Variation of frontal and precipitation structure along a cold front

A diagnostic study is presented to illustrate the sequence of events in which a warm‐conveyor‐belt (WCB) flow is drawn into the circulation of a developing extratropical cyclone and interacts with dry air aloft to produce distinctive precipitation structures. The poleward boundary of the upstream end of the WCB was associated with a classical (ana) cold‐frontal structure. The other end of the WCB flow, closer to the cyclone centre, was overrun by a ‘dry intrusion’ of low wet‐bulb potential temperature air to give a split (kata) cold front. The orientation of the surface cold front (i.e. the WCB boundary) changed significantly between these two parts but the orientation of the surface geostrophic flow within the WCB changed relatively little between the two parts. The surface cold front was characterized in most places by a narrow cold‐frontal rainband composed of shallow line‐convection elements (precipitation cores) aligned nearly parallel to the surface geostrophic flow in the WCB. The changing relationship between the frontal orientation and the direction of the surface flow accounts for the fact that the line‐convection elements occurred end‐to‐end to give almost two‐dimensional line convection along the classical ana part of the cold front, but were non‐aligned and widely spaced along the split cold‐front part.