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Cloud Structure In Satellite Images

The term Baroclinic Boundary is used to describe a front - like cloud band which is located differently when compared to that associated with a front. It can also be distinguished from stationary fronts and air mass boundaries. A Baroclinic Boundary has no significant propagation. The investigation is based on 60 cases.

The appearance of Baroclinic Boundary in different channels:

  • IR imagery:
    • Dark to light grey low and middle level cloud band
    • The Baroclinic Boundary usually appears darker (i.e. warmer cloud tops) than frontal cloud
    • Sometimes a bright jet cloud fibre forms or fibre clouds are embedded within the cloudband
  • WV imagery:
    • The Baroclinic Boundary appears as a dark grey to grey cloud band with sometimes bright high fibres embedded
    • The Baroclinic Boundary generally appears weaker in WV than a frontal cloud band
  • VIS imagery:
    • Depending on the type of cloud the VIS image shows low white patches or cloud bands. If existing, thin high cloud fibres appear grey.

The investigation showed Baroclinic Boundaries at several typical locations (e.g. indicated by the height field at 500 hPa):

  • At the rear of a synoptic scale trough (most frequent type, ~ 50 cases, schematic below left)
  • Baroclinic Boundary associated with ULL (6 cases, schematic below middle)
  • Baroclinic Boundary associated with a deformation zone at a saddle point of upper height fields (3 cases, schematic below right)

 

At the rear of a synoptic scale trough

The set of images below shows a representative case of a Baroclinic Boundary at the rear of a synoptic scale trough extending from the Bay of Biscay to the Tyrrhanian Sea. The position at the rear of a synoptic scale trough is clearly indicated by the heights at 500hPa (below left).

19 October 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
19 October 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models
19 October 2002/06.00 UTC - Meteosat WV image; names of conceptual models
19 October 2002/09.00 UTC - Meteosat VIS image; names of conceptual models

The WV and the VIS images above clearly show the lower cloud tops in the Baroclinic Boundary compared to the CF over the Atlantic. High white fibre clouds can be seen which are reflected in both - WV and IR - images, indicating the jet axis. This type of Baroclinic Boundary can be distinguished as two types, according to development:

 

"In situ" development (~15 cases)

  • Initially low clouds and Stratocumulus sheets exist, which are merging and intensifying (see schematics below, stage 1 - 3)
  • Further development may show either dissipation of the Baroclinic Boundary or a merging with the succeeding frontal system
  • The Baroclinic Boundary cloud band appears either as connected to a WF upstream and/or a CF downstream or can even be a separated cloud feature

18/07.00 - 19/07.00 UTC 3-hourly image loop
18 October 2002/07.00 UTC - Meteosat IR image

The image loop above shows the in situ development of a Baroclinic Boundary starting over N. Spain and protruding into the western Mediterranean.

 

Modification of former frontal cloud (~ 35 cases)

The initiation of this type of Baroclinic Boundary is the modification of a former frontal zone. Such modifications have also been observed from former Occlusion (19 cases), from former elongating Cold Front (12 cases) and from former Warm Fronts or Detached Warm Fronts (4 cases).

The schematics below (stage 1 - 3) outline a typical process: A new frontal system is appproaching the rear of a dissolving and/or elongating old cloud band. The old cloud system splits in two or more parts, where the northern part merges with the new system or is elongating and weakening. The southern part of the old cloud band redevelops to the rear of the trough in the gradient of equivalent thickness. The further development shows a general merging with the succeeding system or weakening and dissolution of the Baroclinic Boundary region.

The set of IR images shown below is characteristic of a development of a Baroclinic Boundary by frontal modification. Below left the initial stage of the process is shown, whereas below right a more developed stage can be seen 12 hours later.

The loop of the IR images shows an old system stretching from Great Britain to Iceland and a newly developed system over the Atlantic. In the loop the splitting of the old cloudband can be observed as well as the redevelopment and the positioning of the southern part at the rear of the upwind trough. The newly developed Baroclinic Boundary is finally positioned over Great Britain and northern France (image below right).

30/06.00 - 31/06.00 UTC 3-hourly image loop
30 January 2001/06.00 UTC - Meteosat IR image
31 January 2001/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

 

Baroclinic boundary associated with ULL

Synoptic scale cloud bands often appear to the rear of and at the leading edge of an Upper Level Low (see Upper Level Low ). They mostly consist of multilayered low and middle clouds and are cyclonically curved. Furthermore, a third zone with high baroclinicity is likely to appear at the north-easternmost part of the Upper Level Low ("Tear - off point"). This part is usually the remains of a former CF and shows as a highly deformed and elongated fibrous cloud band. According to its appearance this cloud band resembles that of the CM of the Deformation Band (see Deformation Band ).

