Joint Centre for Mesoscale Meteorology (JCMM)
The dry intrusion and its effect on the frontal, cloud and precipitation structure of extratropical cyclones
by K A Browning
The dry intrusion is a coherent region of air descending from near tropopause-level. It often has a clear signature in satellite imagery, especially in the water vapour channel where it is seen as a 'dark zone'. The dry intrusion is an indication of upper-level forcing. If it approaches a low-level baroclinic zone, rapid cyclogenesis may be expected to ensue. The leading edge of a dry intrusion is marked by the cold front. At an ana-cold front the dry intrusion mainly undercuts rearward-ascending warm air. At a kata-cold front it overruns the warm air to produce an upper cold front in advance of the surface cold front. Dry intrusions are usually associated with the generation of potential instability and its eventual release as showers or thunderstorms. Identification of dry intrusions provides the forecaster with additional evidence that is helpful when issuing severe weather warnings. The identification of dry intrusions can also form the basis of methods for validating NWP models. Through their relationship to high potential vorticity, dry intrusions provide guidance for bogussing NWP models in situations of potentially severe weather. This article provides an introduction to the structure and behaviour of dry intrusions and their relationship to other aspects of extratropical cyclones.