The Microphysical Structure and Evolution of Hawaiian Rainband Clouds. Part I: Radar Observations of Rainbands Containing High Reflectivity Cores

Abstract

Radar reflectivity factors exceeding 60 dBZ are documented within shallow (<3 km), warm (>0°C), summertime tropical rainbands offshore of the island of Hawaii. Dual-Doppler radar measurements from the Hawaiian Rainband Project are used to document the formation, evolution, and kinematic structure of the high reflectivity cores. The authors show that extremely high radar reflectivities (50?60 dBZ) can develop from echo free regions (?20 dBZ) within approximately 15 min and are preceded by 5?9 m s?1 peak updrafts. High reflectivities (>50 dBZ) typically first formed in the middle or upper part of the clouds. Over the next 10?15 min, the mature high reflectivity cores extended vertically through the cloud depth and then collapsed to the surface as the updrafts weakened. A near-upright orientation of most updrafts producing these high reflectivity cores is conceptually consistent with the idea that large raindrops grow in the highest liquid water content while falling through the updraft core. Strong outflows near the inversion led to the formation of sloped radar echo overhangs surrounding the cells. The bases of the overhangs descended to the surface with time, leading to an overall increase in the width of the rainbands. Short-lived downdrafts were present in the upper part of the clouds in mature and dissipating stages of cells? life cycles but were not observed in the lower parts of the cloud, even in intense precipitation shafts.