Dual Polarization X-band QPE in Support of the OLYMPEX Project

This presentation will give an overview of work by Environment and Climate Change Canada (ECCC) on QPE methods for dual polarization X band systems. The primary focus will be radar activities of during the OLYMPEX project, a NASA Global Precipitation Mission GPM Ground Validation field campaign on the Olympic Peninsula in the US Pacific Northwest during the winter of 2015/16. The Canadian contribution was intended to contribute to microphysical studies of the precipitation formation mechanisms around and to the north of the Olympic Mountains and to evaluate GPM products in complex situations including coastal waters and mountainous terrain. Since ECCC is considering use of X-band radar as backups to network radars across Canada, it is important to understand their processing issues in a variety of situations.

For OLYMPEX, ECCC deployed a new Selex dual-polarization X-band scanning radar on the northern shore of the Strait of Juan De Fuca, which separates Vancouver Island, Canada and Washington state, USA, with the intent to examine precipitation on the leeside of the Olympic mountains. The radar site had a direct view of the enhanced surface instrumentation site at Hurricane Ridge, 45km to the south, and other surface stations contributing to the OLYMPEX project. The radar was configured to perform a 5-minute cycle combining PPI surveillance scans and RHI scans over that site and operated continuously from November 2015 to March 2016.

The post analysis is challenged by a number of factors such as the extreme clutter from the mountain side, sea clutter, attenuation and the number of large ships directly visible from the shore. Primary attenuation correction of Z and Z_DR, particularly with ZPHI methods, are complicated by non-meteorological PhiDP from sea clutter near the radar and extensive ground clutter in the mountainous region. Rainfall accumulation from preliminary dual polarization QPE methods and comparison with rain gauges indicated several ongoing issues to address:- 1) System default coefficients used for attenuation correction may not be optimal, 2) An overall system Z_DR bias to be accounted for and 3) Low melting layers during significant precipitation periods combined with high elevation scanning required to clear mountain clutter, limits the comparison with surface observations. The observed system biases in ZDR during the project will be corrected using observations in light rain. The QPE methodology and solutions developed here are being assessed and refined with an identical X-band radar that is collocated with a C-band dual polarized radar in southern Ontario.