Long-Term Radar Observations of the Melting Layer of Precipitation and Their Interpretation

Abstract

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 radar signature from melting precipitation. Five classes of vertical profiles of reflectivity in rain were identified, with three of them having precipitation undergoing a transition between the solid and liquid phase. Only one of them, albeit the most common, showed a radar brightband 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 dB 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 the observed reflectivity enhancement; the difference could be explained by effects associated with the shape and density of melting snowflakes and, to a smaller extent, by precipitation growth in the melting layer and aggregation in the early stages of the melting followed by breakup in the final stages. The brightband statistics were also significantly different for reflectivities in rain above 2.5 dBZ when observations were made with an X-band radar as opposed to the wind profiler because of the combination of attenuation in the melting layer and the fact that scattering from some of the large hydrometers above and within the melting layer depart from the Rayleigh approximation usually used to compute reflectivity. The bright band is often capped by a thin and faint dark layer, which tends to be more evident at weak precipitation intensifies.