Sting jets and the diagnosis of conditional symmetric instability

A sting jet can be defined as a transient, highly localized low-level jet occurring within rapidly deepening Shapiro-Keyser type extratropical cyclones. It is distinct, in location and life-time span, from other air streams featuring in conceptual models of extratropical cyclones such as the warm and cold conveyor belts. A sting jet originates within the cloud head, exiting at the tip of the cloud head and descending rapidly from there to the surface. Unlike the warm and cold conveyor belts, which are related to their parent cyclones' synoptic structure, sting jets are short-lived events subject to mesoscale processes, with its period of duration being of the order of hours. The study of sting jets is important due to the potential loss of life and damage to property and infrastructure that could occur as a consequence of the strong winds that these phenomena could generate.

The occurrence of sting jets has been linked to observed slantwise circulations near or within the frontal fracture region. Furthermore, these circulations have been associated to the presence and subsequent release of conditional symmetric instability (CSI). The purpose of this work is to understand and quantify the role of CSI in the development of sting jets. The analysis presented in this study is based on various diagnostics for CSI. Computations of moist potential vorticity (MPV) show that regions of negative MPV are indeed related to the initiation of sting jets. We also discuss the use of slantwise convective available potential energy (SCAPE) and downdraft SCAPE in the study of these phenomena. We illustrate these ideas through a case study, namely the analysis of a storm that occurred during the first hours of 26/02/2002 over the UK. This storm was characterized by very strong surface winds over localized regions of Wales and the north of England. We also present some ideas regarding the use of these diagnostics in the development of a climatology of sting jets.