On the Utility of Dual-Polarization Radar Fields in Targeting of Aircraft and Ground-Based Cloud Seeding Operations in the Sierra Nevada Range: Water Year 2014 As an Example

The advent of dual-polarization capabilities (hereafter dual-pol) within the National Weather Service WSR-88D radar network has intriguing implications for operational cloud seeding programs in the western United States, given the potential for dual-pol fields such as differential reflectivity, correlation coefficient and specific differential phase (not to mention the NWS experimental target classification algorithm) to discriminate between liquid and frozen hydrometeors types. Beam blockage and siting issues will provide limitations on the use of these fields in complex terrain, although these effects limit the utility of the reflectivity and Doppler velocity fields. However, because many cloud systems can be resolved adequately by WSR-88D radars on the upwind slopes, the dual-pol information provided on these upwind slopes is still potentially useful, combined with other data sources, theory and intuitive reasoning, in determining whether an approaching cloud system is heavy-laden with supercooled liquid water (SLW) or is predominantly or exclusively in the ice phase.

During Water Year 2014, the Desert Research Institute conducted: a) an operational ground-based cloud seeding program focused on the Central Sierra/Tahoe-Truckee Basin and b) a combined aircraft-ground cloud seeding research program focused south of the Tahoe-Truckee program in the Walker River Basin. WMI seeding aircraft, equipped with liquid water and temperature probes, flew as part of the Walker River Basin project, coordinating closely with the first author and other DRI personnel on calling and targeting aircraft seeding operations. Dual-pol data were heavily utilized in such determinations, with overall favorable results such that a greater amount of seeding material was utilized from the aircraft than in any previous season for which aircraft operations were conducted for the Walker Basin.

In this presentation we will provide an overview of the procedures utilized by DRI and WMI in the Walker Basin project and discuss, in brief, several case studies. We will then discuss how the same methodology was applied to the Tahoe-Truckee ground-based seeding operations and compare/contrast the results of the utilization of dual-pol data between the two projects. We conclude with some general statements as to how effectively dual-pol data was utilized in our Sierra Nevada seeding operations this past year, and lessons learned for the future.