SUMMARYThe interaction of a polar trough with a polar front can lead to a characteristic satellite cloud pattern commonly referred to as an instant occlusion. The detailed structure and development of an event rather resembling an instant occlusion is analysed in this paper using routine data supplemented by additional radiosondes, soundings and cloud imagery from NOAA-7 and Meteosat, and radar network pictures. The event presented differs from an instant occlusion as described by Anderson and others in that a band of polar front cloud developed simultaneously with the band of cloud associated with the polar trough rather than being a pre-existing feature. Thus we refer to it under the different name of pseudo-occlusion in order to keep this distinction in mind. A conceptual model of the pseudo-occlusion is developed in which the key elements are (i) a moist, relatively warm, low-level jet associated with the polar trough, referred to as the 'polar trough conveyor belt', and (ii) a major upper-level jet streak associated with the polar front, referred to as the 'polar front conveyor belt'. The two conveyor belts intersect almost at right angles. Precipitation associated with the polar front is predominantly from stratiform cloud, whereas that associated with the polar trough is a mixture of stratiform and convective. Just on the poleward side of the upper jet the cloud top drops abruptly. Here, dry air having recently descended beneath the tropopause fold, suppresses the depth of the moist air associated with the polar trough conveyor belt. This leads to a minimum in the intensity of precipitation between the rain areas associated with the trough and the front. Although the air overrunning the trough just on the poleward side of the polar front is dry as a result of earlier descent, the air in this location is likely to have begun ascending as part of the indirect circulation in the exit region of the upper level jet streak. The polar trough low-level jet probably forms part of this circulation.
