Polarimetric Radar Characteristics of Shallow Convection in Hawaii during HERO

A Doppler on Wheels (DOW) mobile polarimetric radar was deployed to Oahu from 22 October to 13 November 2013 as part of the Hawaiian Educational Radar Opportunity (HERO). The project was the first dual-polarization field experiment performed on Oahu, and one of the few research radar deployments in Hawaii. Though the primary purpose of HERO was educational, it provided a unique opportunity to observe convection on Oahu at very high spatial and temporal resolution. The life cycle of individual convective cells associated with the two most prevalent weather patterns in Hawaii were analyzed in detail: a trade wind shower on 24 October, and a sea-breeze driven cell on 27 October.

On 24 October, it was found that both the radar reflectivity and cell size grew larger as the shower approached land in the early morning, while the differential reflectivity primarily increased when encountering the Ko'olau mountains. As the 3 km deep shower passed directly over the radar, the vertical velocity and reflectivity of the convective core were observed at ~30 m resolution, and evidence of a weak cold pool was recorded at the surface. The cell evolution suggests an enhancement of precipitation as the shower moved onshore due to land-breeze convergence and increasing friction, with the formation of larger raindrops as the shower interacted with higher terrain.

On 27 October, a single cell within a sea-breeze driven multi-cellular complex was studied from initiation through dissipation. It took ~17 minutes after initiation to reach the convective peak and echo tops near 6 km, with heavy precipitation reaching the surface a few minutes later. Polarimetric radar measurements at ~100 m spatial and 6-minute temporal resolution provide a detailed look at the evolving structure of the cell. These high resolution observations from HERO provide new insight into the mesoscale convective forcing mechanisms and microphysical processes in shallow tropical convection.