Operational detection of rain-snow boundaries

The horizontal transition from rain to snow and vice versa near the ground has received very little attention compared to a similar phase change occurring in the vertical direction. Such horizontal transitions are not uncommon in latitudes as those of Montreal, Canada where sharp temperature gradients during the cold season are of particular concern for ground transportation safety during periods of precipitation. The polarimetric characteristics of light rain are nearly undistinguishable from those of light snow, making it difficult to identify their presence on a point by point basis. However, the transition zone can be easily identified by the relatively high differential reflectivity (Zdr) and lower cross-correlation coefficient (ñhv) values. This zone is also recognizable when it is of considerable vertical extent and reaches the ground within medium radar ranges (< 80 km), thus enabling the identification of regions of surface rainfall or of snowfall from the three-dimensional structure of polarimetric observables. Our operational fast-scanning S-band polarimetric radar is ideally suited to follow the motion of such transition zones, making it possible to devise algorithm for their detection, tracking and short-term forecasting. Several examples are illustrated showing both the simplicity and complexity of such a goal.