Joint Centre for Mesoscale Meteorology (JCMM)

Mesoscale analysis of arc rainbands in a dry slot
K A Browning and N M Roberts

A series of almost concentric, arc-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 Storms Track Experiment (FASTEX). The multiple-rainband event is shown to have been associated with part of a dry intrusion characterised 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 overunning warm-sector air. Continuous wind profiles from a UHF radar, together with a well-located rawinsonde at the "epicentre" of the arc rainbands, showed that the major anomaly in the cold pool was an area of locally stronger winds (>40 ms-1) at 800 hPa 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 multiple nature of the arc rainbands remains unexplained; the possibilities of mesoscale gravity waves and conditional symmetric instability are both considered.

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