The structure of rainbands within a mid‐latitude depression

Abstract 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 θ w 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 θ w 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.