The two IR images below show a case of an Upper Level Low with two well developed Baroclinic Boundaries associated with it, and a Deformation Band near the Tear - off point. The centre of the Upper Level Low is situated over the Aegean Sea and Greece (see Height at 500 hPa, left image below).

05 February 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
05 February 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

The formation of the Baroclinic Boundary takes place in parallel with the development of the Upper Level Low.

 

Baroclinic Boundary associated with a deformation zone at a saddle point of upper height fields

This type is the least common, and can be associated with a "cut - off" process within upper level troughs and ridges. Contrary to the other types described above, the Baroclinic Boundary exists in the frontogenetic/frontolytic areas at the saddle point of four pressure systems. Nearly all cases show a modification of former frontal cloud bands consisting of multilayered low and middle cloudiness which is elongating and gradually dissipating. The two IR images below show a Baroclinic Boundary within a characteristic deformation zone at the saddle point of height fields at 500 hPa (lower left image). It extends from S. France to Ireland and consists of low and middle level multilayered grey clouds. It can clearly be distinguished from all surrounding frontal cloud bands, by its brightness and curvature. The northern part of the old system from which the Baroclinic Boundary is now torn - off can still be seen in front of the trough north of Scotland.

04 April 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
04 April 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

he development of this type of Baroclinic Boundary normally takes place during the "cut - off" process of a trough into an upper level depression.

Meteorological Physical Background

Baroclinicity, in general, is defined as the state of the atmosphere in which surfaces of constant pressure are intersected by surfaces of constant temperature or constant density. The number, per unit area, of isobaric - isosteric solenoids intersecting a given surface is then a measure of the baroclinicity. High baroclinicity shows up as a high gradient of density or temperature which usually accompanies a change of air mass. Consequently a Baroclinic Boundary is - like a front - in principal a boundary between two different air masses. High gradients of ThetaE and equivalent thickness are parameters which indicate baroclinicity. Baroclinic Boundaries show no significant propagation and their position within the synoptic system is different from a classic front.

From the classic front theory a flow from the warm side associated with a ridge of height contours and a cold flow from the rear side of a trough result in significant confluence within the area of the Baroclinic Boundary. Both are indicated by the wind fields and the relative streams.

The dominating physical process from the colder side - the trough side - will be a relative stream at low and middle levels (see schematic below). At upper levels the Warm Conveyor Belt is dominating, which originates within the ridge (or warm side). Elongation and deformation are also well reflected in the wind fields.

In general, a sinking motion at low levels and rising at middle and upper levels can be observed. The advection of cold, dry air at low levels causes superadiabatic stratification above relatively warmer surfaces. Anyway, the vertical expansion of the cold air mass is less than within a CF. Both characteristics can often be seen in the vertical cross sections (see Typical appearance in vertical cross section). The difference in the vertical extension of the cold air may be connected with the difference in propagation between Cold Fronts and Baroclinic Boundaries.

The schematic below shows the typical distribution of the relative streams of a Baroclinic Boundary. The pattern is valid for all three types of the Baroclinic Boundary.

The distribution of vertical streams is similar to a CF in CA. But the sinking of cold air at low levels seems to be contradictory considering the formation of low and middle level cloudiness within the Baroclinic Boundary. One possible explanation may be that the cloudiness forms mainly through condensation at the Boundary between warm and cold air masses, indicated by the slantwise crowding of the isentropes. The sinking at low levels and the rising at upper levels is then compensated by a distinct confluence from different height levels, which is also reflected in the wind fields and the relative streams.

The Baroclinic Boundary to the rear of a synoptic scale trough is the most frequent type and will be therefore be considered as an example in this chapter:

19 October 2002/06.00 UTC - Meteosat IR image; position of vertical cross section indicated
19 October 2002/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The left IR image above shows the first type of a Baroclinic Boundary to the rear of an upper level trough. It reaches from the Bay of Biscay to the Tyrrhanian Sea. The image displays a homogenous grey cloud band with high bright fibre clouds at the rear side, indicating a significant jet streak at the rear of the trough. The white line indicates the orientation of the vertical cross section seen in the image above right. In the cross section the superadiabatic stratification at low levels clearly appears more shallow than in a CF. The high gradient of equivalent potential temperature at mid levels indicates the baroclinic zone of the boundary between both air masses.

The set of images below shows the relative streams at 302K, 314K and 320K. The low levels are dominated by sinking diffluent streamlines, showing an intrusion of cold air from the trough, which leads to rather low (warm) cloud tops. At 314K the upper relative streams and the Warm Conveyor Belt result in the limiting streamline in the centre of the cloud band. Both streams are rising weakly. At 320K a rising Warm Conveyor Belt, originating from the ridge dominates the boundary.

