Mesoscale analysis of arc rainbands in a dry slot

A series of almost concentric, are-shaped narrow rainbands was observed tra velling at 140 km h(-1) and producing severe wind gusts over parts of Engla nd. Their structure and environment have been analysed using radar and sate llite imagery, mesoscale-model diagnostics, surface observations and additi onal rawinsonde ascents obtained during the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The multiple-rainband event is shown to have been ass ociated with part of a dry intrusion characterized by a mesoscale vortex/co ld pool. The event occurred within the 'dry slot' region of a small frontal cyclone where the mesoscale pool of cold dry air was overrunning warm-sect or air. Continuous wind profiles from a UHF radar, together with a well-loc ated rawinsonde at the 'epicentre' of the are rainbands, showed that the ma jor anomaly in the cold pool was an area of locally stronger winds (>40 ms( -1)) at 800 kPa colocated with the temperature minimum at that level. The o bserved orientation of the rainbands is explained in terms of the cold-fron tal structure at this level triggering ascent of the underlying warm moist air. The multi-pie nature of the are rainbands remains unexplained; the pos sibilities of mesoscale gravity waves and conditional symmetric instability are both considered.