Abstract/Summary

Sting jets (SJ) occur as an additional region of low-level strong, and possibly damaging, winds in some Shapiro-Keyser extratropical cyclones. While SJs are widely accepted as being distinct from the warm and cold conveyor belts, this contribution addresses the unresolved questions of the mechanisms responsible for their generation and descent, along with the dependence of their existence and characteristics on environmental conditions. These questions are tackled by using a case study and extending the findings to idealised simulations and related sensitivity experiments, focusing on the generation and release of mesoscale instabilities also from a Lagrangian perspective. This study shows that synoptic-scale frontal dynamics and mesoscale instabilities (e.g. symmetric instability) can both co-exist and drive the SJ evolution. While frontal dynamics can in itself lead to SJs, the formation and eventual release of a succession of mesoscale instabilities can substantially enhance their strength. This analysis outlines, for the first time, the mechanism of generation of dry symmetric instabilities along the SJ. Diabatically-caused frontal motions can lead to the formation, via tilting of horizontal vorticity, of symmetrically unstable regions travelling with the SJ towards the cloud-head tip. SJs form in the majority of idealised experiments, suggesting that they are a common feature of Shapiro-Keyser cyclones. In the control run and in half of the sensitivity experiments, the SJ is associated with a localised symmetrically unstable environment which evolves through the outlined mechanism and enhances the SJ strength, which also depends on jet-stream intensity. Coarser-resolution simulations of both case study and idealised configuration confirm that vertical and horizontal resolution constraints apply to ensure that the release and even generation of mesoscale instabilities is not suppressed. These results represent a substantial step in understanding the mechanisms driving the formation and evolution of SJs, highlighting a likely underestimation of their intensity in coarser-resolution weather/climate models.