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Appearance in Satellite Data

Thickness Ridge Cloudiness is a system with the appearance of a band of cloud which can be found in the thickness ridge ahead of a frontal cloud band. It often contains convective clouds in the summer season. The cloudiness in the thickness ridge may be associated with the frontal system in its northern parts or may be separate. No characteristic life cycle can be observed in the cloud features of a Thickness Ridge Cloudiness.

Appearance in satellite images:

  • IR imagery:
    • Mostly fibrous and/or dense cloudiness, white or light grey;
    • cellular features within the cloud band indicating embedded convection;
  • WV imagery:
    • The cloud features in the IR image can be seen as white features in an extended grey area;
    • for some cases a distinct black zone appears between the frontal cloud band and the Thickness Ridge cloud band.
  • VIS imagery:
    • The VIS image generally shows light grey shades, representing middle to high translucent clouds, with white spots indicating embedded convection cells.

THR cloud - connected type
THR cloud - disconnected type
11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image
11 April 2008/06.00 UTC - Meteosat 9 HRVIS image
11 April 2008/06.00 UTC - Meteosat 9 WV6.2 image

The above three images show a case with the Thickness Ridge Cloudiness situated ahead of the frontal cloud band over the Mediterranean Sea. The THR is mostly separated from the frontal cloud band. Nevertheless, a closer look into the IR and HRVIS images can reveal that in its northern part the THR cloudiness is connected to the front with high fibrous clouds. Due to the fact that it is very thick and reaches higher levels, cloudiness of the Warm Front Shield is much more pronounced then the surrounding cloudiness, especially in the WV image.

13 January 2008/06.00 UTC - Meteosat 9 IR10.8 image
13 January 2008/06.00 UTC - Meteosat 9 WV6.2 image

The example shows pronounced THR cloudiness situated in the warm sector in south-westerly upper-level flow ahead of the Cold Front cloud band. Although in winter cases, such as this one, there is no convective development within the THR and hardly any precipitation, in this case some drizzle over England and North Sea was observed (see Weather Events).

09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image
09 January 2008/12.00 UTC - Meteosat 9 WV6.2 image

The example of the 9th of January shows THR cloudiness situated over the Mediterranean Sea. It consists mostly of high clouds with no weather activity. Also Cold Front part of the frontal cloud band behind the THR is quite weak (this can be seen in the Vertical cross section).

Appearance in Meteosat RGB composite imagery

High resolution of Meteosat HRVIS images makes smaller fine-scale cloud structures more distinct. In case of THR cloudiness, it is especially valuable for the recognition of the embedded cellular cloudiness, hence, possibly convective areas.

THR cloud - connected type
09 January 2008/12.00 UTC - Meteosat 9 HRVIS image

To take into account and combine different types of information retrieved from SEVIRI imagery, such as optical thickness of clouds, particle size and phase, upper and mid level moisture and cloud top temperature, combinations of channels are constructed

09 January 2008/12.00 UTC - Meteosat 9 HRVIS image
13 January 2008/06.00 UTC - Meteosat 9 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8, WV6.2i)

The Airmass RGB (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i) can be used during both day- and night-time and for that reason it is very useful. It is often used for the synoptic overview because one can recognize air-mass types in it (warm, cold as well as dry or humid upper level troposphere) at the first glance. In the case of THR cloudiness it can be observed that it is situated within warm air-mass (greenish colours) and that it consists mostly of thick high level clouds.

13 January 2008/06.00 UTC - Meteosat 9 Dust RGB image (IR12.0-IR10.8, IR10.8-IR8.7, IR10.8)

The Dust RGB is more suited for the detection of dust in the atmosphere but it can also be used for other purposes. In case of the frontal cloudiness as well as THR cloudiness reddish colours are very prominent. The red colour reveals thick high level ice clouds and the dark blue colours superimposed over the red usually reveal thin high level clouds.

13 January 2008/06.00 UTC - Meteosat 9 Night-microphysical RGB image (IR12.0-IR10.8, IR10.8-IR3.9, IR10.8)

Similar to the Dust RGB, the Nightmicrophysical RGB uses channel differences of IR channels and because of that it can be used during the day and the night. One can detect high level clouds which, presented in reddish colours, and thin high level clouds, shown in transparent bluish colour. The THR cloudiness in this case consists, in contrast to the Cold Front, of both high level thick and thin clouds.

Meteorological Physical Background

Generally Thickness Ridge Cloudiness is situated within a rising warm conveyor belt and is generally found within a ridge or a closed maximum of equivalent thickness.

Relative streams provide a good explanation of the physical processes of Thickness Ridge Cloudiness formation:

  • A rising warm conveyor belt is associated with the Thickness Ridge Cloudiness;
  • An upper relative stream/dry intrusion can be seen to the rear of the frontal cloud band;
  • The warm conveyor belt dominates the Thickness Ridge Cloudiness over a thick layer and sometimes influences a part of the frontal cloud band (Drier upper relative streams are far behind the Thickness Ridge Cloudiness and usually not involved in the physics of the cloud itself).

