explained quite well with the conveyor belt theory (see Meteorological physical background ).
    • During the later stages of development, also high cloudiness can develop leading to a similar appearance to the Warm Conveyor Belt type of the Occlusion (see Meteorological physical background ).
  • In the WV image the two cloud bands with different cloud types are seen even more distinctly. A black stripe characterizing the dry air on the cyclonic side of the jet axis extends parallel to the higher cloud band and crosses the cold conveyor belt cloudiness immediately at the transition zone. This effect cannot be detected in the case of an Occlusion of the Warm Conveyor Belt type. The driest air represented by very black pixel values is included in the spiral development. This fact can be observed in both types of Occlusion (see Occlusion: Warm Conveyor Belt Type ). During the later stages of development high and bright pixel values can be observed within the whole area of the cloud spiral which means that the crossing black stripe vanishes.

On the 1st of December a Comma cloud was located over Ireland, and a Cold Front extends from Norway to England and further southwest. The Instant Occlusion process took place during the next 12 hours.
In the following images the initial and final stages are shown in different channels.

21 June 2005/09.00 UTC - Meteosat 8 IR 10.8 image
21 June 2005/09.00 UTC - Meteosat 8 IR 3.9 image
21 June 2005/09.00 UTC - Meteosat 8 Vis 0.6 image
21 June 2005/09.00 UTC - Meteosat 8 HRVIS image
21 June 2005/09.00 UTC - Meteosat 8 WV 7.3 image
21 June 2005/09.00 UTC - Meteosat 8 WV 6.2 image

The penetration of the cloud band of the Cold Conveyor Belt cloudiness from beneath the south-west to east-north-east oriented frontal cloud band is well developed and can be seen at 09.00 UTC above the Atlantic Ocean at approximately 54N/35W.
The IR-images (10.8 and both 3.9 µm) show quite clear the lower temperature of the Cold Conveyor Belt clouds.
In the visible images (Highres Vis and 0.6 µm) the shadows of the Warm and Cold Front clouds on the Occlusion clouds are quite pronounced.
The WV-images (7.3 and 6.2 µm) show a small dark stripe as separation of the Warm and Cold Front cloudiness and the Occlusion.

21 June 2005/00.00 UTC - Meteosat 8 RGB image (3.9, 10.8 and 12.0)
21 June 2005/09.00 UTC - Meteosat 8 RGB image (0.6, 1.6 and 10.8)
21 June 2005/09.00 UTC - Meteosat 8 RGB image (0.6, 0.8 and 10.8)

During nighttime the RGB-image, RGB 3.9 - 10.8 - 12.0 µm will enhance the difference between low warm clouds (brownish) and the high cold cloudiness (whitish).
The daytime RGB-image, RGB 0.6 - 1.6 - 10.8 µm, shows especially the different water and ice clouds (yellow and pink).
The daytime RGB-image, RGB 0.6 - 0.8 - 10.8 µm, enhances the contrast between the low (yellow) and the high cold clouds (bluish white).

20 June 2005 22.00 UTC - Meteosat 8 RGB image (3.9, 10.8 and 12.0)
21 June 2005 06.00 UTC - Meteosat 8 RGB image (0.6, 1.6 and 10.8)
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image   Loop: 01/12.00 - 02/12.00 UTC three-hourly image

In the loop of the RGB-images (0.6-0.8-10.8 Ám) the low cloudiness (yellow) in the center of the low and beneath the high cloudiness of the occlusion can be seen quite good.

21 June 2005 06.00 UTC - Meteosat 8 RGB image (0.6, 0.8 and 10.8)
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image

Meteorological Physical Background

As has been explained in the general remarks the starting viewpoint for the conceptual model Occlusion: Cold Conveyor Belt Type is the typical configuration in the satellite imagery and its development up to an eventually longer lasting mature stage.

The classical Norwegian model

The classical development of an Occlusion is described in the well-known polar front theory after Bergeron as a development from a Wave stage (see Wave) with a well developed Cold Front (see Cold Front, Cold Front In Cold Advection, Cold Front In Warm Advection, Split Front and Arctic Cold Front) and Warm Front (see Warm Front Band, Warm Front Shield and Detached Warm Front). The basic idea is that the Cold Front moves faster than the Warm Front. Therefore the warm sector continously becomes more narrow until finally the Cold Front overtakes the Warm Front completely, thereby lifting the warm air. This is the typical situation for the Occlusion band which turns cyclonically around the core of the cyclone.

