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SUMMARYStrong surface winds often accompany the low-level jets that occur along the cold fronts of extratropical cyclones, but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone. The most damaging extratropical cyclones go through an evolution that involves the formation of a bent-back front and cloud head separated from the main polar-front cloud band by a dry slot. When the cyclone attains its minimum central pressure, the trailing tip of the cloud head bounding the bent-back front forms a hook which goes on to encircle a seclusion of warm air. The most damaging winds occur near the tip of this hook-the sting at the end of the tail.Observations of the Great Storm of October 1987 in south-east England are re-examined in some detail to study this phenomenon. The cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations. Air within these circulations leaves the hooked tip of the cloud head (and enters the dry slot) much faster than the rate of travel of the cloud-head tip, implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds. The circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds. Regardless of how important this role may be, the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds.

SUMMARYThis is the second in a series of papers on 'the sting at the end of the tail'-a mesoscale region of potentially damaging surface winds that can occur close to the evaporating tip of a cloud head wrapped around the bent-back front of an extratropical cyclone. In the first paper the sting phenomenon was identified from a purely observational study of the great storm of October 1987. In the present paper an observationally validated forecast run from a high-resolution numerical weather prediction model is used to examine the evolving three-dimensional structure of the sting phenomenon. It is shown that the damaging surface winds in the October 1987 storm were due to a well-defined mesoscale 'sting jet' (SJ), identifiable as a coherent ensemble of trajectories originating in cloudy air at about 650 hPa. There is evidence of multiple mesoscale slantwise circulations, and the SJ appears to form within the descending part of these circulations. Air within the SJ descends to the 900 hPa level over a period of about 4 h, during which time it accelerates from less than 20 to above 45 m s −1 with extremes greater than 50 m s −1 . Although the wet-bulb potential temperature of air in the SJ remains constant during its descent to 900 hPa, evaporation leads to a reduction of up to 5 K or more in dry-bulb potential temperature in some parts of the jet. The SJ is situated behind the primary cold front, and is distinct from the associated warm-conveyor-belt low-level jet; it is also distinct from the cold-conveyor-belt low-level jet which remains below and behind it as the SJ skirts the bent-back front.

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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

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A diagnostic study of a mid‐latitude cyclone has been carried out using routinely available numerical weather‐prediction model products and imagery. The cyclone was intense but not exceptionally so, and the results are believed to have some generality. The detailed description of the cyclone structure, just before it began to deepen rapidly, integrates concepts from a number of researchers into a common framework. A dominant feature of the cyclone, close to its centre, was the ‘cloud head’: a region of cloud with a sharp convex outer edge, which formed poleward of the main polar‐front cloud band. The cloud head was caused by two flows that entered it from the east, ascending and fanning out within it. One flow (the ‘cold conveyor belt’) brought low wet‐bulb potential temperature (θw) air back into the cloud head from low levels ahead of the warm front. The other flow was due to high‐θw air that peeled off from the base of the main warm‐sector airflow (i.e. part of the ‘warm conveyor belt’) and travelled in the boundary layer back towards the cyclone centre, first undercutting dry air that had earlier descended from the upper troposphere (called a ‘dry intrusion’), and then ascending at the upper boundary of the cloud head, above the cold conveyor belt. The transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts, with a more diffuse frontal region in between (‘frontal fracture’). The two sharp surface cold fronts were associated with narrow cold‐frontal rainbands (‘line convection’), one of the line‐convection segments forming the southern edge of the cloud head. The overrunning of the dry intrusion in the region of the frontal fracture led to a structure (known as a ‘split front’) in which an upper‐level humidity front began to run ahead of the position of the surface cold front. A feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air (the cold conveyor belt) in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper‐level jet.