24 November 2005: 06 UTC

Isentropes

In the pre-merging stage the isentropes of the equivalent temperature show a cold-front-inclined crowding zone (57°N/1°W to 56°N/2°E). In the area of the comma-like feature the isentropes show potential instability in lower layers (58°N/5°W to 59°N/8°W, below 800 hPa). The distribution of the isentropes matches the conceptual model of an instant occlusion.

Isotachs

At this time, the jet came from behind the comma-like structure. The development of the comma took place in the left exit region of the jet streak.

Vorticity Advection

The vorticity advection shows a typical distibution for the pre-merging stage: PVA in the area of the comma and the cold front. One main maximum is found in the area of the comma at 500 hPa (58°N/5°W). A second one is connected with the cold front in 300 hPa (56°N/2°E). A merging of these two maxima can be observed during the next hours.

Temperature Advection

The field of TA shows a situation typical for a cold front. The zero line of temperature advection is situated on its leading edge. The area of the shallow moist zone and the comma are characterized by maxima of cold air advection in lower levels of the troposhere.

Divergence

Strong maxima of convergence (thick lines) are associated with the cold front near the surface. A second maximum of convergence can be found within the area of the comma-like cloud feature in the cold air behind the front. Convergence reaches from low to middle levels within this unstable zone.

Vertical Motion

The maxima of omega indicating ascending air can be found in layers between the maxima of convergence and maxima of divergence. In relation to the cold front we see at 56°N/2°E strongest upward motion around 800 hPa. In relation to the comma (58°N/about 6°W) the most intense upward motion occured at about 700 hPa. These maxima were related to strong convergences near the surface and divergences in upper levels.

Potential Vorticity

A look at the fields of potential vorticity also shows an interesting pattern. As stated in the chapter Airmass RGB, stratospheric air was involved in this development. The model fields show a PV anomaly behind the cold front in the area of cold air. This PV anomaly superimposes the comma structure. However, this anomaly is not typical for an instant occlusion.

This cross section reveals rather clearly the displacement of an upper air PV-maximum (about 57°N/2-0°W/above about 500 hPa) and a lower PV-maximum farer to the Southeast (55°N/5°E, at about 850 hPa). That is in accordance with the conceptual model of a (rapid) cyclogenesis before its onset.