Dual-Polarization Radar Characteristics of Convection in Hawaii during HERO

Abstract

In the fall of 2013 a Doppler on Wheels (DOW) mobile radar was deployed to O`ahu as part of the Hawaiian Educational Radar Opportunity (HERO). The project was one of the first dual-Polarization field experiments to date performed in Hawai`i. Dual-polarization radars send and receive pulses with both horizontal and vertical polarization, allowing them to retrieve hydrometeor Characteristics in two dimensions. With this technology, it is possible to gather information about the size, shape and type of hydrometeors. Though the primary purpose of HERO was educational, it provided a unique opportunity to observe the convective environment of O`ahu at very high spatial and temporal resolution. It is found that the high-resolution Doppler radar data enables a better characterization of the mesoscale characteristics of convection over O`ahu than was available previously. Of the many storms and weather types observed during HERO, two convective cells were chosen to represent some of the most common weather patterns found on the island: a trade wind case on 24 October 2013, and a sea-breeze case on 27 October 2013. For the trade wind shower case, it was found that although both the maximum radar reflectivity (ZH) and overall size grew larger as the shower approached land, the maximum differential reflectivity (ZDR) did not increase much until encountering the mountains. As the storm passed directly over the radar site, the high-resolution vertical velocity structure of the convective core was observed. It is shown that the trade wind shower exhibited a bimodal positive velocity structure as it came onshore. As it passed to the west of the radar, a 7 m s-1 updraft was observed at the center. For the sea-breeze storm, the convective lifecycle from initiation through dissipation was studied for a single cell. From convective initiation, it took 15-19 minutes to reach the convective peak, and another 6 minutes for heavy precipitation to fall at the surface. It is shown from CFADs of ZDR that a large mass of small drops are always located just above the high-reflectivity convective core. Larger drops are located within the core, and fall out quickly following the convective peak of the updraft. Joint PDFs in ZH-ZDR space show that as the updraft reaches its convective peak elevation, drops grow large enough to be affected by breakup. ZH and ZDR were infrequently observed above 52 dBZ or 3 dB.