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SENEAMMeteorología Sinóptica y Análisis I

Trimestre enero marzo 2010

Detached Warm Front - Key Detached Warm Front - Key ParametersParameters

• Equivalent thickness: 

The cloud field of the Detached Warm Front is within the high gradient zone at the eastern branch of a pronounced ridge of the (equivalent) thickness.

• Absolute topography at 500 hPa: 

It is very similar to the (equivalent) thickness, therefore the cloudiness of the Detached Warm Front is also within the high gradient zone at the eastern branch of a pronounced ridge of absolute topography.

• Warm advection (WA):  The cloudiness of the Detached Warm Front is superimposed upon a distinct

(claro) WA maximum. But often two WA maxima can be observed. In this case the northern maximum is associated with the original Warm Front, and the southern one with the Detached Warm Front.

•

Wind Vectors at 500 hPa: The wind field has, in the area of the cloud field of the Detached Warm Front, a more or less strong southern component, and blows normal to the displacement of the whole frontal system of the ridge and frontal zones. Consequently, cloud elements of the Detached Warm Front are moving quickly southward while the complete cloud configuration is displaced eastward, much more slowly.

Shear vorticity at 300 hPa: The zero line coincides with the leading edge of the Warm Front cloud shield.

Isotachs at 300 hPa: The leading edge of the Warm Front cloud shield is superimposed upon a jet streak with intensities varying from case to case.

  04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; blue: geopotential height 500 hPa, green: equivalent thickness 500/850 hPa

04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa

The parameter distribution is very similar to the ideal situation described above. If the wind vectors at 500 hPa (green arrows) are compared to the relative streams (see Meteorological Physical Background), differences in the directions can be observed at a height close to 500 hPa (296K isentrope). While the absolute winds are coming from a north-westerly direction the relative streams are coming from a more north-easterly direction, which is very close to the orientation of the cloud structure.

WARM FRONTDetached Warm FrontWarm Front BandWarm Front Shield

Detached Warm Front - Typical Detached Warm Front - Typical Appearance In Vertical Cross SectionsAppearance In Vertical Cross Sectionsby ZAMGby ZAMG

Vertical cross sections of the Detached Warm Fronts do not differ from the classical band-type Warm Front.

As described before, the isentropes of the equivalent potential temperature across the Detached Warm Front show a high gradient zone through the whole troposphere, which is inclined upwards from low to high levels.

The colder air can be found in front of and below the high gradient zone, the warmer air being in front, and above

The field of humidity shows high values immediately behind and above the frontal surface of the Warm Front. Low values can be found below the high gradient zone of the equivalent potential temperature.

04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

<04 January 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

04 January 2005/00.00 UTC- Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

04 January 2005/00.00 UTC- Vertical cross section; magenta thin: divergence, magenta thick: convergence, cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values

04 January 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

The first cross section shows the high gradient zones of the equivalent potential temperature of a pronounced surface Warm Front with intense WA, especially through the whole troposphere.

The IR radiance values are characterized by high pixel values in the centre of the Warm Front cloudiness (pixel values between approximately 190 and 200 units)

. Lower pixel values, in connection with the dry (anticyclonic) side (and the superimposed jet), can be found in the leading part of the Warm Front (pixel values between approximately 160 and 180 units).

The field of relative humidity shows high values within the lower to mid-levels, also immediately behind the surface front (above 80%).

The third cross section shows, in the lower and mid-levels, intense upward motion within the high gradient zone which is caused by a zone of convergence in low and mid-levels of the troposphere situated within and below the frontal surface.

Divergence can be found above the frontal surface. The fourth cross section is characterized by a pronounced

isotach maximum within the leading part of the Detached Warm Front at approximately 200 hPa. This maximum is accompanied by a decrease of the WV pixel values.

WARM FRONT BAND (Banda de frente WARM FRONT BAND (Banda de frente caliente)caliente)

Warm Front Bands are accompanied by cloud bands which usually are shorter than Cold Front cloud bands

Chapters:Cloud Structure In Satellite ImagesMeteorological Physical BackgroundKey ParametersTypical Appearance In Vertical Cross

SectionsWeather EventsReferences

Warm Front Band - Cloud Structure In Warm Front Band - Cloud Structure In Satellite ImagesSatellite Images

The satellite image shows an anticyclonically curved synoptic scale cloud band which is connected to a Cold Front cloud band.

