Typical Appearance In Vertical Cross Sections

Though there is cloud dissipation in the area of Front Decay, vertical cross sections from the numerical model still show a downward inclined compact zone of moist isentropes (equivalent potential temperature). However, derived parameters of TA, vertical motion, humidity and divergence all indicate cloud dissipation.

  • Temperature advection: The decaying part of the frontal cloud band is completely positioned in CA at all levels, or at least only at upper or lower levels.
  • The relative humidity has a minimum at high and middle levels, which can be interpreted as dry (cold) descending air within the frontal zone
  • There is a pronounced zone of divergence below and within the frontal surface and convergence above, which is in contrast to the classical CF.
  • The field of vertical motion is characterised by positive values within the gradient zone indicating distinct descent. This is in contrast to the classical CF. Cases with remaining low level cloudiness show rising at low and middle levels in front of the gradient zone.

18 December 2006/12.00 UTC - Meteosat 8 IR10.8 image; position of vertical cross section indicated

 

Temperature advection: vertical cross sections from numerical model

18 December 2006/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values

 

Relative humidity: vertical cross sections from numerical model

18 December 2006/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values

 

Divergence: vertical cross sections from numerical model

18 December 2006/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence, orange thin: IR pixel values

 

Vertical motion (Omega): vertical cross sections from numerical model

18 December 2006/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values