Mesoscale aspects of a dry intrusion within a vigorous cyclone

The results of a diagnostic study using radar and satellite imagery, together with surface reports and output from regional and mesoscale models, are presented for an occasion of a rapidly deepening cyclone that crossed the British Isles. The study shows how air descended from near‐tropopause level in the form of mesoscale dry intrusions which appeared to overrun parts of the warm conveyor belt ahead of a surface cold front. One of the dry intrusions is analysed in detail, because it is thought to have led to the large changes that were observed in the character of the wide frontal rain band: in some places the precipitation was entirely suppressed, whilst in other places the precipitation became convective, with a tornadic squall line developing. Although the effect of the dry intrusion was seen most clearly in the precipitation distribution as determined by radar, the approach of the dry intrusion was also clearly detected in the satellite water‐vapour imagery, and foreshadowed in the model forecasts. The mesoscale model, despite some limitations, reproduced many of the observed mesoscale features. It also provided insight into the detailed behaviour of the dry intrusion associated with the tornadic squall line. It showed that the dry intrusion was characterized by a mesoscale filament of high potential vorticity (PV) which was extruded from the lower part of a large region of high PV near a low tropopause. The extruded PV anomaly plunged to within a kilometre of the surface where it overran part of the warm conveyor belt. The dry intrusion was also characterized by relatively low wet‐bulb potential temperature, θw, and it led to potential instability where it overran the warm air. The warm conveyor belt itself was characterized not only by a high θw but also by a separate strip of high PV generated locally, mainly by condensation. The quality of the mesoscale‐model simulation gives grounds for optimism that such models are capable in principle of predicting severe mesoscale weather events when, as in this case, they are orchestrated by resolved larger‐scale dynamics.