Abstract : J.1
Benefit and optimal usage of polarimetry in mountainous terrain

Katja Friedrich, Urs Germann, Pierre Tabary

The implementation of polarimetric Doppler radars in mountainous terrain has already been in the planning or implementation phase by several institutions. Polarimetry has been proven to be useful primarily in terms of (1) quality control distinguishing precipitation from non-precipitation, (2) classifying hydrometeors in particular snow, hail, graupel, (3) correcting data for attenuation and beam shielding, and (4) identifying areas of heavy rain. Rain fall rate estimation improves indirectly by using polarimetry through a better correction and quality control of radar reflectivity. Nevertheless in order to achieve a benefit from polarimetry, the polarimetric quantities need to be measured with high accuracy. Most investigations on polarimetry have neglected the influence of orography. Ground clutter can totally and partially shield the transmitted radar beam and can contaminate directly the receiver signal due to strong backscattering signal. Additionally, many of the measurements in mountains are obtained within or above the melting layer which also decreases the usefulness of polarimetry. In the first part of this study we show how polarimetric phase measurements compared to radar reflectivity can be applied usefully in case of partial radar beam shielding. In the second part we illustrate the sensitivity of polarimetric quantities and rainfall estimation to the presence of ground clutter and contamination by the melting layer. The analysis is based on data obtained during several stratiform and convective precipitation events observed by the Météo-France C-band polarimetric Doppler radar at Trappes near Paris. We determined the critical level of ground clutter and melting layer contamination in order to meet accuracy values required for polarimetric parameters and rainfall rate. Finally a concept on how to optimally use polarimetry in mountainous terrain is presented.