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).