On the Relationship Between Large Hail and Polarimetric Signatures at C-Band: Two Case Studies in the RELAMPAGO-CACTI Domain

During the austral warm season, central part of Argentina is affected by deep moist convection that is responsible for many high impact weather events. Severe weather (i.e., large hail, strong damaging wind gusts, tornadoes, flash flood, and lightning) is responsible for damage in the infrastructure and agriculture sectors. Polarimetric radars can help to detect hail within storms but presents a special challenge due to polarimetric characteristics of hailstones depending on their size, shape, falling behavior, degree of melting and radar wavelength. In particular, strong attenuation and differential attenuation at C-band due to the presence of hail further complicates hail detection and sizing. The C-Band Doppler dual polarization weather radar called RMA1 (Radar Meteorológico Argentino-1) manufactured by INVAP S.E. was used in this study. RMA1 was installed in early 2015 as part of the National Weather Radar System expansion in Argentina. This study represents the first fully-detailed case studies of severe weather analyzed with this radar. RMA1 is the first of a series of 10 C-band dual-polarization radars to be installed in Argentina during 2017 and 2018.

The present work centers around two severe storms that developed over the RELAMPAGO-CACTI area. RELAMPAGO-CACTI is a multi-agency field campaign scheduled for austral spring 2018 that will study convective storms in central Argentina, with a special focus on the Sierras de Córdoba, where these events frequently occur. The first case occurred on 7 November 2015; a severe hail storm (maximum 50 mm diameter) developed near Alta Gracia and Anisacate (31°40′S, 64°26′W), causing in addition to hail damage, flash floods and damaging winds. The second case also produced large hail (maximum 30 mm diameter), flash floods, and damaging winds in Villa Carlos Paz (31°24′S 64°31′W) on 15 February 2017. Both cases were characterized by the presence of isolated convective cells in the early stages of their life cycle, reaching supercellular characteristics in the mature phase.

Two hail identification approaches observed by RMA1 are evaluated: 1) a single-polarization approach based only in the horizontal reflectivity, and 2) a dual-polarization approach applying an hydrometeor identification scheme based on a fuzzy-logic approach with polarimetric radar observables. The first approach cannot distinguish among different types of hydrometeors. However, some features in the vertical profile of horizontal reflectivity, including high value occurrence below specific environmental temperatures, can represent the physical processes related to hail growth. The second approach provides a more objective identification of hail hydrometeors inside the cloud. In both cases, hail signatures and the impact of severe attenuation and differential attenuation at C-band will be discussed in the perspective of forecast applications (e.g., nowcasting, flight safety, etc.). Furthermore, the timing of hail occurrence and other severe weather phenomena are analyzed alongside Alertamos information (a smartphone app for citizen reports) and lightning activity from the University of Washington World-Wide Lightning Network and Vaisala GLD360 cloud-to-ground flash density products.