 

According to this classical theory of the Occlusion band, this front separates the cold air mass, which is situated in front of the former Warm Front, from that behind the former Cold Front with a tongue of warm air within the higher levels of the troposphere (see Typical appearance in vertical cross section ). Following the distribution of the temperature in front of and behind the occlusion, two sub-types can be separated:

  • If the air situated in front of the Warm Front has about the same temperature as the air situated behind, the temperature gradient dissolves during the process of the Occlusion. In this case the Occlusion is characterized only by a line of cyclonic shear and convergence which can be found vertically below the merging point of both fronts.
  • If the air situated in front of the Warm Front is colder than the air behind, a so-called Warm Front Occlusion develops which has a forward inclined frontal surface.
  • If the air situated in front of the Warm Front is warmer than the air behind, a so-called Cold Front Occlusion develops which has a backward inclined frontal surface.
Warm Occlusion Cold Occlusion

During this process, the displacement of the low becomes smaller, until it becomes quasi-stationary.

 

Deviations between cloud configurations in satellite images and the classical Norwegian model

Very soon after the use and study of satellite images it became clear that this idealized theory cannot be observed in every step and detail in reality. In particular, the overtaking of the Warm Front by the Cold Front with very narrow warm sectors can never be seen (left schematic). Instead of this a mergence of Cold and Warm Front cloudiness in the centre of the surface low takes place followed by a westward extension of the Occlusion cloud spiral, while the Warm Front cloudiness becomes shorter (right schematic).

 

Beside this there are Occlusion spirals which contradict the overtaking mechanism completely. Those developments show a lower cloud spiral penetrating westward from below the Cold Front and Warm Front.

As already mentioned for Cold Fronts and Warm Fronts, those contradictions between the typical polar front theory and the appearance in the satellite imagery were also one reason for the conveyor belt theory.

 

Conveyor Belt model

This theory is not an independent new model, but allows a different insight into the same phenomena of the Occlusion process. It shows a much bigger variety of development possibilities and clearly indicates that the Occlusion development described by the Norwegian model represents only a special case.

The conveyor belt theory for the Cold Front deals predominantly with two relative streams: the warm conveyor belt and the dry intrusion. The warm front deals predominantly with the relative streams of the warm conveyor belt and the cold conveyor belt. For the Occlusion all three conveyor belts are relevant:

  • The warm conveyor belt is a rising relative stream from south, south-eastern directions turning to north, north-eastern directions; it transports warm and moist air.
  • The dry intrusion is a sinking relative stream from north-west to south-east, splitting into two branches: a further sinking one to the south-west and a rising one to the north-east. The cold conveyor belt is a rising relative stream from east, south-east which is initially below the warm conveyor belt, but then emerges from below and extends to northern directions.

The vertical relation of these three conveyor belts to each other results in the different cloud types of the Occlusion: the multilayered Warm Conveyor Belt Type of the cloud spiral and the Cold Conveyor Belt Type.

The latter form of the Occlusion can much more often be observed in the satellite image than the Warm Conveyor Belt form of the Occlusion (see Occlusion: Warm Conveyor Belt Type ).It can only be explained with the help of the conveyor belt theory.

The dominant streams in this case are the cold conveyor belt and the warm conveyor belt.

The lower cloud spiral develops in the rising Cold Conveyor Belt, while the higher cloud band of Warm Front and Cold Front are determined by the warm conveyor belt.

In literature different configurations of the cold conveyor belt are mentioned:

Investigations at ZAMG on all Occlusion events of one year clarified and contributed to these ideas.

  • The lower levels of the troposphere are characterized by an ascending cold conveyor belt with two sub-types according to its orientation:
    • In some cases a splitting of the stream lines within the area of the point of the Occlusion can be observed. This type exists when the frontal surface reaches the ground.
    • In other cases the cold conveyor belt first follows the Occlusion band from north, north-east to south-west.
  • Both sub-types show the rising and cyclonically curved cold conveyor belts, which are responsible for the development of the lower cloud spiral. Not as easy to explain is the splitting of the relative stream in one of the schematics above. There are two possible obvious explanations:
    • It is a branch of a splitting cold conveyor belt similar to the classical description.
    • The eastern anticyclonic branch represents the lowest parts of the warm conveyor belt, while the western cyclonic branch represents the cold conveyor belt.
  • At the present time intensive investigations favour the second explanation. Reasons for this are:
    • The fact that on the isentropic surfaces of only one Kelvin distance a smooth growth of the eastern branch into the synoptic scale warm conveyor belt can be observed.
    • The good agreement between Warm Front cloudiness and relative streams there.
  • Reasons against this explanation come from the fact that the relative stream of the cold conveyor belt branch seems to arise from an area to the south of the warm conveyor belt. The key for the explanation can be found in an intensive inspection of vertical cross sections.

