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.
A numerical simulation of an intense, narrow cold-frontal rain band (NCFR) is presented. This front was associated with localized wind damage as it moved eastwards across the United Kingdom on 24 September 2007. The model used was the operational and research Weather Research and Forecasting-Advanced Research (WRF-ARW) mesoscale model, initialized with Global Forecast System (GFS) operational model output data. The simulation produced a front strongly resembling that seen in radar imagery and conforming to previous studies and conceptual models of the anacold front. In the simulation, a strong surface horizontal wind-speed maximum is located along the southern flank of a misocyclone; this is shown to be typical of a number that form along the front. Using trajectory analysis, descending cold air parcels, which originate in the rear inflow jet and accelerate within the circulation of the misocyclone, are identified as the origin of the strongest winds. In addition to the localized strong straight-line winds, circumstantial evidence is presented for the possible occurrence of weak tornadogenesis. The results are discussed in relation to recent studies of NCFRs, squall lines and misocyclones.
This paper investigates the factors that invigorated an outbreak of warm‐sector convection which was instrumental in transporting high momentum downwards to give damaging surface winds. We have re‐examined a wintertime extratropical cyclone from the Fronts and Atlantic Storm‐Track EXperiment for which an earlier study had shown the warm‐sector convection to be in the form of a series of arc‐shaped rainbands. Using a 5 km grid version of the WRF (Weather Research and Forecasting) model, we show that this convection was enhanced as a result of the properties of an airstream at the base of the dry intrusion that overran the shallow moist zone (SMZ) of the warm sector. This airstream, which we refer to as the Diabatically Modified Cloud‐Top Layer (DMCTL), is shown to have originated several hours earlier in part of an ana‐cold frontal cloud layer where a region of previously ascending air began systematically to descend. Air from the DMCTL descended from heights of 2–3 km to heights of 1–2 km over a roughly 5 h period during which sustained evaporation occurred and its potential temperature dropped by up to 5 °C. This substantially enhanced the convective instability where this airstream overran the SMZ, leading to vigorous boundary‐layer convection. The same region of evaporative cooling is also shown to have generated positive potential vorticity (PV) in the upper parts of the DMCTL, with values reaching about 3 PV units where it overran the SMZ. This layer of high PV, although we have not quantified its impact, may have increased the convective instability further by inducing differential advection below it. The cyclone in the present study later underwent frontal fracture and developed a cloud head but the processes diagnosed in this study were underway before the transition occurred.
Three‐dimensional view of snow streamers producing heavy snowfall over southern England obtained from a T + 6h mesoscale model forecast valid at 1200 utc, 1 February 2019, showing a sub‐set of the computational domain 4km high and approximately 240 × 240km as viewed from the west. The 3D isosurface depicts a model reflectivity of 23dBZ, corresponding to a moderate snowfall intensity and has been shaded according to vertical air velocity according to the colour scale. Note the cellular and sloping nature of the streamers within the snow bands and the strong positive vertical velocities consistent with the presence of shallow ‘generating’ cells of upright convection at their top.
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