Radar and lidar observations of the melting process in the bright band

The melting layer of precipitation is known for its high radar reflectivity, and is thus called the bright band. New and unexplained are lidar measurements of the melting layer. This optical instrument receives fewer reflections from the melting layer than from either the ice precipitation above or the rain below. To this phenomenon has been coined the name dark band by Sassen and Chen who published the first clear measurement of this phenomenon. In this article measurements are analysed using lidar together with radar to gain more insight into this dark band. The difference in lidar backscatter between melting layer and its environs is defined as its depth and can amount up to 20 dB compared to the rain (water dark band) and up to 30 dB compared to the ice above (ice dark band). The radar bright band is usually explained by an increase of the radio refractive index of the melting particle at the top of the melting layer and a decrease of particle size and number density (both due to collapse of the melting particle) at the bottom of the melting layer. Aggregation (in the top) and breakup (bottom) work together to increase the particle size in the middle of the melting layer. This enhances the radar reflectivity of the melting layer. There is still a debate on when this is significant. Explanations for the dark band that are discussed are: crystal imperfections, enhanced backscatter of raindrops for vertically pointing lidar, particle aggregation and breakup, collapse of the particle, and enhanced vertical backscatter of the ice precipitation