There is an intensive upward inclination in the cross section of the isentropic surfaces, which leads to the fact that an isentropic surface which reaches the ground in the cross section A1-B1 (schematic bottom left) is already within the middle level Occlusion part in the cross section A2-B2 (schematic bottom right). Consequently in the schematics at the bottom left both relative streams can be seen: the cold conveyor belt which is only in the lowest layers and below the warm conveyor belt. It cuts the cross section perpendicularly, while the warm conveyor belt rises on the isentropic surface above. In the cross section above at the bottom right only the cold conveyor belt can be observed.

  • The mid- and upper levels of the troposphere are characterized by three different conveyor belts. The moist air is within the warm conveyor belt as well as within the upper relative stream at the anticyclonic side of the jet, and the dry air is within the dry intrusion at the cyclonic side of the jet.
  • The warm conveyor belt and the upper relative stream are parallel to the cloud band of the Cold Front, and turn then anticyclonically parallel to the cloud band of the Warm Front (this is a difference from the warm conveyor belt type of Occlusion (see Occlusion: Warm Conveyor Belt Type ).
  • The dry relative stream of the dry intrusion crosses the cloud spiral at the cyclonic side of the jet, which is the area of the Occlusion spiral. This situation restricts the development of higher cloudiness there and leads to the typical appearance in the satellite image (see Cloud structure in satellite image ). Further westwards the tops of the cloud spiral are becoming colder again as the cold conveyor belt ascends there.

The cold conveyor belt described in this chapter develops if the absolute topography within the lower levels of the troposphere shows a closed cyclonic circulation. The mid- and upper levels of the troposphere are characterized by an upper level trough. During the lifetime of such a cloud spiral the trough within the mid- and upper levels of the troposphere might get more and more pronounced ending up in a closed cyclonic circulation. As a consequence of this process the development of high cloudiness within the cloud spiral immediately downstream of the point of the Occlusion can be observed (see cloud structure in satellite image ).

21 June 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section A indicated
21 June 2005/00.00 UTC - Vertical cross section A; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

In cross section A, extending over the Atlantic from approximately 48N/34W to approximately 57N/34W, only the cold conveyor belt on the isentropic surface crosses the cross section perpendicularly. Humidity maxima and IR peaks contribute to the cold conveyor belt.
The relative streams cut the cross section perpendicularly on the isentropic surface of 314 K at a height between 650 and 850 hPa.

21 June 2005/00.00 UTC - Meteosat IR image; magenta: relative streams 314K - system velocity: 239° 18 m/s, yellow: isobars 328K; position of vertical cross section A indicated
21 June 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section B indicated

The cross section B is take perpendicular to the Warm Conveyor Belt. On the isentropic surface of 328K a rising warm conveyor belt can be observed. The relative streams of the Warm Conveyor Belt rise from 600 to 400 hPa and turn to the East and do not go into the direction of the occlusion.

21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
21 June 2005/00.00 UTC - Meteosat IR image (ZAMG); magenta: relative streams 328K - system velocity: 239° 18 m/s, yellow: isobars 328K, position of vertical cross section B indicated

The superimposed model fields of the isotachs and the PVA at 300 hPa are in quite good agreement with the ideal situation and can also be found in the former schematics.

