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Appearance in Satellite Data

  • The satellite image shows an area of increased cloudiness (vertically as well as horizontally) within the frontal cloud band which has superimposed a PVA maximum at 300 hPa (see Key parameters );
  • VIS, IR and WV images show bright grey shades, indicating thick cloudiness (see Typical appearance in vertical cross section );
  • the increased cloudiness can appear in two forms:
    • lumpy structure, which indicates embedded CBs
    • Wave-like configuration, as a consequence of formation by cyclonic vorticity;
  • this cloud feature is clearly brighter than the surrounding frontal cloudiness (see Key parameters );
  • at the rear of the frontal cloud band, WV imagery indicates a jet axis pointing approximately perpendicular to the cloud band by a Black Stripe as well as Cloud Fibres which may be seen also in the IR image.
12 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image
12 August 2005/12.00 UTC - Meteosat 8 WV 6.2 image

The cloud band of a Cold Front leads from the Atlantic along the west coast of Ireland towards the northern Atlantic. The increased area can be observed northwest of Ireland (around approximately 57N/18W).

The approaching jet can be seen in IR and WV images as high Cloud Fibres over the Atlantic (from approximately 55N/34W to 51N/17W). In addition, the dark area in the WV image heading towards Ireland indicates dry air on the cyclonic side of the jet axis.

24 August 2005/03.00 UTC - Meteosat 8 IR 10.8 image
24 August 2005/03.00 UTC - Meteosat 8 WV 6.2 image

A second example shows the FI by Jet around 58N/4W (but hardly seen in the IR image). A cloud band of a Cold Front leads from the Atlantic then turns northwards. In the WV image the increased cloud area is observed with more greyish shades, indicating that most moisture is found within the lower levels of the atmosphere. The Cold Front itself appears as white and reveals a convective character.

The approaching jet can be seen in the IR and WV images by high Cloud Fibres above the Atlantic (from approximately 42N/35W to 48N/27W). In addition darker areas in the WV image can be seen on the cyclonic side of the jet axis which is an indicator of dry air intrusion there.

 

Appearance in AVHRR imagery

  • Images from Channel 1 (fourth row, left) and Channel 2 (fourth row, right) show more details in the cloud structure than in Meteosat VIS, but the cloud patterns do not differ significantly from Meteosat images. Channel 2 images show more contrast between land and sea than Channel 1 or Meteosat images.
  • Cloud patterns in images from Channel 4 or Channel 5 (third row, left) are similar to Meteosat IR images, but show more fine structures in the area of Front Intensification (FI) by Jet Crossing.
  • RGB-combination of channels (below left, second row) provides a quick overview of cloudiness.
  • Channel manipulation (third row, right) highlights cloud patterns, especially the Jet Cloudiness and the Front Intensification.

In the case of the Front Intensification, high and middle level clouds are clearly observed in AVHRR satellite images. The image in the third row right shows the result of subtracting: Channel3B from Channel 1. This artificial image is an effective tool for pattern recognition and shows areas with particle size of >= 10µm. The jet fibres (with ice particles >=10µm) are very pronounced.

07 February 2000/06.52 UTC - NOAA RGB image (channel 3, 4 and 5)
07 February 2000/06.52 UTC - NOAA CH5 image; FI by Jet Crossing at E. Ireland/Irish Sea

In the above images, Jet Fibres lie almost parallel to 50N latitude. The polar jet crosses the polar front close to the Isles of Scilly (SW England).Front Intensification over E. Ireland and the Irish Sea is seen as a concave cloud edge.

08 February 2000/14.28 UTC - NOAA RGB image (channel 3, 4 and 5)
08 February 2000/14.28 UTC - NOAA RGB image (channel 1, 2 and 4); FI by Jet Crossing at E. Ireland/Irish Sea

In the above images, Jet Fibres from the Bay of Biscay to Belgium cross the polar jet at 52N/10W (Germany). Front Intensification is occurring over NE Netherlands and sea area German Bight.

