Signal properties from dual-polarization radars provide information regarding hydrometeor size, shape, orientation, and thermodynamic phase in precipitating storms. Because polarimetric measurements are particularly sensitive to the presence of large, wetted particles that characterize the melting layer, the response signatures in the measurements can be used to designate the freezing level.

A freezing level detection algorithm has been developed that utilizes the prominent melting layer signatures in vertical profiles of radar reflectivity, linear depolarization ratio, and co-polar correlation coefficient. Freezing level determinations are made by comparing observed distributions of the polarimetric variables to modeled profiles. The expected error in the consensus estimate is 0.1-0.2 km.

Algorithm performance will be demonstrated primarily with a dataset collected over the Oregon Cascades in November-December 2001, during the IMPROVE II field program. On 28 November an unusual event with two freezing levels developed in association with an advancing warm front. Initially, a single freezing level existed near the surface. As the warm front advanced, an elevated freezing level appeared. Hydrometeor habits deduced from the polarimetric measurements will also be examined. These retrievals suggest that initial melting of particles in the elevated warm layer was incomplete. Changes in the freezing levels responding to the movement of the synoptic low pressure center and its associated warm front will also be documented.