21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, red: positive vorticity advection (PVA), black: zero line of shear vorticity 300 hPa

Key Parameters

  • Absolute topography at 500 and 1000 hPa:
    The absolute topography at 1000 hPa is characterized by a low in the height field in the centre of the cloud spiral. At 500 hPa in the initial stages an upper level trough exists, which develops into an upper level low during a later stage of development. This is a main difference from the Warm Conveyor Belt Occlusion type where a closed cyclonic circulation already exists very early on (see Occlusion: Warm Conveyor Belt Type ). Consequently the Cold Conveyor Belt Type often changes to a Warm Conveyor Belt Type later on.
  • Equivalent thickness:
    A prominent feature is the ridge of the equivalent thickness which is the consequence of the lifted warm air. In the case of well developed deep lows even a spiral structure of the ridge can be observed. But there are also situations which deviate from the distinct ridge structure. Well developed forms of Warm Front Occlusions and Cold Front Occlusions may lead to a more pronounced thickness gradient in the area of the Occlusion. This can be observed much better in the vertical cross section (see Typical appearance in vertical cross section).
  • Thermal front parameter (TFP):
    The thermal front parameter mostly can be found close to the rear area of the cloud spiral. But the very existence of a front parameter depends on the existence of a pronounced temperature gradient.
  • Temperature advection:
    The whole cloudiness of the Occlusion is under the influence of warm advection. The warm advection maximum can be found in the centre of the cloud spiral, mostly close to the point of the Occlusion. The zero line of the temperature advection follows the rearward edge of the Occlusion cloud spiral
  • Zero line of the shear vorticity at 300 hPa:
    The zero line of the shear vorticity, which also indicates the jet axis, crosses the cloud spiral at the point of the Occlusion.
  • Isotachs at 300 hPa:
    The jet axis crosses the frontal cloud band in the area of the point of the Occlusion as described above. If a well developed jet streak exists, the cold conveyor belt cloud spiral is situated within the area of the left exit region of this jet streak; consequently in the satellite image cellular structured colder cloud tops can be observed (see Cloud structure in satellite image).
  • Vorticity advection at 500 hPa and 300 hPa:
    The cloud spiral of the occlusion is in general superimposed by PVA in the mid-levels and upper levels of the troposphere. As upper level troughs are involved, PVA is caused by both curvature and shear. But as described above the zero line of shear vorticity crosses the cloud spiral. The PVA maximum is very often situated within the left exit region of a jet streak (see Meteorological physical background and Weather events). There is a difference from the Warm Conveyor Belt Type of Occlusion, where curvature vorticity plays the dominant role (see Occlusion: Warm Conveyor Belt Type ).

In this case (21 June 2005) most of the numerical model fields fit quite well to the satellite image.

21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa
21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: thermal front parameter 500/850 hPa, green: equivalent thickness 500/1000 hPa
21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: thermal front parameter 500/850 hPa, red: warm advection 500/1000 hPa
21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: positive vorticity advection (PVA) 500 hPa
21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa, red: positive vorticity advection (PVA) 300 hPa
 
21 June 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: shear vorticity 300 hPa, brown: curvature vorticity 300 hPa

Typical Appearance In Vertical Cross Sections

No notable difference could be found in the configuration of NWP parameters within vertical cross sections between Warm Conveyor Belt Occlusion and Cold Conveyor Belt Type.

The isentropes of the equivalent potential temperature generally show a distinct trough structure indicating the warmer air which has been lifted (see Meteorological physical background ) and the crowding zone of a surface front. In this case the upper level trough is forward inclined with height. The most frequent configuration is a Warm Front inclined crowding zone in front of the isentropic trough structure. The difference between the cross section of an Occlusion cloud band and of a Warm Front cloud band can be seen in the layer above the Warm Front surface: in a Warm Front case the isentropic trough configuration does not exist (see Warm Front Band , Warm Front Shield and Detached Warm Front ).

According to the sub-types Warm Occlusion Type, Cold Occlusion Type and Neutral Occlusion Type (see Occlusion: Warm Conveyor Belt Type - Key Parameters ) the following three vertical distributions of frontal zones can be expected.

In the investigation carried out by ZAMG two main different configurations could be observed:

  • A Warm Front type: in this case the isentropic trough is forward inclined on top of the Warm Front zone.
  • A Warm Front inclined crowding zone in front of and a Cold Front inclined crowding zone behind, the isentropic trough feature. In this case the forward inclination of the isentropic trough is not as strongly pronounced as in the other case. Sometimes also a straight vertical trough axis can be observed.

The field of the relative humidity is characterized by high values, of about 80% and even more, within the area of the isentropic trough. In the case of a forward inclined trough the maximum values of the humidity have forward inclination, two low values of relative humidity can be observed within and below the lower isentropes of the Warm Front - like crowding zone. This minimum typically is situated between approximately 800 and 700 hPa. A second minimum of the relative humidity can be found on the rear side of the isentropic trough within the upper levels of the troposphere at approximately 300 hPa. This corresponds to the following dry intrusion.

