The dry intrusion perspective of extra‐tropical cyclone development

Abstract

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’. Parts of dry intrusions are characterised by high potential vorticity and, upon approaching a low‐level baroclinic zone, rapid cyclogenesis may be expected to ensue. The leading edges of dry intrusions are defined by cold θw‐fronts (moisture fronts). In some places the dry intrusion undercuts rearward‐ascending warm air to give an ana‐cold front. In other places it overruns the warm air to produce an upper cold θw‐front in advance of the surface cold frontHere the dry intrusion is associated with the generation of potential instability and its eventual release as showers or thunderstorms. Identification of dry intrusions provides the forecaster with additional nowcasting evidence that is especially helpful when issuing severe weather warnings. The identification of water vapour dark zones associated with dry intrusions can also form the basis of methods for validating NWP models. Through their relationship to high potential vorticity, they can 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 extra‐tropical cyclones. Copyright © 2004 Royal Meteorological Society

A textbook example of long‐range transport: Simultaneous observation of ozone maxima of stratospheric and North American origin in the free troposphere over Europe

“…For instance, the pseudo-Lagrangian conveyor belt model [Carlson, 1980;Browning, 1990;Roberts, 1994, 1996] describes three characteristic streams in a coordinate system moving with a synoptic system: the warm conveyor belt (WCB), the cold conveyor belt, and the dry intrusion. The WCB is an ascending airstream at the leading edge of a trough [Browning, 1999], the cold conveyor belt is an airflow ahead of a surface warm front, and the dry intrusion is a descending airstream related to tropopause folding [Browning, 1997]. It is not yet known how coherent these airstreams are compared to others.…”

Mesoscale analysis of a comma cloud observed during FASTEX

“…The schematic uses the concept of warm conveyor belts and their relationship to cold fronts described by several authors including Harrold (1973), Young et al (1987) and Browning (1997). The primary warm conveyor belt (W1) is being overrun by the dry intrusion resulting in a kata-cold front with a shallow moist zone (SMZ) at the surface.…”

“…Convective mixing can bring this air right down to the surface in convective down draughts, the convection resulting from the release of potential instability where the low θ w air of the dry intrusion overruns air of high θ w (e.g. in a shallow moist zone (Browning, 1997) -see below). The gusts can be particularly severe if precipitation has fallen from an upper cloud shield into the dry air and evaporated causing cooling and an increased rate of descent in the down draughts.…”

The evolution of an MCS over southern England. Part 1: Observations

“…This shows the very dry (dark) region associated with the low tropopause behind the front. Dark zones such as this in water vapour imagery are known to be associated with upper-level PV maxima (Browning, 1997) corresponding to the depression of the tropopause. Convective cloud is evident over the UK and the MCS itself is evident with a relatively dark ring surrounding it and a relatively dark line through it dividing Line A from Lines C/E (see below).…”