The nature of orographic rain at wintertime cold fronts

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 four 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 are explained using data from series of hourly rawinsonde ascents from a single 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, heavy 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.