Linking rain into ice microphysics across the melting layer instratiform rain: a closure study

Abstract:

This talk examines the relationship between the characteristics of rain and the properties of the ice cloud from which the rain originated. The study is based on the analysis of ground-based radar data in proximity the melting zone and is limited to stratiform precipitation where melting signatures are easy to detect. Our results confirm the widely accepted expectation that the mass flux through the melting zone is well preserved when averaging on long-time scales. For the 6-hour case study period, the total accumulation of rain (2.65mm) and the melted equivalent accumulation of snow (2.60mm) show only a 2% difference. Nevertheless, during some periods large discrepancies between precipitation rates above and below the melting zone are reported. For instance, when large aggregated snowflakes occur above the freezing level the snowfall rate is approximately 33% larger than the corresponding rainfall rate below. In turn, the mass flux below the melting zone was usually greater than above when dense snow is detected in the cloud. We hypothesize that these precipitation rate discrepancies are associated to the relative humidity changes within the melting layer, with the regions dominated by rimed snow more likely to be supersaturated. Moreover, it is shown that, not only the mass flux but also the mean (mass weighted) size of particles below and above the melting zone are strongly linked. On average, the melted equivalent diameter of ice is 21% larger than the diameter of rain underneath, with low uncertainty on this parameter (~0.12 mm only). This prediction leads to slight underestimation of the ice size during aggregation, potentially due to the breakup of melting snowflakes, and to overestimation for dense ice particles. These results are based only on the 6-h case study and it is advisable to confirm them on long-term observations.