Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: A tornadic cold front in amplifying northwesterly flow

Abstract

AbstractThe structure and evolution of a cold front that produced a tornadic narrow cold‐frontal rainband (NCFR) over the UK on 20 November 2013 is explored using reanalysis data, high‐resolution model simulations and observations. Physical links are found to exist across a range of spatiotemporal scales, through which the evolving large‐scale flow field exerts an influence on the timing and location of miso‐scale vortex genesis, and therefore the potential for tornado genesis. The synoptic‐scale flow pattern exhibited amplification, consisting of upstream ridge building and downstream trough extension. A prominent jet streak and associated positive potential vorticity (PV) anomaly and tropopause fold moved rapidly southeastward on the rear flank of the extending upper‐level trough, the leading edge of these features eventually overspreading the surface cold front over the UK. Increasing 850 hPa frontogenesis occurred underneath the left exit of the jet streak and associated, intensifying, PV anomaly. A filament of dry, high‐PV air was extruded from the overlying tropopause fold within an intensifying front‐transverse circulation in this region. This dry filament eventually penetrated to low levels immediately behind the front, where it undercut the upper, rearward parts of the wide cold‐frontal precipitation band. Model fields and observations suggest that diabatic cooling, associated with sublimation of solid hydrometeors falling into the dry filament, led to the development of local downdraft and near‐surface divergence maxima and a prominent cold pool immediately behind the surface cold front over central England. Increases in horizontal convergence, updraft speed, and horizontal temperature gradients along the frontal boundary immediately ahead of the cold pool resulted in development of a locally well‐defined cold front and associated NCFR, where the surface front was formerly (and elsewhere continued to be) relatively weak. Tornadoes occurred in association with miso‐scale vortices that developed rapidly along an intensifying vertical vortex sheet at the NCFR.