The diurnal evolution of rainfall distribution and airflow over the windward side of the island of Hawaii for the 12 strongest and 12 weakest trade-wind days during the Hawaiian Rain Band Project (HaRP) (11 July-24 August 1990) is studied from the 50 PAM (Portable Automatic Mesonet) stations and NCAR Electra flight-level data. It is shown that the rainfall production on the windward side is a complex interaction among island blocking, orographic lifting and the diurnal heating cycle.

For the late afternoon rainfall regime, the orographic rains on the windward slopes are caused by orographic lifting of trades enhanced by anabatic winds. For weak trade-wind cases, the daytime anabatic flow is more significant than strong trade-wind cases with higher rainfall (~0.6-0.8 mm/h) on the slopes . For strong trade-wind cases, more air is forced to move around the island.

For the nighttime flow regime, the convergence between the katabatic flow and stronger incoming trades produce higher nocturnal rainfall (~ 1 mm/h) on the windward lowlands around midnight. When trade winds are weaker, the katabatic flow extends farther toward the coast with less rainfall.

In the early morning, the katabatic flow is more significant when trade winds are weaker. The maximum rainfall (~0.6 mm/h) axis is along the coast for weak trade-wind cases. For strong trades, the coastal rainfall is less but the rainfall is higher on the windward slopes as compared to weak trade-wind cases.