One of the most prominent features for the detection of jet axes are cloud fibres in the IR image accompanied by Black Stripes on the cyclonic (left-hand) side in the WV image.

These fibres are narrow with an order of magnitude of several tens of kilometres but may be very extended with an order of magnitude of several 1000s of kilometres.

They develop on the jet axis with the cloudiness reaching into the anticyclonic side. Upward motion, which is one necessary condition, has several sources:

• a smoothly rising warm conveyor belt with the limiting streamline on the jet axis;
• a very small scale circulation cell at jet level on the jet axis, with the upward branch on the anticyclonic and the downward branch on the cyclonic side.

Whether cloud fibres develop or not is therefore a result of enough humidity in the air mass of the warm conveyor belt, and enough upward motion from a combination of both effects mentioned above. If the air mass in the warm conveyor belt (or a similar relative stream) is very humid, a broader band of high Cloud Fibres develops (see Warm Conveyor Belt and Thickness Ridge Cloudiness ). If humidity is restricted, condensation takes place only where all conditions act together, which is very often the case in the exit region of the jet streak close to the jet axis. Once the Cloud Fibre exists, the high wind velocity at the jet axis transports humidity and cloud particles downstream, which results in the rather long extension of these fibres.

Because of this necessary combination of sufficient upward motion and humidity, Cloud Fibres may also be short-lived cloud configurations but in any case they are a very prominent indication for the jet axis. The Black Stripe in the WV, which is a supplementary satellite feature for this conceptual model indicates sinking dry air on the cyclonic jet side.

Cloud Fibres often form large acute angles with frontal cloud bands. This is the case when a jet streak crosses or is going to cross the frontal cloud band with the possible consequence of a front intensification in the left exit region (see Front Intensfication By Jet Crossing ); therefore a Cloud Fibre can also be used as an indication for the development of a Front Intensification. Similar consequences may also occur for the conceptual model of Front Decay; (see Front Decay ).

Model parameters adding to and supporting this conceptual model are the zeroline of shear vorticity at 300 hPa as indication for the jet axis, and isotachs at the same level as indication for jet intensity and the location of entrance and exit regions. However, very often the zeroline of shear vorticity at 300 hPa is indeed parallel to both Cloud Fibre and the WV Black Stripe but shifted 2 - 3 degrees in the anticyclonic direction, with the result that the Cloud Fibre, or at least bright areas in the WV, appear on the cyclonic side. It is supposed that the cloud feature indicates the jet axis but that either the jet maximum is at higher levels than 300 hPa, leading to an inclination of the zeroline of shear vorticity with height onto the cyclonic side, or that the model over the wide areas of the Atlantic might be somewhat erroneous. Such a discrepancy very often can be observed at the leading edge of Warm Front cloud shields (see Warm Front Shield ), an area where the first explanation might be appropriate.

Appearance in AVHRR imagery

• Jet Fibres are composed of cirrus fibres, which are best seen in NOAA images.
• The most suitable AVHRR imagery for detecting jet clouds are 345-images, in which thin high clouds are seen more distinctly than in 124-images.
• Jet clouds develop near the jet core, quite often in the entrance region, where the clouds are more widespread than in the jet maximum area.

The top left satellite image shows the NOAA 124 image of a jet (cirrus fibres within a jet streak) streak extending from central Sweden down to the Baltic States. The Cirrus is relatively transparent in this image, while in the NOAA 345 image (top right) the jet streak is much easier to detect.

Similar characteristics appear in the individual VIS and IR images (bottom left and bottom right).

Also some transverse short cirrus bands can be seen within the jet fibre, clearest in NOAA 345 image.

The influence of mountains:

This satellite image shows fibrous jet clouds spreading from the Norwegian Sea over middle Scandinavia within a strong northwesterly jet stream. The Norwegian mountains are intensifying the jet cirrus clouds along the east side of the mountain range.

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