MESOSCALE ASPECTS OF A DRY INTRUSION WITHIN A VIGOROUS CYCLONE

The results of a diagnostic study using radar and satellite imagery, t ogether with surface reports and output from regional and mesoscale mo dels, are presented for an occasion of a rapidly deepening cyclone tha t crossed the British Isles. The study shows how air descended from ne ar-tropopause level in the form of mesoscale dry intrusions which appe ared to overrun parts of the warm conveyor belt ahead of a surface col d front. One of the dry intrusions is analysed in detail, because it i s thought to have led to the large changes that were observed in the c haracter of the wide frontal rain band: in some places the precipitati on was entirely suppressed, whilst in other places the precipitation b ecame convective, with a tornadic squall line developing. Although the effect of the dry intrusion was seen most clearly in the precipitatio n distribution as determined by radar, the approach of the dry intrusi on was also clearly detected in the satellite water-vapour imagery, an d foreshadowed in the model forecasts. The mesoscale model, despite so me limitations, reproduced many of the observed mesoscale features. It also provided insight into the detailed behaviour of the dry intrusio n associated with the tornadic squall line. It showed that the dry int rusion was characterized by a mesoscale filament of high potential vor ticity (PV) which was extruded from the lower part of a large region o f high PV near a low tropopause. The extruded PV anomaly plunged to wi thin a kilometre of the surface where it overran part of the warm conv eyor belt. The dry intrusion was also characterized by relatively low wet-bulb potential temperature, theta(w), and it led to potential inst ability where it overran the warm air. The warm conveyor belt itself w as characterized not only by a high theta(w) but also by a separate st rip 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 weathe r events when, as in this case, they are orchestrated by resolved larg er-scale dynamics.