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Abstract Several case studies are presented showing the structure of ana‐cold fronts over the British Isles. One of the cases is analysed in detail using data acquired on the Isles of Scilly to avoid any confusing effects due to topography. All of the fronts described are characterized by a narrow band of shallow but vigorous convection at the surface cold front; this convection is essentially two‐dimensional and is termed line convection. In each case the line convection is bounded on its forward side by a low‐level jet reaching 25 to 30 m s −1 ; behind the line convection the winds decrease abruptly. The low‐level jet is embedded within a ‘convective boundary layer’, reaching its maximum velocity at 900 to 850 mb, and it consists of a tongue of anomalously warm, moist air, which has had a trajectory over an even warmer sea. It is shown that the line convection can be regarded as part of a mesoscale ‘right‐hand corkscrew circulation’ within the low‐level jet. The line convection constitutes one flank of the jet; it is characterized by very strong cyclonic shear and is fed by frictional convergence of air from beneath the jet core within the lowest 100 mb. Some of the air which ascends within the line convection subsequently flows forward and gently subsides within the upper part of the low‐level jet as part of the corkscrew circulation; however, most of the air which ascends within the line convection is extruded from the boundary layer and ascends in a rearward direction as slantwise convection above the inclined cold frontal zone. Precipitation grown in the slantwise convection falls into the cold air behind the surface cold front and the heat sink resulting from the partial evaporation of this precipitation probably accentuates the sharpness of the boundary of the low‐level jet at the surface cold front.
Browning et al. (Mon,) studied this question.