Mesoscale analysis of arc rainbands in a dry slot

A series of almost concentric, are-shaped narrow rainbands was observed travelling at 140 km h(-1) and producing severe wind gusts over parts of England. Their structure and environment have been analysed using radar and satellite imagery, mesoscale-model diagnostics, surface observations and additional rawinsonde ascents obtained during the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The multiple-rainband event is shown to have been associated with part of a dry intrusion characterized by a mesoscale vortex/cold 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-sector air. Continuous wind profiles from a UHF radar, together with a well-located rawinsonde at the 'epicentre' of the are rainbands, showed that the major 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 observed orientation of the rainbands is explained in terms of the cold-frontal structure at this level triggering ascent of the underlying warm moist air. The multi-pie nature of the are rainbands remains unexplained; the possibilities of mesoscale gravity waves and conditional symmetric instability are both considered.