Idealised simulations of cyclones with robust symmetrically-unstable sting jets

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 has been possible to use more realistic environments than in previous idealised studies. As a consequence, this study provides further insight in 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

The role of mesoscale instabilities in the sting-jet dynamics of windstorm Tini

Sting jets (SJ) occur as an additional region of low-level strong winds in some Shapiro-Keyser-type extra-tropical cyclones. While SJs are widely accepted as being distinct from the warm and cold conveyor belts, the mechanisms responsible for their occurrence are still not fully understood. Here we determine the relative importance of the release of mesoscale instabilities and synoptic-scale cyclone dynamics, so addressing an area of current debate. Numerical weather prediction simulations of a SJ-containing windstorm are analysed and Lagrangian trajectories used to assess the evolution of, and mesoscale atmospheric instabilities (e.g. symmetric and inertial instabilities) in, the descending airstream. The SJ undergoes a two-stage descent: cooling via sublimation followed by a large acceleration accompanied by instability release. Combined tilting and stretching of vorticity play a major role in the local onset of instability on the airstream. Vorticity and frontogenesis fields have a narrow slantwise banded structure in the cloud head and around the SJ; the descending SJ modifies the widespread frontolysis expected from the large-scale dynamics alone in the frontal-fracture region. A coarser-resolution simulation also generates strong winds in the frontal-fracture region, although these are significantly weaker than in the higher-resolution simulation. The SJ airstream in the coarser-resolution simulation undergoes a weaker descent without instability generation and descends in a widespread frontolytic region. Hence, while the SJ undergoes a process of destabilisation that enhances its descent and acceleration in the higher-resolution simulation, enhancing the strong winds already generated by the synoptic-scale cyclone dynamics, this destabilisation does not occur in the SJ produced by a coarser-resolution simulation, resulting in weaker winds. This analysis reveals the synergy between the paradigms of SJ occurrence through the release of mesoscale instabilities and synoptic-scale cyclone dynamics and demonstrates that the current debate may in part be a consequence of the model resolutions used by different studies

The role of mid‐tropospheric moistening and land‐surface wetting in the progression of the 2016 Indian monsoon

Accurately predicting the Indian monsoon is limited by inadequate understanding of the underlying processes, which feed into systematic model biases. Here we aim to understand the dynamic and thermodynamic features associated with the progression of the monsoon, using 2016 as a representative year, with the help of convection-permitting simulations of the Met Office Unified Model. Simulations are carried out in a 4 km resolution limited-area model, nested within a coarser global model. Two major processes thought to influence the northwestward progression of the monsoon are: (a) the interaction between the low-level monsoon flow and a mid-tropospheric dry-air intrusion from the northwest, and (b) land–atmosphere interactions. We find that the 4 km limited-area model simulates the mid-tropospheric moistening that erodes the northwesterly dry intrusion, pushing the northern limit of moist convection northwestwards. The surface soil moisture also plays a major role at the leading edge of the monsoon progression. The heavy rains associated with the local onset wet the soil, reducing the sensitivity of surface fluxes to soil moisture and weakening the land influence on further progression of monsoon rains. The 4 km model is tested with an alternative land-surface configuration to explore its sensitivity to land-surface processes. We find that the choice of soil and vegetation ancillaries affects the time-scales of soil moisture–precipitation feedback and the timing of diurnal convection, thereby affecting the local onset. We further compare these simulations with a parametrized convection run at 17 km resolution to isolate the effects of convective parametrization and resolution. The model with explicit convection better simulates the dynamic and thermodynamic features associated with the progression of the monsoon

Interaction of convective organisation with monsoon precipitation, atmosphere, surface and sea: the 2016 INCOMPASS field campaign in India