The whole area of the Warm Front inclined crowding zone and the leading part of the isentropic trough feature is under the influence of pronounced WA. The maximum of WA mostly can be found within the lower levels of the troposphere very close to the area where the Warm Front inclined crowding zone reaches the surface. A secondary pronounced area of WA can be found within the upper levels of the troposphere on the rear side of the isentropic trough. This area is typically situated at approximately 300 hPa and indicates the tropopause. The lower and mid-levels of the troposphere are characterized by pronounced CA. The zero line of temperature advection is close to the minimum of the isentropic trough in the lower and mid-levels of the troposphere. Very often the zero line in these levels is characterized by a forward inclination which leads to the development of a potentially unstable stratification of the troposphere (see Meteorological physical background and Key parameter).

The field of vorticity advection is characterized by PVA within the whole area of the Occlusion. Typically two PVA maxima can be found in the cross section. The main maximum exists in the upper levels of the troposphere on the rear side of the isentropic trough and is connected with the left exit region of a jet streak; a weaker PVA maximum exists within the Warm Front inclined crowding zone in the lower levels of the troposphere at approximately 800 hPa. This maximum is connected with the already pronounced cyclonic circulation in these levels and indicates the propagation of the low area (see Meteorological physical background and Key parameters).

The field of divergence shows pronounced convergence within the area of the Warm Front inclined crowding zone and within the lower part of the leading area of the isentropic trough configuration. In the upper part divergence prevails. The zero line of divergence can be found very close to the trough axis.

According to the distribution of convergence, temperature advection and vorticity advection, the field of vertical motion (omega) shows a wide area of strong upward motion within the area of the occlusion. The strongest upward motion typically can be found within the mid-levels of the troposphere.

In the VIS image the whole frontal cloud band is characterized by high pixel values. In contrast to this the pixel values in the IR and WV images depend on the location of the cross section as well as on the stage of development. During the initial stages the pixel values of the IR and WV images within the area of the dry intrusion, close to the point of the Occlusion, are characterized by lower values typical for low and mid-level cloudiness. Downstream of the cloud spiral the pixel values are increasing continuously. During the maturing stages, when the cloud spiral is also accompanied by high cloudiness within the whole cloud spiral, the pixel values are as high as in case of an Occlusion of the Warm Conveyor Belt type (see Occlusion: Warm Conveyor Belt Type ).

21 June 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated
21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values
21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values
21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence, orange thin: IR pixel values, orange thick: WV pixel values
21 June 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values

The isentropes show an inclined crowding zone similar to a Warm Front with unstable behaviour within the lower levels of the troposphere and an isentropic trough configuration which is forward inclined. High values of humidity can be found forward inclined within the leading part of the trough, humidity minima within the Warm Front inclined crowding zone at approximately 500 hPa and at the rear side of the Occlusion cloud band at approximately 300 hPa. The latter represents the dry intrusion. The field of temperature advection shows strong WA within the leading part of the Occlusion and strong CA within the rear part of the Occlusion. The zero line within the lower and mid-levels of the troposphere has a forward inclination which leads to the development of a potentially unstable stratification of the troposphere. The field of vorticity advection contains only the high level PVA maximum mentioned above: a PVA maximum at approximately 400 hPa at the rearward edge of the cloud band (around 34W48N). However within the lower levels, between approximately 600 and 800 hPa, there is some PVA, but a typical PVA maximum is not seen. The main feature in the field of divergence is a pronounced convergence zone within the lower layers of the Occlusion trough extending upward in the Warm Front - like crowding zone. Pronounced divergence can be found above this zone. The field of omega shows strong upward motion within the whole area of the Occlusion.