In ideal cases:◦ in the VIS image the grey shades are generally white at the rear edge becoming

increasingly grey towards the forward edge;◦ in the IR image the grey shades of the cloud band are grey to white, where the

brighter values appear, in the ideal case, towards the forward and downstream cloud edge.

In reality:◦ very often no continuous cloud band exists but rather several cloud layers with

broken cloudiness, or sometimes even only high cloudiness;◦ in the IR image several white cloud areas are superimposed on grey lower cloud

layers (see Meteorological physical background). High, bright WV pixel values can be observed in the area of the frontal

cloud band. At the leading edge of the cloud band the WV image shows a sharp

gradient from white to black indicating dry air along the cyclonic side of the jet.

In contrast to the Warm Front Shield (see Warm Front Shield), the warm sector of the Warm Front Band is usually cloudless, except in winter and spring time when extended fields of fog and low clouds can exist associated with processes in the lowest layers .

15 September 2004/06.00 UTC - Meteosat 8 IR 10.8 image

15 September 2004/06.00 UTC - Meteosat 8 WV 6.2 image

This case has an appearance very close to the classical description. The Warm Front cloud band can be observed over the Atlantic (just east of approximately 25W) extending to Northern Ireland. Several features mentioned above can be observed:the gradual increase of cloud top temperatures from the rear to the leading edge (in IR image);

the distinct leading boundary of the cloud band (in IR and WV images);

the Dark Stripe (in WV image) along the cyclonic side of the above-mentioned cloud edge;

the cloudless warm sector.

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image

14 November 2004/06.00 UTC - Meteosat 8 WV 6.2 image

This case shows some deviations from the ideal cases. The cloud band of the Warm Front can be located in the IR and WV images from Iceland across the Atlantic towards Norway. In the IR image it consists of several high, i.e. cold, cloud areas and cloud lines superimposed on dark grey, i.e. warmer, cloud tops. Over Iceland the mountains trigger some lee clouds, embedded by the Warm Front Band. In the WV image a broad white band in the area of the Warm Front indicates high water vapour content bounded by Dark Stripes on the northern as well as the southern edges. The northern stripe represents the dry air along the cyclonic side of the jet. The dark grey area extending over Great Britain is an extensive area of Fog and Stratus.

Warm Front Band - Meteorological Warm Front Band - Meteorological Physical BackgroundPhysical Background

n the case of a Warm Front, warm moist air moves against colder dry air. At the boundary of these two air masses the warm air tends to slide up over the wedge of colder air (see Typical appearance in vertical cross section). This process causes the frontal cloud band, and the associated precipitation, found mainly in front of the surface front (or the TFP) (see Weather events).

The idealized structure and physical The idealized structure and physical background of a Warm Front can be background of a Warm Front can be explained with the conveyor belt explained with the conveyor belt theory:theory: Frontal cloud band and precipitation are in general determined by

the ascending Warm Conveyor Belt, which has its greatest upward motion between 700 and 500 hPa. The Warm Conveyor Belt starts behind the frontal surface in the lower levels of the troposphere, crosses the surface front and rises to the upper levels of the troposphere. There the Warm Conveyor Belt turns to the right (anticyclonically) and stops rising, when the relative wind turns to a direction parallel to the front. If there is enough humidity in the atmosphere, the result of this ascending Warm Conveyor Belt is condensation and more and more higher cloudiness.

The Cold Conveyor Belt in the lower layers, approaching the Warm Front perpendicularly in a descending motion, turns immediately in front of the surface Warm Front parallel to the surface front line. From there on the Cold Conveyor Belt ascends parallel to the Warm Front below the Warm Conveyor Belt. Due to the evaporation of the precipitation from the Warm Conveyor Belt within the dry air of the Cold Conveyor Belt, the latter quickly becomes moister and saturation may occur with the consequence of a possible merging of the cloud systems of Warm and Cold Conveyor Belt to form a dense nimbostratus.

DiscussionDiscussion In addition to this idealized structure, the experience from a series of case

studies carried out at ZAMG differs somewhat from the one described above, allowing more differentiation.

In the case of a band type the Warm Conveyor Belt can be observed within the warm sector up to the Cold and Warm Front line.

