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

Abstract

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.

Snow bands over the Gulf of Finland in wintertime

“…At 0000 UTC on 2 February (Fig. 4a), the easterly winds had Â12 m s , and similar to those found by other authors (Andersson and Nilsson, 1990;Norris et al, 2012). Finally, considering the Gulf of Finland as a lake, the morphology of the observed and simulated snow band at this time is quite similar to Type I, which is described by Niziol et al (1995).…”

“…(1 km across the long axis of the gulf, as other authors found in other snow-band events (Laird et al, 2003a;Norris et al, 2012;Savija¨rvi, 2012). During the second part of these events, a southerly wind regime (wind across coasts) prevailed, and the observed and simulated coastal line of precipitation moved inland to the downwind (Finnish) coast.…”

“…Moreover, Alestalo and Savija¨rvi (1987) simulated a single band when wind speed was higher than 10 m s (1 . Norris et al (2012) found winds greater than 10 m s (1 in the investigation of the snow bands over the English Channel and Irish Sea. The ratio between the wind speed (V) and the fetch over the open water (L) was used to determine the lake-effect for idealised cases (Laird et al, 2003a).…”

“…Steenburgh et al, 2000;Onton and Steenburgh, 2001;Steenburgh and Onton, 2001). Outbreaks from inland to the open sea have been detected and studied along the New England coast (Bosart, 1975), off the north coast of Germany (Pike, 1990), over the Sea of Japan (Nagata, 1987), as well as over the English Channel and the Irish Sea (Norris et al, 2012). These last authors performed mesoscale numerical experiments, concluding that the studied snow bands are a response to surface heating of cold-air advection over relatively warm water, and that the role of orography and roughness length differences between sea and land affect only the timing and location of the snow bands, but not their occurrence.…”

Influence of Terrain Resolution on Banded Convection in the Lee of the Rocky Mountains

“…Snowbands have been observed in association with mesoscale gravity waves (e.g., Bosart et al 1998;Koch and Siedlarz 1999). Lake-effect and sea-effect snowbands (e.g., Passarelli and Braham 1981;Niziol et al 1995;Norris et al 2013) can form in the boundary layer associated with horizontal convective rolls, even over land (e.g., Schultz et al 2004). Bands have also been attributed to the release of moist symmetric instability (e.g., Seltzer et al 1985;Wolfsberg et al 1986) or the release of conditional, dry symmetric, and inertial instabilities (e.g., Jascourt et al 1988;Knox and Hoggatt 1996;Hoggatt and Knox 1998;Schultz and Knox 2007).…”