08 February 2000/14.28 UTC - NOAA CH5 image
08 February 2000/14.28 UTC - NOAA CH1 minus CH3B- image

In the image (above left), middle and high level cloudiness are grey and white areas, respectively. Low level cloudiness and land are dark grey or black. In the image above right the grey and white areas represent either ice crystals (Jet Fibres from Bay of Biscay to Belgium) or areas with high risk of precipitation. Frontal cloudiness over the North Sea, Germany and France and showers over the north of Great Britain are clearly seen in this channel manipulation. The Front Intensification shows as a white bulge over sea area German Bight.

08 February 2000/14.28 UTC - NOAA CH1 image
08 February 2000/14.28 UTC - NOAA CH2 image; Front Intensification of front crossing over NE Netherlands and sea area German Bight

In the upper right corner of the above images, twilight can be seen. The frontal zone, jet fibres and the Front Intensification by Jet (FI) are light grey to white with quite remarkable shadows produced by the high clouds lying over lower clouds. The contrast between land/sea is best seen in Channel 2 (image above right).

 

Meteorological Physical Background

If a jet streak approaches and crosses a frontal zone at a large acute angle, horizontal and vertical increase of frontal cloudiness in the left exit region of this crossing jet streak can be observed. These cloud phenomena are produced by the interaction between front and jet streak. The intensification of cloudiness within the left exit region is caused by the following processes:

  • the frontal circulation (for instance an Ana Cold Front) causes lifting processes within the front;
  • the cross circulation in the left exit region of a jet streak causes an intensification of the temperature gradient (frontogenesis);
  • a PVA maximum in the left exit region contributes to upward motion.

The typical distribution of convergence and divergence, which is caused by the vertical circulation in the area of the exit and entrance region of a jet streak, can be explained as follows: As a consequence of the increased wind shear in the centre of the jet streak a maximum of cyclonic relative vorticity can be found on the cyclonic side and a maximum of anticyclonic relative vorticity on the anticyclonic side of the jet axis. As the jet streak propagates downstream, a maximum of positive vorticity advection exists in the left and a maximum of negative vorticity advection in the right exit region. In the area of the entrance region the situation is reversed. On the other hand, as a consequence of the acceleration of the air parcels in the entrance and deceleration in the exit region, ageostrophic wind components can be found. These wind components point, at the jet level, from the anticyclonic to the cyclonic side in the entrance region and from the cyclonic to the anticyclonic side in the exit region. As a consequence of vorticity advection and ageostrophic winds, convergence can be found in the right exit and left entrance, divergence in the left exit and right entrance region at upper levels of the troposhere (at approximately 300 hPa - jet level).

Often it also can be observed that enhanced cloudiness develops within the left exit region of a jet streak in the cold air mass in the upper level trough behind a Cold Front. This kind of cloudiness can be seen in the satellite image either as cellular clusters (see Enhanced Cumulus ) or as a comma-formed cloud band (see Comma ).

The model of the frontal intensification by a jet streak, shown in the figures below, is an idealized model, which describes a straight or only very slightly curved jet streak. But in reality the curvature of the jet is usually stronger. Therefore positive vorticity advection can be found on the cyclonic as well as on the anticyclonic side of the jet axis. But as a consequence of the additional cyclonic shear, the main PVA maximum remains on the cyclonic side close to the jet axis.

 

Key Parameters

  • Positive vorticity advection (PVA):
    • The enhanced cloudiness lies in the left exit region and has superimposed upon it a fairly strong maximum of PVA at 300 hPa.
  • Shear vorticity at 300 hPa:
    • The cloud field of the Front Intensification by Jet Crossing lies at the cyclonic side of the zero line of the shear vorticity at 300 hPa, which indicates the jet axis.
  • Isotachs:
    • The increased cloudiness lies within the left exit region of a well developed jet streak.
  • Thermal front parameter (TFP):
    • The thermal front parameter indicating the Cold Front, and the zero line of the shear vorticity indicating the jet axis, form an acute angle, sometimes nearly perpendicular.
12 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; red: positive vorticity advection (PVA) 300 hPa, yellow: isotachs 300 hPa

 

Typical Appearance In Vertical Cross Sections

Isentropes of equivalent potential temperature show a Cold Front-like gradient zone, which can often be separated into two parts: a surface and an upper level Cold Front. The surface front generally is more pronounced in the cross section than the upper level front.