11 April 2008/06.00 UTC IR - Meteosat 9 IR10.8 image; blue: thermal front parameter, green: equivalent thickness 500/850 hPa, position of vertical cross section indicated

The above image shows Thickness Ridge Cloudiness over the Mediteranean Sea. The position of the front is indicated by the Thermal Front Parameter. The line of the vertical cross section is superimposed.

11 April 2008/06.00 UTC - ECMWF Vertical cross section; black: isentropes (ThetaE)

A maximum of ThetaE is observed in the area of the Thickness Ridge Cloudiness. This maximum is an indication of a tongue of warm air. It consists of a maximum of closed isolines in the lower layers indicating unstable conditions. These unstable conditions only exist in the area just above the ThetaE maximum. Bellow this maximum the atmosphere is conditionally stable because of the increase of ThetaE with height.The height of the maximum differs according to the seasons.

20 June 2000/06.00 UTC - Meteosat IR image, blue: thermal front parameter 500/850 hPa, green: equivalent thickness 500/850 hPa, position of vertical cross section indicated
20 June 2000/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The left image above shows the Thickness Ridge Cloudiness over the Baltic Sea. The position of the Thickness Ridge Cloudiness within the thickness ridge and the frontal system is indicated by the TFP. The line of the cross section is superimposed. The selection of the isentropic surfaces from the vertical cross sections was made following an analysis of the vertical distribution of the isentropes. The surfaces of 318K and 324K represent the relative streams within and above the superadiabatic stratification.

20 June 2000/06.00 UTC - Meteosat IR image; magenta: relative streams 318K - system velocity: 284° 8 m/s, yellow: isobars 318K, position of vertical cross section indicated
20 June 2000/06.00 UTC - Meteosat IR image; magenta: relative streams 324K - system velocity: 284° 8 m/s, yellow: isobars 324K, position of vertical cross section indicated

In both of the above images the distribution of relative streams shows a dominating rising warm conveyor belt at all levels. In the left image the warm conveyor belt at the 318K isentropic surface can be observed, rising from about 500 hPa up to about 450 hPa over the area of Thickness Ridge Cloudiness. In the higher isentropic surfaces of 324K the warm conveyor belt continues to rise from 400 hPa to about 350 hPa.

The tongue of warm air transported in the warm conveyor belt is also indicated by distinct warm temperature advection and a maximum in the ridge of equivalent potential temperature.
The rising warm conveyor belt stream causes condensation.
The embedded convective cloudiness is a bit of a contradiction.
The Thickness Ridge Cloudiness is mostly consisting of fibrous cloudiness and is seldom associated with precipitation.
During the summer season there can be strong convective activity within the thickness ridge (compare Convective cloud features in typical synoptic environments - The warm sector ). Furthermore, because of the thermal energy and the high relative humidity within the thickness ridge this convective activity can be very strong.

Key Parameters

  • Equivalent thickness and Thermal front parameter (TFP):
    Within the ridge of the equivalent thickness (not accompanied by a TFP), the TFP zones are connected to the consecutive CF and the WF if existing.
  • Equivalent potential temperature at 850 hPa:
    The thickness ridge cloud lies within a ridge or a maximum of the equivalent potential temperature at 850 hPa
  • Height Contours at 1000 hPa and 500 hPa:
    The Thickness Ridge Cloudiness is situated within the ridge of the 1000 hPa and the 500 hPa height, in front of a frontal system
  • Temperature advection (TA) at 700 hPa:
    A distinct maximum of WA is superimposed upon the Thickness Ridge Cloudiness.
  • Showalter index:
    There is usually a stable stratification associated with Thickness Ridge Cloudiness, but during the summer season the area within the thickness ridge is often unstable.


Equivalent thickness and Thermal front parameter (TFP)

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP), green: equivalent thickness 500/850 hPa
13 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP), green: equivalent thickness 500/850 hPa
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP), green: equivalent thickness 500/850 hPa

In all three cases THR Cloudiness is found within a ridge of equivalent thickness. In the 13th January case the equivalent thickness ridge is very pronounced and one can also nicely recognize Cold and Warm front Cloud bands. In 9th January case frontal cloudiness is very weak. The first example of 11th of April one can declare as "front-disconnected" type and the other ones as "front-connected" types but without intensive convection because these are winter cases.

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; magenta: equivalent potential temperature 850 hPa
13 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; magenta: equivalent potential temperature 850 hPa
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; magenta: equivalent potential temperature 850 hPa


Height Contours at 1000 hPa and 500 hPa

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa
13 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa


Temperature advection (TA) at 700 hPa

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; red dashed: temperature advection - CA 700 hPa, red solid: temperature advection - WA 700 hPa
13 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; red dashed: temperature advection - CA 700 hPa, red solid: temperature advection - WA 700 hPa
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; red dashed: temperature advection - CA 700 hPa, red solid: temperature advection - WA 700 hPa


Showalter index

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; yellow: Showalter index
13 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; yellow: Showalter index
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: Showalter index

Whereas in the first case the values of Showalter index are nicely related to the cloudiness within the Thickness Ridge, in the last two examples (winter cases) the values of Showalter index are lower than 3, but for the whole frontal region and not specifically for the THR clouds.

