Aspects of melting and the radar bright band

Abstract

The melting of snow as it falls through the 0 °C level is a significant meteorological process that is important for its impact as the bright band of enhanced reflectivity in radar observations. Thus, it is necessary to understand the variability of the phenomena and to determine the factors upon which it depends. This paper reports on preliminary investigations into the observations of the bright band over the UK using vertically pointing radar. These results are compared with output from a simple model of the melting of snowflakes and with other observations from Canada and the Netherlands. The vertical depth of the bright band was determined from the vertical pointing radar data for four cases of widespread frontal rainfall. An increase in the depth of the bright band was seen with increasing background reflectivities. Depths of 100-150 m at 10 dBZ increased to 200-400 m at 25 dBZ. Results from a simple model of the melting of snowflakes were compared with the vertical pointing radar observations. Similar trends were seen in the model output, but in general the model produced deeper but less intense bright bands. Notable in the model results was the lack of strong dependence of the depth on vertical air motions. Indeed, the bright band depth only increased by approximately 30 m in a downdraft of 1 m s^-1. Comparisons of the bright band characteristics with other observations from elsewhere show that the bright band depth was similar to that observed by Klaasen (1988) in the Netherlands, but shallower than those observed by Fabry & Zawadski (1995) in Canada.