Air motion and precipitation growth in a major snowstorm

Abstract

AbstractA mesoscale analysis of the snowstorm of 8–9 January 1982 has been carried out using data from the U.K. weather radar network, the geostationary satellite Meteosat and other more conventional sources. More than 60cm of snow accumulated over S.E. Wales and neighbouring parts of England in association with an intense frontal zone. Much of the snow occurred during a period when synoptic forcing and orographic modulation were essentially steady. The depth of cloud and moist air was not great in the region of heavy snowfall in south Wales, but an orographically enhanced circulation on the mesoscale nevertheless led to efficient growth of precipitation through the seeder‐feeder mechanism. The seeder cloud was generated within a shallow current of air which ascended above the front and which originated from low levels, probably mostly from the boundary layer, on the warm side of the front. Precipitation was initiated within this current repeatedly along a well‐defined and remarkably stationary line just upwind of the south Wales coast. As the rising current travelled inland towards the E.N.E., convective generating cells embedded in it produced precipitation particles which fell into the underlying cold easterly flow. Considerable growth occurred as these particles descended through orographically enhanced feeder cloud, much of the growth occurring in the lowest kilometre or so.