LONG-TERM RADAR OBSERVATIONS OF THE MELTING LAYER OF PRECIPITATION AND THEIR INTERPRETATION

In this study, 600 h of vertically pointing X-band radar data and 50 h of UHF boundary layer wind profiler data were processed and analyzed to characterize quantitatively the structure and the causes of the rad ar signature from melting precipitation. Five classes of vertical prof iles of reflectivity in rain were identified, with three of them havin g precipitation undergoing a transition between the solid and liquid p hase. Only one of them, albeit the most common, showed a radar brightb and signature. In-depth study of the bright band and its dependence on precipitation intensity reveals that the ratio of the brightband peak reflectivity to the rainfall reflectivity is constant at 8 dB below 0 .5 mm h(-1) and then increases to reach 13 dB at 2.5 mm h(-1) and 16 d B at 5 mm h(-1). The equivalent reflectivity factor of snow just above the melting layer is on average 1-2 dB below the reflectivity of rain just below the melting layer, independent of precipitation intensity. The classical brightband explanation accounts for less than half of t he observed reflectivity enhancement; the difference could be explaine d by effects associated with the shape and density of melting snowflak es and, to a smaller extent, by precipitation growth in the melting la yer and aggregation in the early stages of the melting followed by bre akup in the final stages. The brightband statistics were also signific antly different for reflectivities in rain above 25 dBZ when observati ons were made with an X-band radar as opposed to the wind profiler bec ause of the combination of attenuation in the melting layer and the fa ct that scattering from some of the large hydrometeors above and withi n the melting layer depart from the Rayleigh approximation usually use d to compute reflectivity. The bright band is often capped by a thin a nd faint dark layer, which tends to be more evident at weak precipitat ion intensities.