Trade-wind clouds and Hawaiian rainbands

Two case studies of rainband evolution off the windward coast of the Big Island of Hawaii are presented along with an overview of the complete radar and satellite dataset from the Hawaiian Rainband Project conducted in the summer of 1990. These studies reveal that radar-observed rainbands and cells offshore of the windward coastline are nearly always embedded within larger-scale stratocumulus cloud patches and/or cloud lines moving in from the northeastern Pacific Ocean in the easterly trade winds. These cloud patches develop in the tradewind flow downstream of the large shallow stratocumulus cloud mass that covers much of the northeastern Pacific Ocean between Hawaii and California. Tropical cyclones originating in the intertropical convergence zone sometimes move north through this stratocumulus cloud mass, producing, in their wake, cloud-free regions that then advect toward the Hawaiian Islands. These regions contribute to the variability of the organization of cloud patterns and rainband occurrence offshore of Hawaii. Previous studies have employed data analysis and modeling to explain dynamical and thermodynamical processes that lead to the development of a flow separation line upwind of Hawaii and cloud formation along the line. This study shows that most rainbands occurring near the flow separation line first form upstream within cloud patches and/or cloud lines approaching the island and stretch within the deformation flow as the trade winds deflect around the island. Analyses of thermodynamic soundings suggest that lifting associated with convergence at the flow separation line is frequently sufficient to produce clouds but insufficient to trigger free convection and rainbands. A diurnal oscillation in rainband frequency upwind of the island is documented. This diurnal behavior in rainband frequency may be related to the strong diurnal radiational forcing that occurs at the top of the tradewind marine layer.