The field of vorticity advection is characterized by small values of NVA within the area of the surface front up to about 700 hPa. Above 700 hPa, and within the whole frontal area, PVA can be found. The values of PVA are continuously increasing from approximately 700 hPa up to the jet level, which is situated at approximately 300 hPa. The maximum of PVA can be found between 350 and 250 hPa in front of the gradient zone of the isentropes, which is above the surface Cold Front.

In the ideal case the maximum isotachs should be found upstream from the PVA maximum, but according to the position of the cross section and the stage of development it can also be close to the PVA maximum.

The satellite image shows, in the area of Front Intensification in the VIS, IR and WV images, high pixel values indicating multilevel cloudiness (see Cloud structure in satellite image ). The difference in the grey shades between the area of Front Intensification by Jet Crossing and surrounding areas is much more pronounced in the IR image than in the WV image.

12 August 2005/12.00 UTC - Meteosat 8 IR10.8 image; position of vertical cross section indicated
12 August 2005/12.00 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
12 August 2005/12.00 UTC UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

Looking at the satellite image, see where the cross section appears in the image. Equivalent potential temperature plots show a frontal zone which is well developed in upper levels. Top: The PVA maximum and highest IR and WV signals coincide perfectly around 54N/10W. Bottom: The isotach maximum is situated upstream of the PVA maximum above the gradient zone of the equivalent potential temperature, around 57N/18W.

 

Weather Events

Parameter Description
Precipitation
  • Intensifying precipitation, embedded Cbs and thunderstorms.
  • May prolong the rainy period after the passage of the actual frontal precipitation area.
Temperature
  • No significant change.
Wind (incl. gusts)
  • No significant change.


 

References

General Meteorology and Basics

  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Met. Mag., Vol. 114, p. 293 - 317
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather & Forecasting, Vol. 1, p. 23 - 41
  • ROWNING 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
  • 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

Specific 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
  • ZWATZ-MEISE V. (1987): Satellitenmeteorologie; Springer Verlag, Berlin - Heidelberg - New York - London - Paris - Tokyo
  • ZWATZ-MEISE V. and MAHRINGER G. (1990): SATMOD: An interactive system combining satellite images and model output parameters; Weather & Forecasting, Vol. 5, p. 233 - 246

General Satellite Meteorology

  • BROWNING K. A. and PARDOE C. W. (1973): Structure of low-level jet streams ahead of mid-latitude cold fronts; Quart. J. R. Meteor. Soc., Vol. 99, p. 619 - 638
  • MADDOX R. A. and DOSWELL CH. A. III (1982): An examination of jet stream configurations 500 mb vorticity advection and low level thermal advection patterns during extended periods of intense convection; Mon. Wea. Rev., Vol. 110, p. 184 - 197
  • SHAPIRO M. A. (1981): Frontogenesis and geostropically forced secondary circulations in the vicinity of jet stream - frontal zone systems; J. Atmos. Sci., Vol. 38, p. 954 - 973
  • UCCELLINI L. W. and JOHNSON D. R. (1979): The coupling of upper and lower tropospheric jet streaks and implications for the development of severe convective storms; Mon. Wea. Rev., Vol. 107, p. 682 - 703
  • UCCELLINI L. W. (1990): Process contributing to the rapid development of extratropical cyclones; in: Extratropical Cyclones, The Erik Palmen Memorial Volume, Ed. Chester Newton and Eero O Holopainen, p. 81 - 105
  • ZWATZ-MEISE V. (1990): Use of satellite images for diagnosis and prognosis of certain jet streak phenomena; Proceedings of 8th Meteosat Scientific Users' meeting, Norrköping, Sweden, 28 - 31 August 1990, p. 151 - 160