The most damaging winds in a severe extratropical cyclone often occur just ahead of the evaporating ends of cloud filaments emanating from the so‐called cloud head. These winds are associated with low‐level jets (LLJs), sometimes occurring just above the boundary layer. The question then arises as to how the high momentum is transferred to the surface. An opportunity to address this question arose when the severe ‘St Jude's Day’ windstorm travelled across southern England on 28 October 2013. We have carried out a mesoanalysis of a network of 1 min resolution automatic weather stations and high‐resolution Doppler radar scans from the sensitive S‐band Chilbolton Advanced Meteorological Radar (CAMRa), along with satellite and radar network imagery and numerical weather prediction products. We show that, although the damaging winds occurred in a relatively dry region of the cyclone, there was evidence within the LLJ of abundant precipitation residues from shallow convective clouds that were evaporating in a localized region of descent. We find that pockets of high momentum were transported towards the surface by the few remaining actively precipitating convective clouds within the LLJ and also by precipitation‐free convection in the boundary layer that was able to entrain evaporatively cooled air from the LLJ. The boundary‐layer convection was organized in along‐wind rolls separated by 500 to about 3000 m, the spacing varying according to the vertical extent of the convection. The spacing was greatest where the strongest winds penetrated to the surface. A run with a medium‐resolution version of the Weather Research and Forecasting (WRF) model was able to reproduce the properties of the observed LLJ. It confirmed the LLJ to be a sting jet, which descended over the leading edge of a weaker cold‐conveyor‐belt jet.

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One contribution of 16 to a theme issue 'Atmospheric effects of solar eclipses stimulated by the 2015 UK eclipse' .

On 1 July 2015, severe hailstorms developed over northern England. One storm tracked across an area with a dense network of privately owned (i.e. 'home') automatic weather stations (AWSs), permitting analysis of surface wind, pressure and temperature fields on the storm scale. The home AWS data were filtered and corrected by comparison with data from the nearest United Kingdom Met Office AWS, where measurements are made with calibrated sensors having known error characteristics. A time-compositing technique was applied to the corrected home AWS data, before interpolation onto a 1 km grid using Delaunay triangulation. The resulting analyses were compared with radar data to assess their quality and provide insights into storm evolution and structure. Surface analyses resolved a pressure anomaly couplet on the right, rear flank of the storm, gust fronts, and regions of inflow and outflow. The pressure couplet was closely collocated with the radar-observed mid-level updraught position, with the mesolow (mesohigh) situated underneath the downshear (upshear) flank. The mesolow was also collocated with the strongest inflow winds. Structural features in the surface analyses (e.g. forward-and rear-flank gust fronts and a prominent inflow notch) compared well with radar-observed structures, and conformed closely to established conceptual models of supercell storm morphology. Further insight into storm structure was gained by synthesis of the surface analyses, Doppler radar data, crowd-sourced hail reports and eyewitness photographs. Collectively, the results demonstrate that the gridded home AWS data may be of sufficient quality for use in post-event studies of severe thunderstorms and, potentially, in the operational forecasting environment.Fine-scale analysis of a hailstorm

In December 2018, the Danish Meteorological Institute organised an international meeting on the subject of crowdsourced data in numerical weather prediction (NWP) and weather forecasting. The meeting, spanning 2 days, gathered experts on crowdsourced data from both meteorological institutes and universities from Europe and the United States. Scientific presentations highlighted a vast array of possibilities and progress being made globally. Subjects include data from vehicles, smartphones, and private weather stations. Two groups were created to discuss open questions regarding the collection and use of crowdsourced data from different observing platforms. Common challenges were identified and potential solutions were discussed. While most of the work presented was preliminary, the results shared suggested that crowdsourced observations have the potential to enhance NWP. A common platform for sharing expertise, data, and results would help crowdsourced data realise this potential.

Article:Clark, MR and Parker, DJ (2014) On the mesoscale structure of surface wind and pressure fields near tornadic and nontornadic cold fronts. Monthly Weather Review, 142 (10). https://doi.org/10.1175/MWR-D-13-00395.1 eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. ABSTRACTObservations from a mesoscale network of automatic weather stations are analyzed for 15 U.K. cold fronts exhibiting narrow cold frontal rainbands (NCFRs). Seven of the NCFRs produced tornadoes. A timecompositing approach is applied to the minute-resolution data using the radar-observed motion vectors of NCFR precipitation segments. Interpolated onto a 5-km grid, the analyses resolve much of the smallmesoscale structure in surface wind, temperature, and pressure fields. Postfrontal winds varied substantially between cases. Tornadic NCFRs exhibited a near-908 wind veer and little or no reduction in wind speed on NCFR passage; these attributes were generally associated with large vertical vorticity, horizontal convergence, and vorticity stretching at the NCFR. Nontornadic NCFRs exhibited smaller wind veers and/or marked decreases in wind speed across the NCFR, and weaker vorticity, convergence, and vorticity stretching. In at least four tornadic NCFRs, increases in vorticity stretching preceded tornadogenesis. Doppler radar observations of two tornadic NCFRs revealed the development of misocyclones, some tornadic, during the latter stages of vorticity-stretching increase. The presence of cyclonic vortices only, in one case occurring at regular intervals along the NCFR, provides limited circumstantial evidence for horizontal shearing instability (HSI), though other vortex-genesis mechanisms cannot be discounted. Vorticity-stretching increases were associated with coherent mesoscale structures in the postfrontal wind field, which modified the crossfrontal convergence. Where cross-frontal convergence was large, extremely narrow, intense shear zones were observed; results suggest that tornadoes occurred when such shear zones developed in conjunction with conditional instability in the prefrontal environment.

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High-resolution model simulations and radar observations are used to investigate the onset of vortex-genesis in a tornadic narrow cold-frontal rain band (NCFR). The timing and location of vortex-genesis was strongly constrained by a developing frontal wave, which tracked northeast across the United Kingdom and Ireland on 17 October 2011. In the simulations, vortices occurred preferentially during the early stages of wave development and just down-front of the wave centre, where large increases in vertical vorticity occurred in concert with decreases in the cross-frontal confluence. Vortex-genesis ceased as the frontal wave matured, due to the onset of frontal fracture. Two distinct scales of vortex-genesis are documented: primary vortex-genesis on the meso-γ-scale, and secondary vortex-genesis on the miso-scale. We show that horizontal shearing instability is the most likely vortex-genesis mechanism, consistent with previous theoretical work on the stability of vertical vortex strips in the presence of horizontal stretching deformation. Secondary vortices occurred along the braid regions between primary vortices where the shear zone became particularly narrow and intense. In the model, these vortices developed extremely rapidly (from small perturbations to maximum vertical vorticity in 5-15 min) and the strongest exhibited near-surface vertical vorticity maxima approaching 10 −1 s −1 .Vortices of both scales were associated with characteristic local perturbations in the NCFR and we show, by comparison with radar reflectivity data, that primary and secondary vortices were likely present in the real NCFR. Tornado reports were associated with small NCFR perturbations like those associated with the secondary vortices in the model simulations. Analysis of the sub-structure of individual simulated vortices suggests that tornado-genesis is most likely within a region of intense near-surface vertical vorticity stretching at the north or northwest flank of the secondary vortices.