Role of eddy circulations in early formation of island-induced tropical rainbands as determined from dual Doppler radar analysis

Hilo, Hawaii annually receives an average of over 300 cm of precipitation. observations and modeling studies based on data collected during the Joint Hawaiian Warm Rain Project in 1985 identified a low level convergence zone just upstream of the island. Lifting over this convergence zone was assumed to be the major generator of convection offshore of the island. Radar data collected in 1990 during the Hawaiian Rainband Project (HaRP) showed that convective cells and rainbands were consistently present well upstream of this convergence zone. As these offshore rainbands were advected toward the island in the trade wind flow, they sometimes were enhanced upon interaction with the low level. convergence zone. Many of the offshore rainbands developed very high reflectivities and became well organized with embedded cells before they reached the convergence zone. Others were enhanced over the convergence zone. Our recent analyses of the dual-Doppler radar data have found that several of the cells are associated with small-scale eddies or vortices in the flow. The eddies form upstream of the island aand are advected toward the island. The eddies then appear to be steered along the island shoreline to either side of the island. This paper briefly describes the presence and structure of these small-scale eddies within the context of air flow perturbed by the island, focusing on the 10 August 1990 case study.