If there is high cloudiness in front of and parallel to the Warm Front line, it is situated within an (at least in the area of the fronts) ascending conveyor belt from the rear side of the Cold Front extending from south-west to north-east, the so-called upper relative stream.

While the high cloudiness can thus be explained, the Warm Front cloudiness in the lower levels of the troposphere develops, as in the ideal case described above, within the Cold Conveyor Belt.The conveyor belt situation, especially of the Warm Conveyor Belt and the upper relative stream, described above can be found in a thick layer of isentropic surfaces, but there is some tendency for the Warm Conveyor Belt in lower layers to overrun the surface Warm Front to a small extent.

The ascending Warm Conveyor Belt in the warm sector is not accompanied by appreciable cloudiness either because of too dry air masses or/and too little lifting. But it can be observed that in the ascending Warm Conveyor Belt cloudiness may develop leading to a second Warm Front type, the Warm Front Shield.

14 November 2004/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 302K - system velocity: 295° 10 m/s, yellow: isobars 300K, position of vertical cross section indicated

The 302K isentropic surface chosen for the relative streams in the figure below is very close to the upper boundary of the higher gradient, inclined Warm Front zone.

The analysis shows two Conveyor Belts: there is one from eastern directions across The Netherlands,

Belgium, France, turning northward in an anticyclonic direction over the Atlantic. This is a typical example of a Warm Conveyor Belt extending across the warm sector to the leading edge of the Cold Front and the rear edge of the Warm Front cloudiness.

A second relative stream originates from the cold air mass over the Atlantic behind the Cold Front and approaches the Warm Conveyor Belt over Iceland, extending from there on across the Atlantic (approx. 66N/2W) towards Norway parallel to the Warm Conveyor Belt.

Generally speaking, the Warm Conveyor Belt rises from about 800 hPa over the Atlantic up to 400 hPa over Norway. The relative stream from the Atlantic exists in a higher layer, rising from about 600 to 450 hPa. The high cloudiness of the Warm Front exists in this latter relative stream.

15 September 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

Warm Front Band - Key ParameteWarm Front Band - Key Paramete

Equivalent thickness: The cloud band of the Warm Front Band is within the higher gradient zone of equivalent thickness.

Thermal front parameter (TFP): The TFP can be found close to and parallel to the rear edge of the Warm Front cloud band. This is in contrast to the Ana Cold Front where the TFP accompanies the leading edge of the cloud band.

Warm advection (WA): The whole frontal cloudiness lies within a WA maximum, which increases towards the occlusion point. Consequently, the maximum is in front of the frontal line.

Shear vorticity at 300 hPa: The zero line coincides with the leading edge of the Warm Front cloud band.

Isotachs at 300 hPa: The leading edge of the Warm Front is superimposed upon a jet streak

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; red: temperature advection 500/1000 hPa, green: equivalent thickness 500/850 hPa

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa

Fin de Banda del frente caliente

WARM FRONT SHIELDby ZAMG

Warm Front Shields are accompanied by cloud shields

comprising the areas of the warm sector and the Warm Front.

ChaptersCloud Structure In Satellite Images

Detailed discussion of typical cloud configurations.

Meteorological Physical Background Key Parameters Typical Appearance In Vertical Cross

Sections Weather Events References

Warm Front Shield - Cloud Structure In Satellite Imagesby ZAMG The satellite image shows a synoptic scale cloud shield which lies in

front of a Cold Front, and comprising the area of the Warm Front and the warm sector.

In the VIS image the grey shades, in a well developed case, are generally white within the warm sector and near to the surface Warm Front,while the grey shades become gradually more and more grey towards the forward cloud edge (see Warm Front Band).

In the IR image the grey shades of the cloud shield are usually white indicating cold cloud tops, but very often they show substructures differing from case to case.

The pixel values in the WV image within the area of the cloud shield are high and bright.

At the leading edge of the cloud shield the WV image shows, along the jet axis, a sharp gradient from white to black indicating dry air at the cyclonic jet side (see Warm Front Band).

In contrast to the Warm Front Band,(see Warm Front Band - Cloud structure in satellite image ) the warm sector of the Warm Front Shield is overcast with cloudiness within the ascending Warm Conveyor Belt, which can, in a well developed case, extend through a rather deep layer of the troposphere.

