UPPER LEVEL LOW - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS
by ZAMG and FMI
- Pixel values of IR and WV image:
The pixels of the IR image are characterized by low values representing low clouds (cold air cloudiness, core convection) in the centre of the Upper Level Low. The WV image shows low pixel values representing dry air above the lower clouds. - Isentropes:
The isentropes are characterized by a ridge structure which represents the cold air in or near the centre of the Upper Level Low. Often the lower levels of the troposphere within the centre of the low show decreasing values of equivalent potential temperature with height, indicating a conditional unstable stratification (see Meteorological physical background and Weather events ). - Relative Vorticity:
According to the strong circulation, the relative vorticity field shows a pronounced maximum which is nearly coincident with the ridge structure of the isentropes representing the centre of the Upper Level Low. - Temperature Advection:
The air within an Upper Level Low is colder than in the surroundings. Therefore, there is distinct cold advection ahead of the Upper Level Low, and warm advection behind it. - Potential Vorticity:
There is a maximum of potential vorticity above the centre of the Upper Level Low. Therefore, higher values of PVU are found on the lower levels in the troposphere within the area of an Upper Level Low than in the surroundings.
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24 January 1998/12.00 UTC - Meteosat IR image; position of vertical cross section indicated
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Pixel values in IR and WV, Isentropes
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24 January 1998/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV
pixel values
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Relative Vorticity
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Right: 24 January 1998/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative vorticity, orange thin: IR
pixel values, orange thick: WV pixel values
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Temperature Advection
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24 January 1998/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick:
temperature advection - WA, orange thin: IR pixel values, orange thick: WV pixel values
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Potential Vorticity
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09 May 2001/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta: potential vorticity, orange thin: IR pixel
values, orange thick: WV pixel values
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