The effects of the island of Hawaii on offshore rainband evolution

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.