Idealised simulations of cyclones with robust symmetrically unstable sting jets

Abstract

Abstract. Idealised simulations of Shapiro–Keyser cyclones developing a sting jet (SJ) are presented. Thanks to an improved and accurate implementation of thermal wind balance in the initial state, it was possible to use more realistic environments than in previous idealised studies. As a consequence, this study provides further insight into SJ evolution and dynamics and explores SJ robustness to different environmental conditions, assessed via a wide range of sensitivity experiments. The control simulation contains a cyclone that fits the Shapiro–Keyser conceptual model and develops a SJ whose dynamics are associated with the evolution of mesoscale instabilities along the airstream, including symmetric instability (SI). The SJ undergoes a strong descent while leaving the cloud-head banded tip and markedly accelerating towards the frontal-fracture region, revealed as an area of buckling of the already-sloped moist isentropes. Dry instabilities, generated by vorticity tilting via slantwise frontal motions in the cloud head, exist in similar proportions to moist instabilities at the start of the SJ descent and are then released along the SJ. The observed evolution supports the role of SI in the airstream’s dynamics proposed in a conceptual model outlined in a previous study. Sensitivity experiments illustrate that the SJ is a robust feature of intense Shapiro–Keyser cyclones, highlighting a range of different environmental conditions in which SI contributes to the evolution of this airstream, conditional on the model having adequate resolution. The results reveal that several environmental factors can modulate the strength of the SJ. However, a positive relationship between the strength of the SJ, both in terms of peak speed and amount of descent, and the amount of instability occurring along it can still be identified. In summary, the idealised simulations presented in this study show the robustness of SJ occurrence in intense Shapiro–Keyser cyclones and support and clarify the role of dry instabilities in SJ dynamics.

Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics

“…This quasi-horizontal PV dipole structure is a robust response of convective updrafts in the presence of vertical wind shear (Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017Hitchman and Rowe, 2017). Such convectively generated PV dipoles were previously identified in idealized simulations of isolated cumulus-scale convection (Chagnon and Gray, 2009), in case studies of mesoscale convective sys-tems (Davis and Weisman, 1994;Chagnon and Gray, 2009;Hitchman and Rowe, 2017;Clarke et al, 2019), and in midlatitude convective updrafts with varying large-scale flow conditions (Weijenborg et al, 2015(Weijenborg et al, , 2017. The amplitude of the horizontal PV dipoles can strongly exceed the typical amplitude of synoptic-scale PV.…”

“…where ω h denotes the horizontal vorticity (ω h = ξ i + ηj ). Previous studies showed that the horizontal diabatic heating gradients play a major role in the PV modification in isolated convective updrafts (e.g., Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017 and in narrow smaller-scale heating regions embedded in a larger-scale flow (Harvey et al, 2020). The localized diabatic heating in a vertically sheared environment generates upper-level horizontal PV dipoles centered around the convective updraft ( Fig.…”

“…PV is frequently considered for synoptic-scale dynamics (e.g., Hoskins et al, 1985;Stoelinga, 1996) but is also suited for the analysis of mesoscale convective systems (e.g., Conzemius and Montgomery, 2009;Shutts, 2017;Clarke et al, 2019) and has already been applied at the scale of individual convective cells (Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017. While for PV modification in synoptic-scale systems the horizontal gradient of diabatic heating is frequently neglected (as in Eq.…”

“…The amplitude of the horizontal PV dipoles can strongly exceed the typical amplitude of synoptic-scale PV. In strong convective updrafts horizontal PV dipoles of ±10 PVU can be generated (Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017, resulting in regions of absolute negative PV. These regions can be hydrostatically, inertially, or symmetrically unstable (e.g., Hoskins, 2015) and can form mesoscale circulations associated with, e.g., frontal rainbands (Bennetts and Hoskins, 1979;Schultz and Schumacher, 1999;Siedersleben and Gohm, 2016), sting jets (Clark et al, 2005;Volonté et al, 2018Volonté et al, , 2019, enhanced stratosphere-troposphere exchange (Rowe and Hitchman, 2015), and local jet accelerations and northward displacements (Rowe and Hitchman, 2016).…”

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

“…1 Negative PV values in the northern hemisphere are related to either hydrostatic, inertial or symmetric instability (Hoskins, 2015, e.g.,), and have been associated with mesoscale circulations (e.g., Bennetts and Hoskins, 1979;Schultz and Schumacher, 1999;Volonté et al, 2020). 4 https://doi.org/10.5194/wcd-2020-49 Preprint.…”