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    A vertical cross section taken across the Cold Front - Thickness Ridge Cloudiness - Warm Front shows the typical frontal gradient of isentropes connected to the Cold Front and Warm Front. In between a maximum of isentropes is observed in the area of the Thickness Ridge Cloudiness. This maximum is an indication of a tongue of warm air. It consists of a maximum of closed isolines in the lower layers indicating unstable conditions. The height of the maximum differs according to the seasons and can be very extreme in summer. The upper part over the maximum has some similarities with an "isentropic trough" usually associated with an Occlusion, but corresponding to a rising warm conveyor belt which is superimposed upon the unstable zone. Depending on the season the layer of the warm conveyor belt can be rather deep.
  • Temperature advection:
    As the Thickness Ridge Cloudiness appears in front of a Cold Front, it generally exists within positive temperature advection (WA) at all levels. This is, in effect, the dominating warm conveyor belt.
  • Relative humidity:
    Although there is a maximum of humidity at the low levels, the rising air of the warm conveyor belt leads to a secondary relative humidity maximum associated with Thickness Ridge Cloudiness in middle and upper levels.
  • Divergence:
    Although there is a distinct convergence maximum in lower levels, there is a broad zone of convergence representing the warm conveyor belt stream at middle and higher levels.
  • Vertical motion (Omega):
    Convergence in lower levels leads to a distinct maximum of upward motion in middle and even higher levels.

11 April 2008/06.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP), green: equivalent thickness 500/850 hPa; position of vertical cross section indicated
09 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa; position of vertical cross section indicated
13 January 2008/06.00 UTC - Meteosat 9 IR10.8 image; blue: thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa; position of vertical cross section indicated


Isentropes, equivalent potential temperature

11 April 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE)

The above diagram shows a typical appearance of isentropes associated with Thickness Ridge Cloudiness in summer time. The tongue of warm air represented by a maximum, or at least a bulge of the isentropes within the centre of a vertical cross section, is the most evident feature with Thickness Ridge Cloudiness.

09 January 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE)
13 January 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE)

The lower of the diagrams shows a common distribution of the isentropes for the winter season, where the warm air tongue is not so strong. The cross-sections show the typical distribution of the isentropes in the summer and the winter season.


Temperature advection

11 April 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick: temperature advection - WA
09 January 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick: temperature advection - WA
13 January 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick: temperature advection - WA

In the upper of the above cross-sections, distinct WA, representing the rising warm conveyor belt, can clearly be seen in the right half of the cross-section. The maximum located at the right-hand side both at lower levels and at upper levels can clearly be linked to the Thickness Ridge Cloudiness.


Relative humidity

11 April 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity
09 January 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity
13 January 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity

A broad zone of increased relative humidity in all levels is connected to the Cold Front, whereas a maximum in the right-hand side of the image, in the upper levels, is connected to the Thickness Ridge Cloudiness.


Divergence

11 April 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thick: convergence, magenta thin: divergence
09 January 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thick: convergence, magenta thin: divergence
13 January 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thick: convergence, magenta thin: divergence


Vertical motion (Omega)

11 April 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion
09 January 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion
13 January 2008/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion

The correspondence between upward motion and THR cloudiness is the best in the last example, where negative values of omega are found in the region of Thickness Ridge. In the first two examples there is hardly any upward motion in the region of the Thickness Ridge.

Weather Events

The Thickness Ridge cloudband is multi-layered and normally without significant precipitation except in cases with embedded convection.

Parameter Description
Precipitation
  • Usually no or very few surface reports of precipitation. Light drizzle or light rain can occur during the winter season.
  • Rain, rain showers or even thunderstorms are possible during the summer season in connection with an unstable stratification during the daytime.
Temperature
  • Rising temperatures in warm sector
Wind
  • Mostly weak winds with a southerly component
Other relevant information
  • Multi layered middle and high cloudiness


13 January 2008/06.00 UTC - Meteosat 9 IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm, yellow: fog, black: no precipitation)

In the example of 13 January 2008, 06 UTC, light drizzle and light rain were reported over the UK. There is also light to moderate southerly and south-westerly wind in the region of the Thickness Ridge Cloudiness. Synop measurements are shown for the region in red rectangulare.

References

General Meteorology and Basics

  • KURZ M. (1990): Synoptische Meteorologie - Leitfäden für die Ausbildung im Deutschen Wetterdienst; 2. Auflage, Selbstverlag des Deutschen Wetterdienstes
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General Satellite Meteorology

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