An area associated with the cold front of a depression, where the equatorward branch of the cold conveyor belt causes cold, dry air from the upper troposphere and lower stratosphere to descend towards the surface. When the eastwards motion of the warm conveyor belt and the cold air are approximately equal, the dry intrusion reaches the surface and produces a clearly defined cloud-free zone behind the cold front. This initially appears as a notch in the rear of a leaf cloud, and develops into a distinct clear sector at the comma-cloud stage. The area is not only conspicuously clear of cloud and sharply defined on water-vapour satellite images, but has gusty winds because the air originates near the jet stream, which normally crosses the system at this position. The dry intrusion is associated with the sting jet found in explosive cyclogenesis. When the dry intrusion overruns the warm conveyor belt it creates a split front, with a distinct pattern of cloud cover and precipitation that differs considerably from the ‘classic’ cold front first described by Vilhelm Bjerknes.
Subjects: Meteorology and Climatology.