Snow falling through the freezing level will melt and reach the ground as rain. On this image, at the top we can see a CAPPI of precipitations. At the bottom, there is a cross-section of the radar data from ground to 15 km. One can see a level where echos are stronger: the bright-band. The melting begin to occur just above this band and continue until its lower part.

This effect is due to the fact that the reflectivity depend on the 6th power of the diameter of targets multiplied by its ability to reflect. Snow flakes are much larger than rain drops but they reflect much less so echos from snow and rain have intensities relatively close. However, when snow melt, it keeps its diameter for a while and get coated with water. The radar beam sees something similar to a very larger water dropplet and the return is producing this very strong band.