Diagnosing Sting Jet Formation in European Windstorms Using the Sawyer-Eliassen Circulation Equation

The dynamics of sting jet formation are examined through comparison of the ageostrophic Sawyer-Eliassen circulations diagnosed in sting jet-producing and non-sting jet producing cyclones. For both cases, upper-level geostrophic cold-air advection in cyclonic shear forced subsidence through the jet core, facilitating a rapid transfer of momentum from the upper to the lower troposphere. Partitioning of the circulation into its geostrophic and diabatic contributions indicates that the geostrophic component accounted for a significant fraction of the circulation. Importantly, in the sting-jet case, the diabatic heating forced subsidence very near the location of the sting jet. This forcing was much more substantial, particularly near the time of sting jet production, than that observed in the non-sting jet case. Thus, we hypothesize that sting jet formation is governed by the interaction of cyclone-scale and meso-scale processes. On the cyclone-scale, geostrophic shearing deformation drives upper tropospheric momentum downward through the jet core into the lower troposphere where diabatic circulations deliver that momentum to the surface. We suggest that use of the Sawyer-Eliassen circulation diagnostic can be used to determine the degree to which this proposed mechanism is characteristic of the sting jet environment.