Modeling Radar Attenuation by a Low Melting Layer With Optimized Model Parameters at C-Band

At northern latitudes, it is not uncommon for a melting layer of precipitation to touch or be close to the ground. For a low elevation angle, radio waves from a surveillance weather radar scan can travel a long distance through a melting layer. The resulting attenuation can be significant and must be taken into account when radar observations are interpreted. In this paper, we use a melting layer model to derive the relations between the specific attenuation caused by propagation through a melting layer and the reflectivity factor associated with this layer. The relations derived in this paper are based on modeled attenuation values for a variety of conditions and input parameters, i.e., rain rate, snow density, and rain drop size distribution parameters. The model parameters were constrained by vertically pointing Doppler C-band radar measurements of two events. The fitting procedure is presented for two different cases, of unrimed and rimed snow, and case-specific estimates of the expected attenuation of the horizontal scanning are suggested. Based on measurements of precipitation collected on December 10, 2011, by the University of Helsinki Kumpula radar, we also demonstrate that radar signal attenuation due to propagation through a low melting layer can be on the order of 7 dB or higher over a distance of 40 km.