Weather Events

Parameter Description
Precipitation
  • Intensive precipitation within and especially ahead of the Occlusion point.
  • Rainbands typically oriented parallel to the occluded front (5-10 km wide).
Temperature
  • Rising temperature associated with the passage of the secondary Occlusion.
Wind (incl. gusts)
  • Strong winds circulating around secondary vortex, veering of the wind associated with the passage of the secondary Occlusion.
Other relevant information
  • rear part of cloud spiral close to Occlusion point, possibly situated to the left exit region of a jet streak, is predestined for convective development due to potentially unstable troposphere.
28 March 1997/12.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, purple: freezing rain, red: thunderstorm with precipitation, orange: hail, black: no actual rain or thunderstorm with precipitation)

References

General Meteorology and Basics

  • BENNETTS D. A., GRANT J. R. and MCCALLUM E. (1988): An introductory review of fronts. Part I: Theory and observations; Met. Mag., Vol. 117, p. 357 - 370
  • BENNETTS D. A., GRANT J. R. and MCCALLUM E. (1989): An introductory review of fronts. Part II: A case study; Met. Mag., Vol. 118, p. 8 - 12
  • 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

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
  • CARLSON T. N. (1987): Cloud configuration in relation to relative isentropic motion; in: Satellite and radar imagery interpretation, preprints for a workshop on satellite and radar imagery interpretation - Meteorological Office College, Shinfield Park, Reading, Berkshire, England, 20 - 24 July 1987, p. 43 - 61
  • EVANS M. S., KEYSER D., BOSART L. F. and LACKMANN G. M. (1994): A satellite derived classification scheme for rapid maritime cyclogenesis; Mon. Wea. Rev., Vol. 122, p. 1381 - 1416

Specific Satellite Meteorology

  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Met. Mag., Vol. 114, p. 293 - 317
  • BROWNING K. A. and HILL F. F. (1985): Mesoscale analysis of a polar trough interacting with a polar front; Quart. J. R. Meteor. Soc., Vol. 111, p. 445 - 462
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather&Forecasting, Vol. 1, p. 23 - 41
  • BROWNING K. A. (1990): Organization of clouds and precipitation in extratropical cyclones; in: Extratropical Cyclones, The Erik Palmen Memorial Volume, Ed. Chester Newton and Eero O Holopainen, p. 129 - 153
  • CARLSON T. N. (1980): Airflow trough mid-latitude cyclones and the comma cloud pattern; Mon. Wea. Rev., Vol. 108, p. 1498 - 1509
  • GREEN J. S. A., LUDLAM F. H. and MCILVEEN J. F. R. (1966): Isentropic relative - flow analysis and the parcel theory; Quart. J. R. Meteor. Soc., Vol. 92, p. 210 - 219
  • 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
  • THEPENIER R.-M. and CRUETTE D. (1981): Formation of cloud bands associated with the American subtropical jet stream and their interaction with mid-latitude synoptic disturbances reaching Europe; Mon. Wea. Rev., Vol. 109, p. 2209 - 2220

Special Investigation: T-Bone

A T-bone is a frontal structure seen during a special development of a deepening low. It was first identified in 1990 by Shapiro and Keyser. The main feature of the cloudiness in a T-bone is that the Cold and Warm Fronts are separated from each other; the Cold Front near the low centre weakens, whereas the Warm Front and the Cold Front farther away strengthen. T-bones develop usually over the sea in wintertime in a strong zonal flow with a flat and confluent upper level trough. The name "T-bone" comes from the frontal fracture stage, where the Cold Front and the Warm Front are separated and perpendicular to each other.

From satellite image and conceptual model points of view, the T-bone is a special case of a Cold Conveyor Belt Occlusion. Both start as a Wave in a baroclinic zone and end as an Occlusion. There are, however, significant differences in the stages in between. T-bones can also be seen as a type of Rapid Cyclogenesis (see Rapid Cyclogenesis ), because a deep surface low with strong winds and a typical cloud head development are observed. There is also a dissipation of cloudiness in the gap between the fronts due to the effect of a right exit region of a jet, which is similar to the conceptual model Front Decay (see Front Decay ). Also, because the Cold Front is weak at lower levels, T-bone has features similar to an upper Cold Front (see Cold Front ).

In this chapter two cases are compared: a Cold Conveyor Belt Occlusion 05 - 06 January 2003 and a T-bone 01 - 02 April 2001; the main differences are presented.

The T-bone development stages are presented in the following schematics:

As there are no significant differences in the Wave and occluded stages, only the changes that occur during the cloud head and cloud hook stages are discussed here.

 

Satellite Images

Appearance of a Cold Conveyor Belt Occlusion in satellite images:

In IR images a lower cloud spiral gradually appears from below the higher cloud band. There is a dark area between Cold and Warm frontal cloudiness and the cloud head.