12 February 2005/12.00 UTC - Meteosat 8 IR 10.8 image

The IR and the WV image of 12.00 UTC show a cloud shield of a Warm Front extending from south Sweden over the Baltic Sea into Poland and further south. The VIS image from 12.00 UTC shows the shield as greyish. Not related to the Warm Front Shield but still remarkable, are the contrails seen at the rear edge of the Cold Front, southeast of Ireland. Additionally, several features mentioned above can be observed:

bright grey shades in the centre of the cloud shield in the IR and WV images;

high albedo values in the VIS image in the centre of the cloud shield and gradually decreasing values to the forward edge;

the black area in the WV imagery in front of the leading edge of the cloud shield along the cyclonic side of the jet;

the cloud shield encompasses the warm sector and the Warm Front area, which is in contrast to the band-form of the Warm Front.

Warm Front Shield - Meteorological Physical Background

The classical physical Warm Front model as well as the conveyor belt theory of Warm Fronts do not discriminate between the Warm

Front Band and the Warm Front Shield. But the main difference between the Warm Front Band and the Warm Front Shield is the

cloudiness within the warm sector (see Cloud structure in satellite image).

It is not easy to understand this with the classical model. The conveyor belt theory better explains this feature:

The cloudiness within the warm sector is associated with a more or less pronounced upper level front (see Typical

appearance in vertical cross section) and there is a pronounced ascending Warm Conveyor Belt on the various surfaces of these frontal zones. It overruns the Cold and

Warm Front surface lines (which is different from the band type) and mostly exists in a deep layer of the troposphere.

This may be the reason for thick cloudiness and precipitation in the warm sector behind the surface front.

Beside (junto a) this upper level frontal zone there also exists a surface level frontal zone, which is comparable to the band type. Cloudiness on this frontal zone cannot normally be differentiated in the satellite image, but can be explained in a similar way as for the band type

With this type a different relative stream from behind the Cold Front has already been described, forming the high Warm Front Band cloudiness. In the shield type this relative stream can only be seen in the area of the leading edge of the shield. Therefore this stream is either overrun or shifted forward by the dominant Warm Conveyor Belt.

In the case of the band type, it was stated that there is not enough ascending motion with the Warm Conveyor Belt, which is the reason that no cloudiness develops. In the shield type the ascent is much stronger: either this is the characteristic of the special situation or it is the consequence of a further development of the frontal system. If the frontal surfaces approach, then the isentropic surfaces become more and more inclined leading to a stronger ascent of the relative stream. This process can often be followed in the satellite images by the development of cloudiness in the warm sector of a Warm Front band culminating in the formation of the complete cloud shield (see Cloud structure in satellite images).

03 January 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

There is a distinct surface level and a developing upper level front.

The 300K isentropic surface chosen for the relative streams in the first of two figures belongs to the pronounced surface frontal zone.

The 308K isentropic surface in the second figure belongs to the upper level frontal zone.

3 January 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: relative streams 300K - system velocity: 260° 8 m/s, yellow: isobars 300K

3 January 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 308K - system velocity: 260° 8 m/s, yellow: isobars 308K

This isentropic surface is characteristic of the situation and cloud development at the surface frontal zone. The most striking difference to the upper level surface is the northward shift of the ascending Warm Conveyor Belt (left image). This causes an overrunning of the frontal line at a layer of about 700 to 500 hPa. The second relative stream from behind the Cold Front cannot be observed on this isentropic surface. This isentropic surface is characteristic of the situation at the upper level front as well as for the warm sector. Although this situation is rather complex, representing the development and merging of several synoptic scale systems, a lot of features described in the meteorological physical background above can be detected. Both satellite images with superimposed relative streams show a pronounced ascending warm conveyor belt (ascent from about 600 to 400 hPa). The increase of cloudiness within the warm sector is manifested in the relevant area from Iceland to southern Norway.

12 February 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The isentropic surfaces which are used for the relative streams are 300K, representing the situation within the lower levels of the troposphere, and 312K for the upper levels.

