Radar and Remote Sensing

Utilizing a Polarimetric Size Sorting Signature to Develop a Convective Nowcasting Algorithm

Joey Picca

School of Meteorology

24 March 2016, 1:30 PM

National Weather Center, Room 1350
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The nationwide implementation of polarimetric radar continues to fuel a resurgence in microphysical studies of convective cells and other atmospheric phenomena. More specifically, recent research has targeted polarimetric signatures of particle size sorting, in which high differential reflectivity factor (ZDR) is collocated with low-to-moderate reflectivity factor at horizontal polarization (ZH). The cause of this signature is the presence of medium/large rain drops (as well as water-coated ice particles in some instances) in low concentration, in addition to a relative deficit of small drops. Such particle-size distributions (PSD) are not uncommon in convection, as strong horizontal and vertical motion, such as that of an updraft, can promote these PSDs. The associated polarimetric signature can thus assist a forecaster in diagnosing the location and perhaps even the intensity of updrafts. Furthermore, recent work has shown that vigorous storm-relative flow (which is related to storm-relative helicity) can manifest in the ZDR arc signature, also the result of size sorting.

Despite their somewhat ubiquitous nature, relatively little work has attempted to leverage the diagnostic and prognostic potential of this signature for operational purposes. While Snyder et al. (2015) developed a ZDR column algorithm for identifying updrafts above the environmental 0 C level, it can suffer from poor WSR-88D spatiotemporal resolution at higher altitudes. Moreover, besides their work, little else exists for harnessing the operational utility of size sorting signatures in an operational environment. As such, we have developed a novel algorithm that targets this signature by synthesizing polarimetric variables into a single output to identify growing convection. At the core of the algorithm, a unique Z-ZDR relationship is created for each elevation scan and positive ZDR outliers (indicative of size sorting) are identified. By normalizing the Z-ZDR relationship, the algorithm is immune to systemic ZDR biases—a common issue in the WSR-88D network. This presentation will provide an overview of the algorithm design, results from preliminary test cases, and topics for future exploration within the framework of hydrometeor size sorting.

Radar and Remote Sensing Seminar Series website