According to this classical theory of the Occlusion band, this front separates the cold air mass, which is situated in front of the former Warm Front, from that behind the former Cold Front with a tongue of warm air within the higher levels of the troposphere. Ideal schematics were shown in Polar Front theory.
This idealized process cannot be observed in every step and detail in reality. In particular, the overtaking of the Warm front by the Cold front with very narrow warm sectors are rare. Instead of this a mergence of Cold and Warm Front cloudiness in the centre of the surface low takes place followed by a westward extension of the Occlusion cloud spiral, while the Warm Front cloudiness becomes shorter. Beside this there are Occlusion spirals which contradict the overtaking mechanism completely. Those developments show a lower cloud spiral penetrating westward from below the Cold Front and Warm Front.
 |
Schematic image of the Occluded front (Cold Conveyor Type). |
The occluded fronts and occluded spirals are often very distinct features in both the NWP models and in the satellite images. In numerical parameters one should particularly check the Vertical Cross sections, which typically show a pronounced isentropic trough at middle levels of the troposphere. The isentropic trough denotes the warm and moist air cut off the surface layers by the cold advection at the rear of the cold front. The most intensive ascending motion and precipitation takes place within this area. Even without Vertical cross sections plots of the Equivalent Potential Temperature at middle levels of the troposphere show the tongue of warm air protruding into the circulation and spiralling around the low pressure centre.
 |
Schematic image of the Vertical Cross Section through an Occluded Front. |
References