Sting jets (SJs) occur as an additional region of low‐level strong winds in some Shapiro–Keyser‐type extratropical 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.

During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of north‐eastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavy‐rainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common large‐scale patterns and a similar mesoscale setting, characterised by southerly low‐level flow interacting with the Alpine orography, but the precipitation distribution was very different. During Intensive Observing Period IOP18 (31 October–1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4/5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavy‐precipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A high‐resolution (2 km grid spacing) non‐hydrostatic NWP model and available observations are used for this purpose. The two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the Alps. Depending on the thermodynamic profile, convection can be triggered when the impinging flow is forced to rise over a pre‐existing cold‐air layer at the base of the orography. In this situation a persistent blocked‐flow condition and upstream convergence are responsible for heavy rain localized over the plain. Conversely, if convection does not develop, flow‐over conditions are established and heavy rain affects the Alps. Numerical parameters proposed in the literature are used to support the analysis. Finally, the role of evaporative cooling beneath the convective systems is evaluated. It turns out that the stationarity of the systems upstream of the Alps is mainly attributable to persistent blocked‐flow conditions, while convective outflow slightly modifies the location of precipitation.

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 h 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 4 km 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.

The Indian summer monsoon is a vital source of water and a cause of severe impacts for more than a billion people in the Indian subcontinent. The INCOM-PASS project investigates the mechanisms driving its onset and progression through an observational field campaign supplemented by high-resolution numerical simulations for the 2016 season using UK Met Office models. A 4.4 km resolution convection-permitting limited-area model simulation (driven at its boundaries by a daily-initialised global model) is used in this study, and verified against observations, along with short-lead-time operational global forecasts. These data show that the monsoon progression towards northwest India in June 2016 is a non-steady process, modulated by the interaction between moist low-level southwesterly flow from the Arabian Sea and a northwesterly incursion of descending dry air from western and central Asia. The location and extent of these two flows are closely linked to midlatitude dynamics, through the southward propagation of potential vorticity streamers and the associated formation of cyclonic circulations in the region where the two airmasses interact. Particular focus is devoted to the use of Lagrangian trajectories to characterise the evolution of the airstreams and complement the Eulerian monsoon progression analysis. The trajectories confirm that the interaction of the two airstreams is a primary driver of the general moistening of the troposphere associated with monsoon progression. They also indicate the occurrence of local diabatic processes along the airstreams, such as turbulent mixing and local evaporation from the Arabian Sea, in addition to moisture transport from remote sources. In summary, this combined Eulerian-Lagrangian analysis reveals the non-steady nature of monsoon progression towards northwest India. This process is driven by the interaction of different airmasses and influenced by a synergy of factors on a variety of scales, such as midlatitude dynamics, transient weather systems and local diabatic processes. K E Y W O R D S diabatic processes, dry-air incursion, high-resolution numerical modelling, Indian summer monsoon, Lagrangian trajectories, PV streamers This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.