Spatial Variability of the Melting Layer in an Alpine Valley Using a Polarimetric X-band Radar

Spatial variability of the melting layer in an alpine valley using a polarimetric X-band radar.

F. van den Heuvel, A. Berne, M. Gabella

Measuring precipitation with radar in complex terrain such as the Alps, is complicated by many factors such as partial and total beam shielding by terrain, the influence of orography on the dynamics and microphysics of precipitation as well as the shallow depth of precipitation during cold seasons. The extrapolation of measurements aloft to the ground level, or vertical profile (VPR) correction, is a commonly applied technique to compensate for the lack of direct visibility with the radar. Moreover, in order to increase the availability of radar information, MeteoSwiss has extended its weather radar network with two polarimetric C-band radars at high altitude locations (~ 3000 m asl) in the Swiss Alps. This poses a new challenge of the applicability of existing vertical profile correction techniques to these high-altitude measurements but also provides an opportunity to use polarimetric radar variables for the improvement of quantitative precipitation estimates (QPE).

The melting layer (ML) is a transition region from solid precipitation to liquid precipitation and an important feature of stratiform precipitation. It is characterised by a high reflectivity factor due to the increase in dielectric constant as solid hydrometeors are coated by a thin layer of water. Algorithms for QPE and VPR extraction often assume that the ML is uniform in space and time, however for events with rain-snow transitions and in an orographic context spatio-temporal variations of the ML can be considerable.

In order to better characterise the variability of the ML in the Alps, the Environmental Remote Sensing Laboratory (LTE) of EPFL, Lausanne deployed an X-band Doppler dual polarization radar (MXPol) at the ground level (460 m asl) in the main valley of the Swiss Alps and in the vicinity (~40 km) of the operational MeteoSwiss C-band radar. During the Winter season from November 2016 to March 2017 the MXPol radar measured 15 precipitation events corresponding to over 250 hours of precipitation, and covering various temperature and synoptic conditions. In the considered RHI scans, the melting layer detection algorithm detected the presence of a ML at some point during all of the events. This study presents the statistics of the ML in an inner alpine valley in a Winter context and stratified for different weather conditions.