ETT Model Fields - 12 UTC 7 August 2004

Isentropes
The isentropes θe show increasing lower level instability to the northwest of the system. To the southwest we see very little change in the isentropes at low levels with some ridging now at higher levels.

Orange thin: IR pixel values

Orange thick: WV pixel values

The high values in the IR image are clearly reflected in the pixel values in this cross section. The values of WV are higher than the IR values which indicates that there an intrusion of drier stratospheric air to the rear of the development.

The VCS was laid from position A to B.

Convergence/Divergence:
The field of divergence shows convergence at low levels ahead of the trough, the area of convergence has now become narrower with significantly high values associated with the extension and sharpening of the frontal zones. In the upper part divergence prevails. The zero line of divergence is found close to the trough axis.

Isotachs
The Isotachs fields show a maximum of velocity at mid to lower levels approximately 22 m/s (45 KT). to the east of the center decreasing to approximately 18 m/s 35 KT to the west of the center. The Jet stream is shown to the north with speeds of approximately 28 m/s (60 KT).

Vertical motion (omega):
Mass ascent is occurring throughout the atmosphere directly ahead of the trough with descent is occurring the the rear. The zero line of Ω marks the frontal boundary and trough axis.

Potential Vorticity:
The dynamic tropopause marked by PV=2 has descending behind the trough and both the WV and IR images show significant dips coincident with this feature.

Vorticity advection:
A PVA maximum exists close to the trough, at this stage the trough is at it's sharpest and the PVA is at it's strongest.

Relative humidity
A maximum of relative humidity is found within the developing system. Significantly drier polar air is seen to the rear on the development.

Temperature advection:
WA is found throughout the whole troposphere ahead of the frontal zones, coincident with the maximum of Ω. Significant cold advection is also apparent to the area with the zero of advection marking the cold frontal boundaries.