RADAR OBSERVATIONS OF SNOW FORMATION IN A WARM PREFRONTAL SNOWBAND

Radar reflectivity measurements and sounding data were analyzed to inv estigate snowfall production in a long-lasting snowband that formed in advance of a warm surface front moving across Alberta. The sounding d ata indicated that the band could have been forced by slantwise overtu rning during the release of moist symmetric instability combined with frontogenesis. The stability analysis presented here is novel in that it includes ice phase thermodynamics, neglected in previous studies of slantwise convection. Radar reflectivity fields were analyzed to dete rmine the total snow content and the mass outflow rate as factors of t ime. The peak value of total snow content was 17 kilotons per km of sn owband, and the peak mass outflow rate was 10 tons s(-1) km(-1). The s nowfall rate averaged across the cloud base was about 0.8 cm h(-1), an d the average snow content remained close to 0.2 g m(-1). The characte ristic rime (defined as the ratio of total snow content over mass outf low rate) was about 30 minutes, which is approximately the rime needed for the growth of snowflakes by aggregation in the observed temperatu re range. The precipitation efficiency of the snowband, defined as the ratio of snow mass outflow to water vapour inflow was estimated to be 14%. The precipitation production values observed in the Alberta snow band are compared with previous estimates reported for frontal rainban ds and Alberta thunderstorms.