Radar characterization of the melting layer in alpine precipitation

Melting snowflakes and/or ice crystals in stratiform precipitation increase the radar backscatter. The melting layer hence produces an enhancement known as the bright band and has particular signatures in the polarimetric radar variables. A correct identification of the freezing level, as well as other characteristics such as top, bottom, thickness, and intensity, are important especially to obtain accurate accumulated rainfall estimates.

A mobile polarimetric X-band radar (MXPol) was deployed in the area of Davos (Switzerland) from September 2009 to July 2011 to study alpine precipitation. During the observation period, numerous summer and spring events evidence the presence of a melting layer.

Range-height-indicator (RHI) scans were part of the scan strategy and were performed approximately every 3 minutes. Scan parameters include range resolution of 75 m and angular resolution of 1.45 deg. To avoid the beam broadening to affect the characterization of the melting layer, the maximum range considered in this study is 5 km (the maximum recorded is 30km). The developed algorithm is based solely in the polarimetric variables of reflectivity (Zh) and cross-polar correlation coefficient (Rhohv).

Statistics of the melting layer depth and its spatial variability were computed. Additionally, the distributions of the polarimetric variables of reflectivity, differential reflectivity, and cross-polar correlation coefficient within the melting layer were calculated for both seasons. For example, the melting layer is on average approximately 300 m deep, and 80% of the cases are within 150 and 450 m. Such statistical descriptions of the melting layer are useful to parameterize/constrain melting layer models and could be used to evaluate melting layer simulations from numerical weather models.