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.

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A case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small‐scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low θw in the middle troposphere overran low‐level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large‐scale, and thermally‐direct, circulation associated with the active warm front, the air which ascended as small‐scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward‐sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations. The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low θw in the middle troposphere above the moist warm‐sector air in a region where the resulting instability could be realized by large‐scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre‐frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.

Some of the largest falls of orographic rain in the western parts of the British Isles are associated with wintertime cold fronts. This paper contains foul case studies of wet cold fronts which were all characterized by south-westerly prefrontal low-level jets but were widely different in other respects. The distribution of rainfall in England and Wales is analysed using a network of autographic raingauges, specially augmented near the coast and over the hills of south Wales, and the orographic effects areexplained using data fromseries of hourly rawinsonde ascents fromasingle station. The rain is considered in three distinct regions: pre-frontal, the surface cold front, and post-frontal. Behind the front orographic effects were found to be well defined but rather slight. At the surface front orographic effects were negligible, heivy rain tending to occur regardless of topography. Ahead of the front, orographic effects varied from small to very large, depending on the existence of a moist low-level feeder cloud and seeding particles. The occurrence of heavy orographic rain ahead of the cold front is favoured by the presence of the low-level jet, since this ensures the replenishment of a high liquid water content in the feeder cloud despite its rapid depletion by washout. Low-level ascent, or at least the absence of descent, in the general airflow upwind of the hills is also needed if the feeder cloud is to have a high liquid water content. The required seeding particles in some cases originate from melted ice crystals grown aloft; alternatively they may be generated within the low-level cloud itself even in the absence of the ice phase.

SUMMARYA case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small-scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low Ow in the middle troposphere overran low-level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large-scale, and thermally-direct, circulation associated with the active warm front, the air which ascended as small-scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward-sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations.The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low 0, in the middle troposphere above the moist warm-sector air in a region where the resulting instability could be realized by large-scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre-frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.