FRONT DECAY - KEY PARAMETERS

• Height contours at 1000 hPa and 500 hPa:
  
  • Upper level and low level trough corresponding to the frontal system in all stages of life cycle of Front Decay
  • Acute angle between isolines of surface low and Upper Level Low indicating cold advection
• Thermal front parameter (TFP) and equivalent thickness:
  
  • Zone of high gradient of equivalent thickness to the rear of the cloud band
  • Maximum of the TFP accompanies the cloud band
  • Weakening of the thermal front parameter at the end of the life cycle of a Front Decay is possible
• Temperature advection (TA) at 700 hPa:
  
  • Very distinct maximum of CA over the dissipation zone within the frontal cloud band
  • Zero line of TA appears almost at right angles to the front line
  • No difference between the 3 types of conceptual submodel
  • CA remains mostly constant in the life cycle, although a few cases show some weakening at the mature stage of Front Decay
• Isotachs at 300 hPa:
  
  • Isotachs at 300 hPa show a pronounced jet streak along the rear edge of the frontal cloud band
  • Jet axis mostly parallel to the rear edge of frontal cloud band over the dissipation zone (only 30% of all cases show acute angle between jet axis and cloud band)
  • The cloud gap of the Front Decay is always located on the anticyclonic side of the jet axis, near the centre and/or the right exit region of a jet streak
• Vorticity advection at 500 and 300 hPa (VA):
  
  • Field of vorticity advection shows an NVA at 500 as well as at 300 hPa, maximum, mostly associated with the right exit region of the jet streak
  • Maxima of NVA at 500 hPa and 300 hPa intensify at a more mature stage in the lifecycle of the Front Decay
  • Higher coincidence between NVA maxima and the cloud gap, if the dissipation zone is situated in the right exit region of a jet streak

Height contours at 1000 hPa and 500 hPa

	09 October 1999/01.00 UTC - Meteosat IR image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa

Thermal front parameter (TFP) and equivalent thickness

	09 October 1999/01.00 UTC - Meteosat IR image; blue: thermal front parameter 500/850 hPa, green: equivalent thickness 500/850 hPa

Temperature advection (TA) at 700 hPa

	09 October 1999/01.00 UTC - Meteosat IR image; red dashed: temperature advection - CA 700 hPa, red solid: temperature advection - WA 700 hPa

Isotachs at 300 hPa and vorticity advection at 500 and 300 hPa (VA)

	09 October 1999/01.00 UTC - Meteosat IR image; blue: negative vorticity advection 500 hPa, cyan: negative vorticity advection 300 hPa, yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa