A Case Study of Nocturnal Rain Showers over the Windward Coastal Region of the Island of Hawaii

Abstract

Nocturnal rain showers over the windward side of the island of Hawaii were investigated from the late afternoon of 2 August to the early morning of 3 August 1990 during the Hawaiian Rainband Project (HaRP). Three types of rainbands produce rainfall peaks over the lowland/coastal region during the evening, after midnight, and along the coast in the early morning. In the early evening, an inland rainband develops over the lower slopes as a result of orographic lifting and low-level forcing along the drainage front. As the drainage front progresses toward the coast, new rain cells continue to develop along the drainage front. These cells move westward with the trade winds aloft and dissipate. After the drainage front moves to the Hilo coast, new cell generation along the drainage front ceases. It appears that in the absence of orographic lifting, the shallow (?0.2 km) katabatic flow offshore is not deep enough to lift the low-level air to saturation. Thus, the rainband weakens and dissipates. It lasts about 6 h and produces the heaviest rainfall over the lowland/coastal region during the analysis period. During the evening transition, rain evaporative cooling aloft deepens the cool pool behind the drainage front over land. After the drainage front moves off the coast around 2300 HST, cold air continues to move from lower slopes and lowlands toward the coast. The offshore flow gradually deepens. By ?0207 HST, the depth of the katabatic flow over the coastal region reaches 0.48 km, which is above the level of free convection. A rainband develops along the drainage front offshore. As the rainband moves westward over the deepened katabatic flow, the katabatic flow is replaced by easterly winds. The rainband weakens and dissipates over the lower slopes. During the early morning, two groups of trade wind rain showers move into the coastal region. They are enhanced offshore in the convergent zone between the offshore flow and the trade winds. When the first group of rain showers moves over the katabatic flow, the outflow associated with the rain showers deepens the offshore flow (?0.6 km) east of the rain showers. The second group of rain showers is more intense than the first group as the rain showers move into the convergent zone and interact with the deep offshore flow. Nevertheless, they weaken rapidly over the deep offshore flow and produce little rainfall along the coast.