The effects of the island of Hawaii on offshore rainband evolution

The diurnal cycle of airflow on the island of Hawaii and the structure of the low-level flow separation line between the island-induced offshore flow and incoming trade winds are reasonably well understood. This study examines the formation and organization of nocturnal and early morning offshore Hawaiian rainbands, the flow patterns associated with these rainbands, and their relationship to the flow separation line. This investigation uses the complete radar and aircraft dataset from the 1990 Hawaiian Rainband Project. The formation and evolution of nocturnal and early morning rainfall on the windward side of the island were found to be related to four major factors: (a) the existence or absence of trade wind cloud patches arriving from the northeast Pacific, (b) the effect of island blocking on the trade wind flow as it approaches the island, (c) the low-level convergence and thermal and moisture stratification near the flow separation line, and (d) the position of the flow separation line, controlled by both the Froude number and diabatic processes (nocturnal radiative cooling and evaporative cooling) occurring over the island. If trade wind cloud patches are absent over the ocean upstream of the island, rainbands normally will not form upstream of the flow separation line. Under normal Froude number conditions, if the ratio of the low-level trade wind speed to the Brunt–Väisälä frequency, U/N, is greater than the height of the level of free convection, rainbands will form offshore over the flow separation line. Otherwise, rainbands will not form at this location. When the Froude number is elevated (typically, strong trade wind cases) rainbands will form over the windward shore independent of this ratio. If trade wind cloud patches are present, which is the typical case, light rain normally falls far from the island. These rain regions are enhanced by the weak convergence associated with island blocking and are organized into shore-parallel linear features by the deformation flow induced by the island. The enhancement generally occurs about 35–50 km from the island. Rainbands undergo a second enhancement over the flow separation line. Rainbands will also form over the flow separation line if U/N is greater than the level of free convection.