19 October 2002/06.00 UTC - Meteosat IR image; magenta: relative streams 302K - system velocity: 250° 9 m/s, yellow: isobars
19 October 2002/06.00 UTC - Meteosat IR image; magenta: relative streams 314K - system velocity: 250° 9 m/s, yellow: isobars
19 October 2002/06.00 UTC - Meteosat IR image; magenta: relative streams 320K - system velocity: 250° 9 m/s, yellow: isobars

In all types of Baroclinic Boundaries the relative streams are, in general, similar to those within Cold Fronts. The most striking difference is the location of the Baroclinic Boundary, which results in an inverted orientation of the relative streams, when compared to a CF.

The crowding of isentropes is an indication of high baroclinicity, both in fronts and in the Baroclinic Boundary. Because of frontal propagation the decline of the isentropes is steep within fronts. In contrast, Baroclinic Boundaries show a more flat decline, which can also be a result of their stationarity.

Key Parameters

  • Height contours at 500 hPa:
    • Distribution of height at 500 hPa indicates boundary between closed synoptic systems:
    • To the rear of a trough
    • To the rear and in front of an ULL, in the tear - off stage of development
    • At the deformation zone between two troughs and two ridges
  • Equivalent thickness:
    • A distinct gradient of equivalent thickness accompanies the cloud band
    • Mostly weaker than "classical" fronts
    • The appearance of this parameter is similar within all 3 types of Baroclinic Boundary
  • Thermal front parameter (TFP):
    • Maximum of TFP associated with the cloud band
    • Normally, weaker than "classical" fronts
    • No significant propagation
    • The appearance of this parameter is similar within all 3 types of Baroclinic Boundary

 

Height contours at 500 hPa

  1. At the rear of synoptic scale trough
  2. 19 October 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
  3. Baroclinic boundary associated with ULL
  4. 05 February 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
  5. Baroclinic Boundary associated with deformation zone at a saddle point of upper height Fields
  6. 04 April 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa

 

Thermal front parameter (TFP) and equivalent thickness

29 November 2002/12.00 UTC - Meteosat IR image; blue: thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa

Typical Appearance In Vertical Cross Sections

A crowding of isentropes indicates an air mass boundary and a baroclinic zone. Though similar to fronts, the cloud band of the Baroclinic Boundary shows no front like appearance in a vertical cross section. The appearance is similar throughout all types of Baroclinic Boundary.

  • Isentropes (equivalent potential temperature):
    • Zone of high gradient of equivalent potential temperature, gradient mostly weaker than in fronts
    • Flat decline of crowded isentropes - a distinct difference from a front
    • Superadiabatic stratification only at low levels (associated with diabatic heating or cold air advection)
  • Divergence:
    • Zone of convergence within the highest gradient of isentropes, usually weaker than within a frontal zone
  • Vertical motion (Omega):
    • Upward motion within the gradient zone indicating ascent, but usually weaker than in a frontal zone
    • All cases show weak ascent at middle levels
19 October 2002/06.00 UTC - Meteosat IR image; position of vertical cross section indicated

 

Isentropes (equivalent potential temperature)

19 October 2002/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

 

Divergence

19 October 2002/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence, orange thin: IR pixel values, orange thick: WV pixel values

 

Vertical motion (Omega)

19 October 2002/06.00 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

Weather Events

In general observed precipitation is significantly weaker than in fronts, or there may be no precipitation at all.

Parameter Description
Precipitation
  • No active front: large areas with no precipitation
  • Modification of old systems: remains of precipitation activity, light rain and drizzle
  • Upper Level Low: Precipitation at tear - off stage
Temperature No connection (stationary)
Wind (incl. gusts) No connection (stationary)

Example: To the rear of a synoptic scale trough, in the area of northern Spain and SW France.

19 October 2002/06.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm, yellow: fog, black: no precipitation)

References

General Meteorology and Basics

  • CARLSON T. N. (1980): Airflow through 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
  • HARROLD T. W. (1973): Mechanisms influencing the distribution of precipitation within baroclinic disturbances; Quart. J. R. Meteor. Soc., Vol. 99, p. 232 - 251
  • HERZEGH P. H. and HOBBS P. V. (1981): The mesoscale and microscale structure and organization of clouds and precipitation in mid-latitude cyclones. Part IV: Vertical air motions and microphysical structures of prefrontal surge clouds and cold frontal clouds; J. Atmos. Sci., Vol. 38, p. 1771 - 1784
  • KURZ M. (1998): Synoptic Meteorology - 2nd complete revised edition, Deutscher Wetterdienst

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

  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Quart. J. R. Meteor. Soc., Vol. 114, p. 293 - 319
  • HOSKINS B. J. and HECKLEY W. A. (1981): Cold and warm fronts in baroclinic waves; Quart. J. R. Meteor. Soc., Vol. 107, p. 79 - 90