Anomalous Orographic Rains of Hawaii

Abstract

Observations, on the island of Oahu, Hawaii, of unusual rains and clouds in marine air during fresh easterly winds show that rain can form within and fall continuously from a shallow layer of warm stratocumulus in about 5 to 13 minutes. This time estimate is shown to be dependent upon where, in the rapidly moving marine cloud system, an island effect initiating raindrop growth is assumed to begin. The clouds are associated with frontal passage nearby, and they form light rains of about 1 to 8 mm/h, averaging 3.5. On the windward shore the clouds produce sparsely distributed showers, suggesting a cell-like structure of the rain development processes over the sea, whereas a few kilometers inland over the Koolau Mountain Range they produce continuous rain and then quickly dissipate. The raindrop-generating processes seem to occur largely over the island rather than over the windward sea, as has been previously suggested concerning the more common orographic showers from the northeasterly tradewind cumuli. The unusually short raindrop-formation times revealed are thought to require some modification of current ideas about the collision-coalescence process of raindrop growth in these oceanic clouds. It is suggested that turbulence in the clouds caused by passage over Oahu may accelerate the collision-coalescence process and thus reduce the raindrop growth times.