The sting at the end of the tail: Model diagnostics of fine-scale three-dimensional structure of the cloud head

This 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.