Anomalous Orographic Rains of O'ahu (Hawai'i) Revisited: An Over-sea Origin Indicated

Abstract

The first study of anomalous orographic rains of O'ahu, Hawai'i, utilized the average daily amounts and shower frequency characteristics as an indication of where they were first formed. In this renewed study I show that the rain rates, rather than the amounts and shower frequency, are more appropriate for revealing rain origin. Hourly amounts at gage sites alined about parallel and normal to the winds are graphically presented. The graphs reveal that many of the rains, although concentrated over the island, apparently originated as light rains over the windward sea along a crosswind line, intensifying as the wind-borne overcast of cumulus clouds was carried inland. This finding adds further support to early suggestions of an over-sea origin of many of O'ahu's orographic rains. It is hypothesized that all of these anomalous rain lines, and perhaps the trade-wind cumulus showers as well, originate upwind of the island. The idea could be readily tested experimentally.