The INCOMPASS field campaign combines airborne and ground measurements of the 2016 Indian monsoon, towards the ultimate goal of better predicting monsoon rainfall. The monsoon supplies the majority of water in South Asia, but forecasting from days to the season ahead is limited by large, rapidly developing errors in model parametrizations. The lack of detailed observations prevents thorough understanding of the monsoon circulation and its interaction with the land surface: a process governed by boundary-layer and convective-cloud dynamics. INCOMPASS used the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft for the first project of this scale in India, to accrue almost 100 hours of observations in June and July 2016. Flights from Lucknow in the northern plains sampled the dramatic contrast in surface and boundary layer structures between dry desert air in the west and the humid environment over the northern Bay of Bengal. These flights were repeated in pre-monsoon and monsoon conditions. Flights from a second base at Bengaluru in southern India measured atmospheric contrasts from the Arabian Sea, over the Western Ghats mountains, to the rain shadow of southeast India and the south Bay of Bengal. Flight planning was aided by forecasts from bespoke 4km convection-permitting limited-area models at the Met Office and India's NCMRWF. On the ground, INCOMPASS installed eddy-covariance flux towers on a range of surface types, to provide detailed measurements of surface fluxes and their modulation by diurnal and seasonal cycles. These data will be used to better quantify the impacts of the atmosphere on the land surface, and vice versa. INCOMPASS also installed ground instrumentation supersites at Kanpur and Bhubaneswar. Here we motivate and describe the INCOMPASS field campaign. We use examples from two flights to illustrate contrasts in atmospheric structure, in particular the retreating mid-level dry intrusion during the monsoon onset

Dynamics of sting jets and their relation to larger-scale drivers

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

Characterising the interaction of tropical and extratropical air masses controlling East Asian summer monsoon progression using a novel frontal detection approach

The East Asian summer monsoon (EASM) is a complex phenomenon, influenced by both tropical and midlatitude dynamics and by the presence of the Tibetan Plateau. The EASM front (EASMF) separates tropical and extratropical air masses as the monsoon marches northwards. Although the different factors behind EASM progression are illustrated in a number of studies, their interactions, in particular between tropical and extratropical air masses, still need to be clarified. In this study we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal progression and interannual variability of the EASM, and we highlight the dynamics of the air masses converging at its front. A frontal detection algorithm is used to perform a front-centred analysis of EASM progression. The analysis highlights the primary role of the subtropical westerly jet (STWJ) and of the Western North Pacific subtropical high (WNPSH) in controlling the strength and the poleward progression of the EASMF, in particular during Mei Yu, the primary stage of EASM progression. These forcings act to steer the southerly advection of low-level moist tropical air, modulated by the seasonal cycle of the Asian monsoon. The Mei Yu stage is distinguished by an especially clear interaction between tropical and extratropical air masses converging at the EASMF. The analysis of composites based on the latitude of the EASMF during Mei Yu reveals the influence exerted by the STWJ on the cool extratropical flow impacting on the northern side of the EASMF, whose progression is also dependent on the location of the WNPSH. In turn, this affects the extent of the warm moist advection on its southern side and the distribution and intensity of resultant rainfall over China. This study shows the validity of an analysis of EASM progression focused on its front and on the related low-~and mid-level airstreams, at least in the Mei Yu stage. The framework highlighted shows how the regional flow over East Asia drives the low-level airstreams that converge at the EASMF, thus controlling the shape of EASM progression. This framework provides a basis for studies of climate variability and extreme events and for model evaluation

A climatology of summer-time Arctic cyclones using a modified phase space

We perform a climatological analysis of summer-time Arctic cyclone structure in reanalysis data using a phase space approach. A classification scheme for Arctic cyclones is proposed, dependent on whether vorticity structure during development is low-level-dominant (LLD) or upper-level-dominant (ULD). During growth, LLD cyclones (65.5%) exhibit warm-core asymmetric structures, whereas ULD cyclones (34.5%) have cold-core asymmetric structures. LLD cyclones typically have greater thermal asymmetry during growth. However, a transition to a persistent cold-core axisymmetric structure after maturity is characteristic of summer-time Arctic cyclones, regardless of structure during growth. LLD cyclones are typically stronger and preferentially track on the Russian coastline where there is high baroclinicity, whereas ULD cyclones tend to be longer-lived and preferentially track in the Pacific sector, where they can interact with tropopause polar vortices. This study provides a platform for further research into different classes of Arctic cyclones and associated hazardous weather, and ultimately for developing conceptual models