Snowbands over the English Channel and Irish Sea during cold-air outbreaks.

Persistent northerly-to-easterly cold-air outbreaks affected the UK during the winters of 2009-10 and 2010-11, with the resulting convection frequently organizing into snowbands over the English Channel and Irish Sea. Sounding data and composite radar reflectivity images from the Met Office Nimrod precipitation radar network reveal that these bands formed along the major axis of each body of water (or sea) when the boundary-layer flow was roughly parallel to each of those axes (along-channel). For both seas, a band was present the majority of times that the 850 hPa flow was along-channel. Of these times of along-channel flow, the 850 hPa wind speed and surface-to-850 hPa temperature difference were significantly greater when bands were present than when they were not. For the English Channel only, the land-sea temperature difference was also significantly greater when bands were present than when they were not. In a real-data Weather Research and Forecasting model (WRF) control simulation of a typical band over the English Channel, a trough develops over the water and offshore air streams from either side converge along it. In the absence of surface fluxes, the trough, convergence and organized precipitation fail to develop altogether. Orography and roughness-length variations are less important in band development, affecting only the location and morphology. [ABSTRACT FROM AUTHOR]