18 - 19 FEBRUARY 1996 - CLOUD FIBRE AT JET AXIS

by ZAMG


18 February 1996/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models; SatRep menu: key parameters for Jet Cloudiness
18 February 1996/06.00 UTC - Meteosat WV image; yellow: isotachs 300 hPa, black: shear vorticity 300 hPa
There are two fibrous structures diagnosed in the IR image as the conceptual model "Jet Cloudiness (Fibre)" (compare Conceptual Models: Jet Cloudiness (Fibre) ): one over the Atlantic west of Iceland and a second one over the Atlantic south-west of Ireland crossing the Cold Front cloud band at a very pronounced angle. Comparing with the WV image these fibres are respectively at the eastern and northern boundary of a broad stripe of humid air with an adjacent narrow Black Stripe. This is a typical situation for a jet stream with the sharp boundary in the WV image and the fibres in the IR image close to the jet axis.

Consequently the key parameters from the conceptual model are contained in the following combination:

Isot300 + Scher300=0 This parameter combination shows the isotachs at 300 hPa greater than 30 units (m sec-1) and the zero line of shear vorticity at 300 hPa which marks the jet axis; one can get information about the existence and strength of jet streaks and the course of the jet axis

However, although the 300 hPa level should be very close to the jet level, differences may occur. Therefore the bright boundary in the WV image need not automatically coincide with the zero line of shear vorticity at 300 hPa; on the contrary there can be significant differences as can be noticed by comparing the zero lines of shear vorticity at 500 and 300 hPa. Such a situation exists also in this case study; the humid air reaches to the anticyclonic side of the zero line (black line) of shear vorticity.

Cloud fibres which form within a rising conveyor belt should appear, according to the conceptual model, immediately at the jet axis but reaching into the anticyclonic side; consequently a comparison between the location of the fibre and the zero lines of shear vorticity at different levels can give some information about the height level of the real jet core. In this example it should be higher than 300 hPa over the Atlantic, but very close to 300 hPa at the point where it crosses the Cold Front cloud band.


SUB-MENU OF 18 - 19 FEBRUARY 1996
WARM FRONT
INTENSIFICATION OF COLD FRONT THROUGH JET STREAK CROSSING