12 February 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 300K - system velocity: 325° 10 m/s, yellow: isobars 300K, position of vertical cross section indicated

12 February 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 312K - system velocity: 325° 10 m/s, yellow: isobars 298K

The upper left figure shows an ascending Cold Conveyor Belt from Switzerland/Austria across Germany and then further north forming the cloudiness within the lower layers of the troposphere. The second figure shows, within the Warm Front Shield, an ascending Warm Conveyor Belt originating in front of the Cold Front south of the Alps (at approximately 650 hPa). Above Italy (at approximately 550 hPa) it turns from south-western to south-eastern directions where it stops ascending above Croatia (approximately between 550 to 500 hPa).

Warm Front Shield - Key Warm Front Shield - Key ParametersParametersby ZAMGby ZAMG

Equivalent thickness: The higher gradient zone of the equivalent thickness typical for a frontal zone can be found at the leading part of the cloud shield.

Thermal front parameter (TFP): The TFP has its maximum close to the area of the surface front, which is situated within the cloud shield at the warm side of the higher gradient zone of the equivalent thickness. Consequently, no indication for the location of the surface Warm Front from the cloud image alone is possible.

Warm advection (WA): The whole cloudiness of the Warm Front Shield (frontal cloudiness as well as the cloudiness of the warm sector) is within more or less strongly pronounced WA. Like the Warm Front described before (see Warm Front Band - Key parameters ), the field of WA increases towards the Occlusion point. Therefore its maximum can be found, in the case of an eastward moving frontal system, in the northern part of the cloud shield. A second, usually less pronounced, WA maximum can be found within the central part of the cloud shield (the warm sector). While the first maximum represents the Warm Conveyor Belt ascending to the surface frontal zone, the second maximum represents the Warm Conveyor Belt ascending to the upper level frontal zone (see Typical appearance in vertical cross section).

03 January 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: thermal front parameter 500/850 hPa, green: equivalent thickness 500/850 hPa

Shear vorticity at 300 hPa: The zero line coincides with the leading edge of the Warm Front cloud shield.Isotachs at 300 hPa: The leading edge of the Warm Front cloud shield is superimposed upon a jet streak.

03 January 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: thermal front parameter 500/850 hPa, red: temperature advection 500/1000 hPa

03 January 2005/12.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa

Warm Front Shield - Typical Appearance In Vertical Cross Sections

The isentropes of the equivalent potential temperature across the Warm Front Shield show two higher gradient zones: a surface Warm Front zone and an upper level Warm Front zone. Both zones are inclined from low to high levels and merge within the upper level of the troposphere. In both cases the colder air can be found in front of, and below, the warmer air behind and above the zones

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

The field of humidity shows high values immediately behind and within both frontal surfaces of the Warm Front. Lower values can be found below the higher gradient zones. Of special interest in this connection are the lower humidity values between the two frontal zones, indicating air masses of different origin.

Like the distribution of humidity, the field of temperature advection can also be separated into two parts. WA exists above and within the crowding zone of the surface warm front. CA can be found below and ahead of the surface front. In contrast, the upper level front is completely within WA, with higher values of WA above and within, and lower values below, the higher gradient zone. The maximum of the WA can be found within the higher gradient zones of both frontal surfaces. In the case of the surface warm front the WA maximum is situated at approximately 800 hPa while in the case of the upper level front it can be found at approximately 600 hPa. These two maxima correspond to the two WA maxima in the isobaric fields, the main one in front of and the second one behind the TFP

All three channels of satellite images across the Warm Front Shield show a broad area of high values. In the ideal case typical distributions can be observed. While the IR image shows, across the Warm Front Shield, only weakly increasing values of grey shades from the rear to the leading edge, the distribution of the grey shades in the VIS image is inverse (see Cloud structure in satellite image). The most pronounced feature in the WV image is the minimum associated with the dry air at the leading edge of the shield.

The sharp decrease of IR and WV pixel values at the leading edge of the Warm Front Shield is associated with a jet streak, which can be found at approximately 300 hPa.

03 January 2005/12.00 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

The first cross section shows temperature advection in red. The main maximum is connected with the surface Warm Front zone, the second weaker one can be found in the upper level frontal zone. According to the position of the surface front line, or the TFP, the main maximum is in front of it, the second one behind. The second figure shows relative humidity (blue). A zone of lower values discriminates the two zones of high humidity on top of the two frontal surfaces. The third and last figure shows a good coincidence between the jet maximum at 300 hPa and the sharp decrease of WV and IR pixel values, already mentioned before.

03 January 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

03 January 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values