In WV images a Dark Stripe behind the Cold Front crosses the cold conveyor belt near the Occlusion point.

05 January 2003/18.00 UTC - Meteosat IR image
05 January 2003/18.00 UTC - Meteosat WV image

Appearance of a T-Bone in satellite images:

In tha IR images below, a weak lower cloud spiral seems to exist before the Occlusion. In the cloud head and cloud hook stages the cold frontal clouds clearly dissolve near the Warm Front due to the frontal fracture and the effect of the right exit area of the jet stream.

In the WV images below, there is a Dark Stripe behind the Cold Front, but it does not cross it as in the case of the Cold Conveyor Belt Occlusion. The Cold Front near the surface low is very dark grey in WV images.

01 April 2001/18.00 UTC - Meteosat IR image
01 April 2001/18.00 UTC - Meteosat WV image

The Cold Conveyor Belt Occlusion case:

The loop shows IR images from 05 January 12.00 UTC to 06 January 00.00 UTC. In the upper left corner there is a Wave that moves eastwards and develops into a Cold Conveyor Belt Occlusion, although there are also some features af Rapid Cyclogenesis present. The cloud head stage is reached on 05 January 18.00 UTC and the cloud hook stage on 06 January 00.00 UTC. The depression occludes on 06 January 00.00 UTC.

05 January 2003/12.00 UTC - Meteosat IR image
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image

The T-bone case:

The loop below shows IR images from 01 April 15.00 UTC to 02 April 10.00 UTC. There is a Wave over the Atlantic that moves towars the British Isles and develops into a Warm Occlusion (for the of Occlusions see Meteorological Physical Background). The cloud head stage is reached at 18.00 UTC on 01 April and the cloud hook stage between 00.00 - 06.00 UTC on 02 April. By 02 April 12.00 UTC the depression has occluded.

01 April 2003/15.00 UTC - Meteosat IR image
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image

 

Numerical Parameters

The height contours of 1000 and 500 hPa are similar in both cases: i.e. a rapidly deepening surface low with an upper trough to the west.

Temperature advection is, likewise, similar: the Warm Front and the cloud head cloud systems are within warm advection in both cases.

Jet stream:

In the case of the Cold Conveyor Belt Occlusion there is a strong jet following the cyclonic edge of the frontal cloudiness. In the T-Bone, there are two jets: one ahead of the Warm Front and a second behind the Cold Front. In the frontal fracture stage they are separated, in the Occlusion stage the two jets merge.

Zero line of shear vorticity at 300 hPa:

In the Cold Conveyor Belt Occlusion the zero line of shear vorticity is a relatively straight line following the jet streak. In the T-Bone the zero line is curveed within the cloud head and cloud hook. The curve straightens somewhat in the Occlusion stage.

05 January 2003/18.00 UTC - Meteosat IR image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa
01 April 2001/18.00 UTC - Meteosat IR image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa

 

Vertical Cross Sections

The cross sections below are taken during the cloud head stage. They show the properties of the partly dissolved Cold Front, which is the main difference between Cold Conveyor Belt Occlusion and a T-bone.
The cross section is taken through the partly dissolved Cold Front near the surface low. The Cold Front is seen on the left, approximately in the area between longitudes 18W and 12W. The cross sections show:

  • Weak Cold Front structure in the isentropes below 600 hPa between longitudes 18W and 16W with low IR and WV signals, and a weak upper Cold Front between 16W and 12W with higher IR and WV signals
  • Warm advection in the area of Cold Front in the lower troposphere, very weak cold advection in the area of the upper Cold Front
  • Very strong rising motion in the area of the front, not ahead and behind it as in a Cold Front (see Cold Front - Typical appearance in vertical cross sections )
  • High humidity throughout the frontal zone

It can be concluded that although there is a weak cold frontal structure, the parameters do not fit a Cold Front.

01 April 2001/18.00 UTC - Meteosat IR image; position of vertical cross section indicated
01 April 2001/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values
01 April 2001/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values
01 April 2001/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

 

Weather Events

T-Bones are frontal structures that are usually associated with rapidly deepening lows. Heavy rain occurs ahead of the Warm Front, and stormy winds around its tail, especially in systems over the ocean. Contrary to a Cold Conveyor Belt Occlusion, there is no precipitation associated with the Cold Front located near to the Warm Front.